CHAPTER OVERVIEW 20: Biogeochemical Cycles Learning Objectives Summarize the concept of biogeochemical cycles Explain the hydrologic cycle and its importance for Ecology Distinguish between the biological and biogeochemical cycles of carbon Describe the nitrogen cycle and its various pools and fluxes Illustrate the phosphorus cycle and the effects of phosphorus on the environment Define eutrophication and dead zones and their importance for Ecology Explain the sulfur cycle Topic hierarchy 20.1: Biogeochemical Cycles 20.2: The Water (Hydrologic) Cycle 20.3: The Carbon Cycle 20.4: The Nitrogen Cycle 20.5: The Phosphorus Cycle 20.6: Eutrophication and Dead Zones 20.7: The Sulfur Cycle Summary This chapter discusses earths major biological and biogeochemical cycles including cycling of water, carbon, nitrogen, phosphorus, sulfur. After a brief introduction to biogeochemical cycles and a review of the hydrologic cycle, the pools and fluxes of the major elemental cycles are introduced. Effects of the changes in the carbon, nitrogen, and phosphorus cycles caused by humans are also discussed including the concepts of eutrophication and dead zones in aquatic and marine systems. 20: Biogeochemical Cycles is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
20.1: Biogeochemical Cycles Key Points Carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur are conserved and recycled in the atmosphere, on land, in water, or beneath the earth's surface. Materials are recycled via erosion, weathering, water drainage, and the movement of tectonic plates. Water is essential to all living processes, while carbon is found in all organic (carbon-based) macromolecules. Nitrogen and phosphorus are major components of nucleic acids and play major roles in agriculture. Sulfur plays a role in the three-dimensional folding of proteins and is released into the atmosphere by the burning of fossil fuels. Biogeochemical Cycles: Introduction Energy flows directionally through ecosystems, entering as sunlight (or inorganic molecules for chemoautotrophs) and leaving as heat during the many transfers between trophic levels. However, the matter that makes up living organisms is conserved and recycled. The six most common elements associated with organic (carbon-based) molecules (carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur) take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath the earth's surface. The cycling of these elements is interconnected. Geologic processes, such as weathering, erosion, water drainage, and the movement of the continental plates, all play a role in this recycling of materials. Because geology and chemistry have major roles in the study of this process, the recycling of inorganic matter between living organisms and their environment is called a biogeochemical cycle. It is important to remember that while matter and energy are processed in cycles, they aren't necessarily moving in a simple circle and don't really have a beginning or an end. Today, anthropogenic (human) activities are altering all major ecosystems and the biogeochemical cycles they drive. The components of organic molecules are constantly being stored and recycled as part of their biogeochemical cycle. Water, which contains hydrogen and oxygen, is essential to all living processes. The hydrosphere is the area of the earth where water movement and storage occurs. Water can be liquid on the surface and beneath the surface or frozen (rivers, lakes, oceans, groundwater, polar ice caps, and glaciers) or exist as water vapor in the atmosphere. Carbon, found in all organic macromolecules, is an important constituent of fossil fuels. Nitrogen, a major component of our nucleic acids and proteins, is critical to human agriculture. Phosphorus, a major component of nucleic acid (along with nitrogen), is one of the main ingredients in artificial fertilizers used in agriculture and their associated environmental impacts on our surface water. Sulfur, critical to the 3-D folding of proteins (as in disulfide binding), is released into the atmosphere by the burning of fossil fuels, such as coal.
Earth has a hydrosphere, where water movement and storage occurs. The cycle is important for leaching
certain components of organic matter into rivers, lakes, and oceans, and is a reservoir for carbon.
The cycling of all of these elements is interconnected. For example, the movement of water is critical for the leaching of nitrogen and phosphate into rivers, lakes, and oceans. Furthermore, the ocean itself is a major reservoir for carbon. Thus, mineral nutrients are cycled, either rapidly or slowly, through the entire biosphere, from one living organism to another, and between the biotic and abiotic world. What pops into your mind when you hear the word reservoir? Probably a body of water, small or large, that is impounded behind a dam. In Earth systems science, the term reservoir is used for a distinctive kind of place where a certain kind of material is stored, or resides, for some period of time. This part of a cycle that holds an element or water for a short period of time is sometimes also called an exchange pool, or pool. For example, the atmosphere is an exchange pool for water. It usually holds water (in the form of water vapor) for just a few days. Some other examples of reservoirs or pools for water you will encounter in the course include glaciers; the soil layer; the aggregate of bodies of fresh water on the continents (rivers and lakes). Material moves into and out of reservoirs. The rate at which a given material moves between reservoirs is called a flux. If the flux of material into and out of a given reservoir is the same for some period of time, that reservoir is said to be in a steady state. Commonly, however, the flux in and the flux out are not equal. Contributors and Attributions Modified by Kyle Whittinghill (University of Pittsburgh) from the following sources Section 10.1 from The Environment of the Earth's Surface by John Southard Professor Emeritus (Earth, Atmospheric and Planetary Sciences) at Massachusetts Institute of Technology Sourced from MIT OpenCourseware Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Original content by OpenStax (CC BY 4.0; Access for free at https://cnx.org/contents/b3c1e1d2-83...4e119a8aafbdd). 20.2: Biogeochemical Cycles by OpenStax, is licensed CC BY 6.6: Water Cycle by CK-12: Biology Concepts, is licensed CC BY-NC General Microbiology Provided by: Boundless.com. License: CC BY-SA: Attribution-ShareAlike 20.1: Biogeochemical Cycles is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts. 12.1: Introduction - Systems, Cycles, Reservoirs, and Fluxes by John Southard is licensed CC BY-NC-SA 4.0. Original source: https://ocw.mit.edu/courses/12-090-the-environment-of-the-earths-surface-spring-2007/.
Key Points Water cycling affects the climate, transports minerals, purifies water, and replenishes the land with fresh water. Water with a longer residence time, such as water in oceans and glaciers, is not available for short-term cycling, which occurs via evaporation. Surface water evaporates (water to water vapor) or sublimates (ice to water vapor), which deposits large amounts of water vapor into the atmosphere. Water vapor in the atmosphere condenses into clouds and is eventually followed by precipitation, which returns water to the earth's surface. Rain percolates into the ground, where it may evaporate or enter bodies of water. Surface runoff enters oceans directly or via streams and lakes.
Water is essential for all living processes. The human body is more than one-half water and human cells are more than 70 percent
water. Thus, most land animals need a supply of fresh water to survive. However, when examining the stores of water on earth,
97.5 percent of it is non-potable salt water (Figure
). Of the remaining water, 99 percent is locked underground as water or
as ice but this water is inconveniently located, mostly in Antarctica and Greenland. Shallow groundwater is the largest reservoir of
usable fresh water. Less than one percent of fresh water is present in lakes and rivers, the most heavily used water resources. If all
of world's water was shrunk to the size of 1 gallon, then the total amount of fresh water would be about 1/3 cup, and the amount of
readily usable fresh water would be 2 tablespoons.
: Bar charts of the Distribution of Earth's Water including total global water, fresh water, and surface water and other
fresh water and pie charts of water usable by humans and sources of usable water reveal that only 2.5 percent of water on Earth is
fresh water, and less than 1 percent of fresh water is easily accessible to living things. Source: United States Geographical Survey
Igor Skiklomanov's chapter "World fresh water resources" in Peter H. Gleick (editor), 1993, Water in Crisis: A Guide to the World's
Many living things, such as plants, animals, and fungi, are dependent on the small amount of fresh surface water supply, a lack of which can have massive effects on ecosystem dynamics. Humans, of course, have developed technologies to increase water availability, such as digging wells to harvest groundwater, storing rainwater, and using desalination to obtain drinkable water from the ocean. Although this pursuit of drinkable water has been ongoing throughout human history, the supply of fresh water is still a major issue in modern times.
Water is the only common substance that occurs naturally on Earth in three forms: solid, liquid and gas. The hydrosphere is the area of Earth where water movement and water storage occurs. Water reservoirs are the locations where water is stored. (Note that this term can also refer to artificial lakes created by dams.) Water is found as a liquid on the surface (rivers, lakes, oceans) and beneath the surface (groundwater), as ice (polar ice caps and glaciers), and as water vapor in the atmosphere. Figure illustrates the average time that an individual water molecule may spend in the Earth's major water reservoirs. Residence time is a measure of the average time an individual water molecule stays in a particular reservoir.
: Average residence time that water remains in each reservoir. Water remains in organisms for about one week, in the
atmosphere for 1.5 weeks, in rivers for two weeks, as soil moisture from two weeks to a year, in swamps for 1-10 years, in lakes for
10 years, in oceans and seas for 4,000 years, as groundwater for 2 weeks to 10,000 years, and in glaciers or as permafrost for 1,000-
10,000 years. Image from OpenStax (CC-BY).
Water cycling is extremely important to ecosystem dynamics as it has a major influence on climate and, thus, on the environments of ecosystems. For example, when water evaporates, it takes up energy from its surroundings, cooling the environment. When it condenses, it releases energy, warming the environment. The evaporation phase of the cycle purifies water, which then replenishes the land with fresh water. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the earth through processes including erosion and sedimentation.
The various processes that occur during the cycling of water are illustrated in Figure
: Water from the land and oceans enters the atmosphere by evaporation or sublimation, where it condenses into clouds
and falls as rain or snow. Precipitated water may enter freshwater bodies or infiltrate the soil. The cycle is complete when surface
or groundwater reenters the ocean (credit: modification of work by John M. Evans and Howard Perlman, USGS).
The water cycle is driven by the Sun's energy as it warms the oceans and other surface waters. This leads to evaporation (water to water vapor) of liquid surface water and sublimation (ice to water vapor) of frozen water, thus moving large amounts of water into the atmosphere as water vapor. As the water vapor rises in the atmosphere, it cools and condenses. Condensation is the process in which water vapor changes to tiny droplets of liquid water. Over time, this water vapor condenses into clouds as liquid or frozen droplets and eventually leads to precipitation (rain or snow), which returns water to Earth's surface. Most precipitation falls into the ocean. Some frozen precipitation becomes part of ice caps and glaciers. These masses of ice can store frozen water for hundreds of years or longer. Rain reaching Earth's surface may evaporate again, flow over the surface, or percolate into the ground. Most easily observed is surface runoff: the flow of fresh water either from rain or melting ice. Runoff can make its way through streams and lakes to the oceans or flow directly to the oceans themselves. In most natural terrestrial environments rain encounters vegetation before it reaches the soil surface. A significant percentage of water evaporates immediately from the surfaces of plants. What is left reaches the soil and begins to move down. Surface runoff will occur only if the soil becomes saturated with water in a heavy rainfall. Infiltration is the process through which water sinks into the ground and is determined by the soil or rock type through which water moves. Most water in the soil will be taken up by plant roots. The plant will use some of this water for its own metabolism, and some of that will find its way into animals that eat the plants, but much of it will be lost back to the atmosphere through a process known as evapotranspiration. Water enters the vascular system of the plant through the roots and evaporates, or transpires, through the stomata of the leaves. Water in the soil that is not taken up by a plant and that does not evaporate is able to percolate into the subsoil and bedrock. Here it forms groundwater.
Groundwater is a significant reservoir of fresh water. It exists in the pores between particles in sand and gravel, or in the fissures in rocks. Shallow groundwater flows slowly through these pores and fissures and eventually finds its way to a stream or lake where it becomes a part of the surface water again. Streams do not flow because they are replenished from rainwater directly; they flow because there is a constant inflow from groundwater below. Some groundwater is found very deep in the bedrock and can persist there for millennia. Most groundwater reservoirs, or aquifers, are the source of drinking or irrigation water drawn up through wells. In many cases these aquifers are being depleted faster than they are being replenished by water percolating down from above. Rain and surface runoff are major ways in which minerals, including carbon, nitrogen, phosphorus, and sulfur, are cycled from land to water. More precipitation falls near the equator, and landmasses there are characterized by a tropical rainforest climate (Figure ). Less precipitation tends to fall near 20-30° north and south latitude, where the world's largest deserts are located. These rainfall and climate patterns are related to global wind circulation cells. The intense sunlight at the equator heats air, causing it to rise and cool, which decreases the ability of the air mass to hold water vapor and results in frequent rainstorms. Around 30° north and south latitude, descending air conditions produce warmer air, which increases its ability to hold water vapor and results in dry conditions. Both the dry air conditions and the warm temperatures of these latitude belts favor evaporation. Global precipitation and climate patterns are also affected by the size of continents, major ocean currents, and mountains.
: The false-color map above shows the amount of rain that falls around the world. Areas of high rainfall include
Central and South America, western Africa, and Southeast Asia. Since these areas receive so much rainfall, they are where most of
the world's rainforests grow. Areas with very little rainfall usually turn into deserts. The desert areas include North Africa, the
Middle East, western North America, and Central Asia. Source: United States Geological Survey Earth Forum, Houston Museum
An important part of the water cycle is how water varies in salinity, which is the abundance of dissolved ions in water. The saltwater in the oceans is highly saline, with about 35,000 mg of dissolved ions per liter of seawater. Evaporation is a distillation process that produces nearly pure water with almost no dissolved ions. As water vaporizes, it leaves the dissolved ions in the original liquid phase. Eventually, condensation forms clouds and sometimes precipitation. After rainwater falls onto land, it dissolves minerals in rock and soil, which increases its salinity. Rain and surface runoff are major ways in which minerals, including phosphorus and sulfur, are cycled from land to water. Freshwater (such as lakes, rivers, and near-surface groundwater) has a relatively low salinity.
The steps of the water cycle are also explained in the video below.
Humans alter the water cycle by extracting large amounts of freshwater from surface waters as well as groundwater. Freshwater
supply is one of the most important provisioning ecosystem services on which human well-being depends. By 2000, the rate of our
water extraction from rivers and aquifers had risen to almost 4000 cubic kilometers per year. The greatest use of this water is for
irrigation in agriculture, but significant quantities of water are also extracted for public and municipal use, as well as industrial
applications and power generation (Figure
: The water cycle including human use of water. "The Water Cycle" by Atmospheric Infrared Sounder is available in the public domain.
Other major human interventions in the water cycle involve changes in land cover and infrastructure development of river networks. As we have deforested areas for wood supply and agricultural development we have reduced the amount of vegetation, which naturally acts to trap precipitation as it falls and slow the rate of infiltration into the ground. As a consequence, surface runoff has increased. This, in turn, means flood peaks are greater and erosion is increased. Erosion lowers soil quality and deposits sediment in river channels, where it can block navigation and harm aquatic plants and animals. Where agricultural land is also drained these effects can be magnified. Urbanization also accelerates streamflow by preventing precipitation from filtering into the soil and shunting it into drainage systems.
Additional physical infrastructure has been added to river networks with the aim of altering the volume, timing, and direction of
water flows for human benefit. This is achieved with reservoirs, weirs, aqueducts and diversion channels (Figure
example, so much water is removed or redirected from the Colorado River in the western United States that, despite its
considerable size, in some years it is dry before reaching the sea in Mexico. In an extreme example, the Aral Sea in Central Asia
has decreased to only 10% of its initial size after water was diverted for agriculture (see this case study for more details).
: The California Aqueduct carries water needed for agriculture from Northern California to Southern California. Image by USGS (public domain).
We also exploit waterways through their use for navigation, recreation, hydroelectricity generation and waste disposal. These activities, especially waste disposal, do not necessarily involve removal of water, but do have impacts on water quality and water flow that have negative consequences for the physical and biological properties of aquatic ecosystems.
The water cycle is key to the ecosystem service of climate regulation as well as being an essential supporting service that impacts the function of all ecosystems. Consider the widespread impacts on diverse natural and human systems when major droughts or floods occur. Consequently, human disruptions of the natural water cycle have many undesirable effects and challenge sustainable development. There are two major concerns. First, the need to balance rising human demand with the need to make our water use sustainable by reversing ecosystem damage from excess removal and pollution of water. Traditionally, considerable emphasis has been on finding and accessing more supply, but the negative environmental impacts of this approach are now appreciated, and improving the efficiency of water use is now a major goal. Second, there is a need for a safe water supply in many parts of the world, which depends on reducing water pollution and improving water treatment facilities.
Although glaciers represent the largest reservoir of fresh water, they generally are not used as a water source because they are
). Melting glaciers do provide a natural source of river water and groundwater.
During the last Ice Age there was as much as 50% more water in glaciers than there is today, which caused sea level to be about
100 m lower. Over the past century, sea level has been rising in part due to melting glaciers. If Earth's climate continues to warm,
the melting glaciers will cause an additional rise in sea level.
: Mountain Glacier in Argentina Glaciers are the largest reservoir of fresh water but they are not used much as a water resource directly by society because of their distance from most people. Source: Luca Galuzzi - www.galuzzi.it.
Further "Reading" For more information on the water cycle you might want to watch this water cycle video from USGS.
Contributors and Attributions Modified by Kyle Whittinghill and Melissa Ha from the following sources Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Original content by OpenStax (CC BY 4.0; Access for free at https://cnx.org/contents/b3c1e1d2-83...4e119a8aafbdd). 20.2: Biogeochemical Cycles by OpenStax, is licensed CC BY from Biology 2e by OpenStax (CC-BY). Access for free at openstax.org. 6.6: Water Cycle by CK-12: Biology Concepts, is licensed CC BY-NC Biogeochemical Cycles and the Flow of Energy in the Earth System and Water Cycle and Fresh Water Supply from Sustainability: A Comprehensive Foundation by Tom Theis and Jonathan Tomkin, Editors (licensed under CC-BY). Download for free at CNX. Water Cycle and Fresh Water Supply, Water Supply Problems and Solutions, and Biogeochemical Cycles from Environmental Biology by Matthew R. Fisher (licensed under CC-BY) The Water Cycle, Water Use and Distribution, and Groundwater from An Introduction to Geology by Chris Johnson et al. (licensed under CC-BY-NC-SA) General Microbiology Provided by: Boundless.com. License: CC BY-SA: Attribution-ShareAlike
20.2: The Water (Hydrologic) Cycle is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts. 20.2: Biogeochemical Cycles by OpenStax is licensed CC BY 4.0.
Key Points Carbon is present in all organic molecules; carbon compounds contain large amounts of energy, which humans use as fuel. The biological carbon cycle is the rapid exchange of carbon among living things; autotrophs use carbon dioxide produced by heterotrophs to produce glucose and oxygen, which are then utilized by heterotrophs. The biogeochemical cycle occurs at a much slower rate than the biological cycle since carbon is stored in carbon reservoirs for long periods of time. Carbon dioxide from the atmosphere dissolves in water, combining with water molecules to form carbonic acid, which then ionizes to carbonate and bicarbonate ions. Most of the carbon in the ocean is in the form of bicarbonate ions, which can combine with seawater calcium to form calcium carbonate (CaCO3), a major component of marine organism shells. Carbon can enter the soil as a result of the decomposition of living organisms, the weathering of rocks, the eruption of volcanoes, and other geothermal systems.
The Carbon Cycle Carbon, the second most abundant element in living organisms. Carbon is present in all organic molecules, and its role in the structure of macromolecules is of primary importance to living organisms. Carbon compounds contain energy, and many of these compounds from plants and algae have remained stored as fossilized carbon, which humans use as fuel. Since the 1800s, the use of fossil fuels has accelerated. As global demand for Earth's limited fossil fuel supplies has risen since the beginning of the Industrial Revolution, the amount of carbon dioxide in our atmosphere has increased as the fuels are burned. This increase in carbon dioxide has been associated with climate change and is a major environmental concern worldwide.
The carbon cycle is most easily studied as two interconnected subcycles: one dealing with rapid carbon exchange among living
organisms and the other dealing with the long-term cycling of carbon through geologic processes. The entire carbon cycle is shown
. The overall effect is that carbon is constantly recycled in the dynamic processes taking place in the atmosphere, at
the surface and in the crust of the earth. The vast majority of carbon resides as inorganic minerals in crustal rocks. Other reservoirs
of carbon, places where carbon accumulates, include the oceans and atmosphere. Some of the carbon atoms in your body today
may long ago have resided in a dinosaur's body, or perhaps were once buried deep in the Earth's crust as carbonate rock minerals.
: Carbon dioxide gas exists in the atmosphere and is dissolved in water. Photosynthesis converts carbon dioxide gas to
organic carbon, while respiration cycles the organic carbon back into carbon dioxide gas. Long-term storage of organic carbon
occurs when matter from living organisms is buried deep underground and becomes fossilized. Volcanic activity and human
emissions bring this stored carbon back into the carbon cycle.
Carbon Cycles Quickly between Organisms and the Atmosphere Cells run on the chemical energy found mainly in carbohydrate molecules, and the majority of these molecules are produced by one process: photosynthesis. Through photosynthesis, certain organisms convert solar energy (sunlight) into chemical energy, which is
then used to build other organic molecules like complex carbohydrates (such as starch), proteins and lipids. The energy stored in the bonds to hold these molecules together is released when an organism breaks down food. Cells then use this energy to perform work, such as movement. The energy that is harnessed from photosynthesis enters the ecosystems of our planet continuously and is transferred from one organism to another. Therefore, directly or indirectly, the process of photosynthesis provides most of the energy required by living things on Earth. Photosynthesis also results in the release of oxygen into the atmosphere. In short, to eat and breathe humans depend almost entirely on the organisms that carry out photosynthesis.
: (a) Plants, (b) algae, and (c) certain bacteria, called cyanobacteria, are can carry out photosynthesis. Algae can grow
over enormous areas in water, at times completely covering the surface (credit a: Steve Hillebrand, U.S. Fish and Wildlife Service;
credit b: "eutrophication&hypoxia"/Flickr; credit c: NASA; scale-bar data from Matt Russell).
Some organisms can carry out photosynthesis, whereas others cannot. An autotroph is an organism that can produce its own food.
The Greek roots of the word autotroph mean "self" (auto) "feeder" (troph). Plants are the best-known autotrophs, but others exist,
including certain types of bacteria and algae (Figure
). Oceanic algae contribute enormous quantities of food and oxygen to
global food chains. Carbon dioxide is the basic building block that most autotrophs use to build multi-carbon, high-energy
compounds, such as glucose. Most terrestrial autotrophs obtain their carbon dioxide directly from the atmosphere, while marine autotrophs acquire it in the dissolved form (bicarbonate, HCO3-).
Heterotrophs are organisms incapable of photosynthesis that must therefore obtain energy and carbon from food by consuming
other organisms. The Greek roots of the word heterotroph mean "other" (hetero) "feeder" (troph), meaning that their food comes
from other organisms. Even if the organism being consumed is another animal, it traces its stored energy back to autotrophs and the
process of photosynthesis. Heterotrophs acquire the high-energy carbon compounds from the autotrophs by consuming them and
breaking them down by respiration to obtain cellular energy, such as ATP. The most efficient type of respiration, aerobic
respiration, requires oxygen obtained from the atmosphere or dissolved in water. Thus, there is a constant exchange of oxygen and
carbon dioxide between the autotrophs (which need the carbon) and the heterotrophs (which need the oxygen). Humans are
heterotrophs, as are all animals and fungi. A deer obtains energy by eating plants. A wolf eating a deer obtains energy that originally
came from the plants eaten by that deer (Figure
). Using this reasoning, all food eaten by humans can be traced back to
autotrophs that carry out photosynthesis.
: The energy stored in carbohydrate molecules from photosynthesis passes through the food chain. The predator that
eats these deer is getting energy that originated in the photosynthetic vegetation that the deer consumed (credit: Steve VanRiper,
Plants, animals, and other organisms break down organic molecules during the process of aerobic cellular respiration, which consumes oxygen and releases energy, water and carbon dioxide. Carbon dioxide is returned to the atmosphere during gaseous exchange. Another process by which organic material is recycled is the decomposition of dead organisms such as leaf and root litter. During this process, bacteria and fungi break down the complex organic compounds into simpler organic compounds (Figure ). Decomposers may use aerobic or anaerobic respiration (decomposition with or without oxygen) thereby releasing carbon dioxide or methane (CH4).
: "Soil carbon cycle through the microbial loop" by Dan Naylor, Natalie Sadler, Arunima Bhattacharjee, Emily B.
Graham, Christopher R. Anderton, Ryan McClure, Mary Lipton, Kirsten S. Hofmockel and Janet K. Jansson is licensed under CC
Photosynthesis and respiration are actually reciprocal to one another with regard to the cycling of carbon: photosynthesis removes
carbon dioxide from the atmosphere and respiration returns it (Figure
). A significant disruption of one process can therefore
affect the amount of carbon dioxide in the atmosphere.
: This equation means that six molecules of carbon dioxide (CO2) combine with six molecules of water (H2O) in the presence of sunlight. This produces one molecule of glucose (C6H12O6) and six molecules of oxygen (O2).
Within marine ecosystems (Figure by zooplankton.
) phytoplankton photosynthesize and then are either decomposed by bacteria or consumed
: "Marine Carbon Cycle" by NASA is available in the public domain.
Cellular respiration is only one process that releases carbon dioxide. Physical processes, such as the eruption of volcanoes and release from hydrothermal vents (openings in the ocean floor) add carbon dioxide to the atmosphere. Additionally, the combustion of wood and fossil fuels releases carbon dioxide. The level of carbon dioxide in the atmosphere is greatly influenced by the reservoir of carbon in the oceans. The exchange of carbon between the atmosphere and water reservoirs influences how much carbon is found in each.
Carbon Cycles Slowly between Land and the Ocean
The movement of carbon through land, water, and air is complex, and, in many cases, it occurs much more slowly geologically
than the movement between living organisms (Figure
). Carbon is stored for long periods in what are known as carbon
reservoirs, which include the atmosphere, bodies of liquid water (mostly oceans), ocean sediment, soil, rocks (including fossil
: Global Carbon Cycle including storage and fluxes. "Carbon Cycle" by NASA is available in the public domain.
As stated, the atmosphere is a major reservoir of carbon in the form of carbon dioxide that is essential to the process of photosynthesis. The level of carbon dioxide in the atmosphere is greatly influenced by the reservoir of carbon in the oceans. The exchange of carbon between the atmosphere and water reservoirs influences how much carbon is found in each, and each one affects the other reciprocally. Atmospheric carbon dioxide also dissolves in the ocean, reacting with water molecules to form carbonic acid and then dissociating into bicarbonate (HCO3-) and carbonate ions (CO32-). More than 90 percent of the carbon in the ocean is found as bicarbonate ions. Some of these ions combine with calcium ions in the seawater to form calcium carbonate (CaCO3), a major component of the shells of marine organisms. These organisms eventually form sediments on the ocean floor. Over geologic time, the calcium carbonate forms limestone, which comprises the largest carbon reservoir on Earth.
: Formation of bicarbonate---carbon dioxide reacts with water to form bicarbonate and carbonate ions.
On land, carbon is stored in soil as organic carbon as a result of the decomposition of living organisms or from weathering of terrestrial rock and minerals. This carbon can be leached into the water reservoirs by surface runoff. Partially decomposed plants and algae are sometimes buried and compressed between layers of sediments. After millions of years fossil fuels such as coal, oil, and natural gas are formed. Fossil fuels are considered a non-renewable resource because their use far exceeds their rate of formation. A non-renewable resource is either regenerated very slowly or not at all. Another way for carbon to enter the atmosphere is from land (including land beneath the surface of the ocean) by the eruption of volcanoes and other geothermal systems. Carbon sediments from the ocean floor are taken deep within Earth by the process of
subduction: the movement of one tectonic plate beneath another. The ocean sediments are subducted by the actions of plate tectonics, melted and then returned to the surface during volcanic activity. Carbon is released as carbon dioxide when a volcano erupts or from volcanic hydrothermal vents.Plate tectonics can also cause uplifting, returning ocean sediments to land.
The carbon cycle is crucially important to the biosphere. If not for the recycling processes, carbon might long ago have become
completely sequestered in crustal rocks and sediments, and life would no longer exist (Figure
makes energy and carbon available to higher trophic levels, but it also releases gaseous oxygen (O2). Gaseous oxygen is necessary
for cellular respiration to occur. Photosynthetic bacteria were likely the first organisms to perform photosynthesis, dating back 2-3
billion years ago. Thanks to their activity, and a diversity of present-day photosynthesizing organisms, Earth's atmosphere is
currently about 21% O2. Also, this O2 is vital for the creation of the ozone layer, which protects life from harmful ultraviolet
radiation emitted by the sun. Ozone (O3) is created from the breakdown and reassembly of O2.
: Decomposers will break down the organic compounds in this fallen tree at Cliffs of the Neuse State Park in Wayne
County, North Carolina, releasing carbon dioxide into the atmosphere. Decomposition ensures that carbon dioxide will be available
in the atmosphere for photosynthetic organisms, which then provide carbon for consumers. "Fallen" by Gerry Dincher is licensed
The global carbon cycle contributes substantially to the provisioning ecosystem services upon which humans depend. We harvest approximately 25% of the total plant biomass that is produced each year on the land surface to supply food, fuel wood and fiber from croplands, pastures and forests. In addition, the global carbon cycle plays a key role in regulating ecosystem services because it significantly influences climate via its effects on atmospheric CO2 concentrations.
Human Alteration of the Carbon Cycle Atmospheric CO2 concentration increased from 280 parts per million (ppm) to 413 ppm between the start of industrial revolution in the late eighteenth century and 2020. This reflected a new flux in the global carbon cycle--anthropogenic CO2 emissions--where humans release CO2 into the atmosphere by burning fossil fuels and changing land use. Fossil fuel burning takes carbon from coal, gas, and oil reserves, where it would be otherwise stored on very long time scales, and introduces it into the active carbon cycle. Land use change releases carbon from soil and plant biomass pools into the atmosphere, particularly through the process of deforestation for wood extraction or conversion of land to agriculture. Carbon dioxide is also added to the atmosphere by the breeding and raising of livestock. In 2018, the additional flux of carbon into the atmosphere from anthropogenic sources was estimated to be 36.6 gigatons of carbon (GtC = 1 billion tons of carbon)--a significant disturbance to the natural carbon cycle that had been in balance for several thousand years previously. High levels of carbon dioxide in the atmosphere cause warming that results in climate change.
Contributors and Attributions Modified by Kyle Whittinghill (University of Pittsburgh) and Melissa Ha from the following sources: Biogeochemical Cycles, Energy, and Energy Enters Ecosystems Through Photosynthesis from Environmental Biology by Matthew R. Fisher (licensed under CC-BY) Carbon Cycle from Biology by John W. Kimball (licensed under CC-BY) Energy Flow through Ecosystems from General Biology by OpenStax (licensed under CC-BY) Soil and Sustainability and Biogeochemical Cycles and the Flow of Energy in the Earth System from Sustainability: A Comprehensive Foundation by Tom Theis and Jonathan Tomkin, Editors (licensed under CC-BY). Download for free at CNX. Cycling of Matter from AP Environmental Science by University of California College Prep, University of California (licensed under CC-BY). Download for free at CNX. Nutrient Cycles from Life Sciences Grade 10 by Siyavula (CC-BY) Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Original content by OpenStax (CC BY 4.0; Access for free at https://cnx.org/contents/b3c1e1d2-83...4e119a8aafbdd). 20.2: Biogeochemical Cycles by OpenStax, is licensed CC BY Essentials of Environmental Science by Kamala Dorsner is licensed under CC BY 4.0. General Microbiology Provided by: Boundless.com. License: CC BY-SA: Attribution-ShareAlike 20.3: The Carbon Cycle is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts. 20.2: Biogeochemical Cycles by OpenStax is licensed CC BY 4.0.
Key Points Bacteria, such as cyanobacteria, convert nitrogen into nitrogen gas via nitrogen fixation. Nitrogen fixation occurs in three steps: ammonification, nitrification, and denitrification. Human activity can release nitrogen into the environment by the combustion of fossil fuels and by the use of artificial fertilizers in agriculture. Atmospheric nitrogen is responsible for acid rain, the release of greenhouse gasses, and eutrophication. Nitrogen fixation can be performed by marine bacteria; nitrogen falls to the ocean floor as sediment and is then moved to land, becoming incorporated into terrestrial rock.
Key Terms denitrification: process of converting nitrates into nitrogen gas, especially by the action of bacteria nitrification: the conversion of ammonium into nitrites (NO2-) by nitrifying bacteria ammonification: the formation of ammonia or its compounds from nitrogenous compounds, especially as a result of bacterial decomposition
All organisms require nitrogen because it is an important component of nucleic acids, proteins, and other organic molecules.
Getting nitrogen into the living world is difficult. Plants and phytoplankton are not equipped to incorporate nitrogen from the
atmosphere (which exists as tightly-bonded, triple-covalent N2), even though this molecule comprises approximately 78 percent of
the atmosphere. Nitrogen enters the living world through nitrogen fixation (Figure
gas into ammonia (NH3), which spontaneously becomes ammonium (NH4+). Ammonium is found in bodies of water and in the soil
: Nitrogen enters the living world from the atmosphere through nitrogen-fixing bacteria. This nitrogen and
nitrogenous waste from animals is then processed back into gaseous nitrogen by soil bacteria, which also supply terrestrial
food webs with the organic nitrogen they need (credit: modification of work by John M. Evans and Howard Perlman, USGS).
: In the nitrogen cycle, nitrogen-fixing bacteria in the soil or legume root nodules convert nitrogen gas (N2) from the
atmosphere to ammonium (NH4+). Nitrification occurs when bacteria convert ammonium to nitrites (NO2-) and then to nitrates
(NO3-). Nitrates re-enter the atmosphere as nitrogen gas through denitrification by bacteria. Plants assimilate ammonium and
nitrates, producing organic nitrogen, which is available to consumers. Decomposers, including aerobic and anaerobic bacteria and
fungi, break down organic nitrogen and release ammonium through ammonification. "Nitrogen cycle" by Johann Dréo & Raeky is
Three processes are responsible for most of the nitrogen fixation in the biosphere. The first is atmospheric fixation by lightning. The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth. Atmospheric nitrogen fixation probably contributes some 5-8% of the total nitrogen fixed. The second process is industrial fixation. Under great pressure, at a temperature of 600°C (1112°F), and with the use of a catalyst (which facilitates chemical reactions), atmospheric nitrogen and hydrogen can be combined to form ammonia (NH3). Ammonia can be used directly as fertilizer, but most of it is further processed to urea and ammonium nitrate (NH4NO3).
The third process is biological fixation by certain free-living or symbiotic bacteria, which incorporate nitrogen into their
macromolecules. Cyanobacteria live in most aquatic ecosystems where sunlight is present; they play a key role in nitrogen fixation.
Nitrogen-fixing cyanobacteria are essential to maintaining the fertility of semi-aquatic environments like rice paddies. Free-living
bacteria, such as Azotobacter, are also important nitrogen fixers. Some nitrogen fixing bacteria form a symbiotic relationship with
plants in the legume family, which includes beans, peas, soybeans, alfalfa, and clovers (figure
symbiotically in the root nodules of legumes (such as peas, beans, and peanuts) and provide them with the organic nitrogen they
need. Some nitrogen-fixing bacteria even establish symbiotic relationships with animals, e.g., termites and "shipworms" (wood-
eating bivalves). Although the first stable product of the process is ammonia, this is quickly incorporated into protein and other
: Nitrogen-fixing bacteria live in the spherical nodules of this soybean root. Image by United Soybean Board is licensed under CC BY 2.0.
Organic nitrogen is especially important to the study of ecosystem dynamics since many ecosystem processes, such as primary
production and decomposition, are limited by the available supply of nitrogen. Plants and other producers directly use ammonium
and nitrates to make organic molecules through the process of assimilation (Figure
consumers. Consumers excrete organic nitrogen compounds that return to the environment. Additionally dead organisms at each
, the nitrogen that enters living systems by nitrogen fixation is eventually converted from organic
nitrogen back into nitrogen gas by bacteria. This process occurs in three steps in terrestrial systems: ammonification, nitrification,
and denitrification. First, the ammonification or nitrogen mineralization process converts nitrogenous waste from living organisims or the remains of dead organisms into ammonium (NH4+ ) by certain bacteria and fungi. Second, this ammonium is then
converted to nitrites (NO2-) and then nitrates (NO3-) by nitrifying bacteria and archaea, such as Nitrosomonas or Nitrobacter, through the process of nitrification. In addition, both soil and the ocean contain archaeal microbes, assigned to the Crenarchaeota, that convert ammonia to nitrites. They are more abundant than the nitrifying bacteria and may turn out to play an important role in the nitrogen cycle.. Like ammonium, nitrites and nitrates are found in water and the soil. Some nitrates are converted back into nitrogen gas, which is released into the atmosphere. The process, called denitrification, is conducted by bacteria, such as Pseudomonas and Clostridium, which use nitrate when decomposing organic matter in the absence of oxygen. In the process of denitrification several intermediates are formed and may be released to the atmosphere including nitric oxide (NO) and nitrous oxide (N2O, a greenhouse gas). Under anaerobic conditions in marine and freshwater systems other species of bacteria are able to oxidize ammonia with nitrite forming nitrogen gas in a process called anammox (anaerobic ammonia oxidation). In marine ecosystems, nitrogen compounds created by bacteria, or through decomposition, collects in ocean floor sediments. It can then be moved to land in geologic time by uplift of Earth's crust and thereby incorporated into terrestrial rock. Although the movement of nitrogen from rock directly into living systems has been traditionally seen as insignificant compared with nitrogen fixed from the atmosphere, a recent study showed that this process may indeed be significant and should be included in any study of the global nitrogen cycle (Morford et al., 2011).
Human Alteration of the Nitrogen Cycle Human activity can release nitrogen into the environment by two primary means: the combustion of fossil fuels, which releases different nitrogen oxides, and by the use of artificial fertilizers (which contain nitrogen and phosphorus compounds) in agriculture, which are then washed into lakes, streams, and rivers by surface runoff. Humans are also increasing the amount of reactive nitrogen in the environment by the cultivation of nitrogen fixing crops, such as soybeans. If the nitrogen fixation from leguminous crops (e.g. beans, alfalfa) is included, then the anthropogenic flux of nitrogen from the atmosphere to the land exceeds natural fluxes to the land. Atmospheric nitrogen (other than N2) is associated with several effects on Earth's ecosystems including the production of acid deposition (as nitric acid, HNO3), also known as acid rain. Acid deposition damages healthy trees, destroys aquatic systems and erodes building materials such as marble and limestone. Like carbon dioxide, nitrous oxide (N2O) is a greenhouse gas, potentially causing climate change when released during denitrification.
Humans are primarily dependent on the nitrogen cycle as a supporting ecosystem service for crop and forest productivity. Nitrogen
fertilizers are added to enhance the growth of many crops and plantations (figure
agriculture was a key feature of the green revolution that boosted global crop yields in the 1970s. The industrial production of
nitrogen-rich fertilizers has increased substantially over time and now matches more than half of the input to the land from
biological nitrogen fixation (90 megatons = 1 million tons of nitrogen each year). If the nitrogen fixation from legume crops is
included, then the anthropogenic flux of nitrogen from the atmosphere to the land exceeds natural fluxes to the land. Fertilizers are
washed into lakes, streams, and rivers by surface runoff, resulting in saltwater and freshwater eutrophication, a process whereby
nutrient runoff causes the overgrowth of algae, the depletion of oxygen, and death of aquatic fauna. Excess nitrates in water
supplies have also been linked to human health problems.
: Fertilizer containing nitrogen is conventionally applied at large scales in agriculture. Image by Bob Nichols, USDA Natural Resources Conservation Service is available in the public domain.
Efforts to reduce nitrogen pollution focus on increasing the efficiency of synthetic fertilizer use, altering feeding of animals to reduce nitrogen content in their excreta, and better processing of livestock waste and sewage sludge to reduce ammonia release. At the same time, increasing demand for food production from a growing global population with a greater appetite for meat is driving greater total fertilizer use, so there is no guarantee that better practices will lead to a reduction in the overall amount of nitrogen pollution.
References Morford, S. L., Houlton, B. Z., & Dahlgren, R. A. (2011). Increased forest ecosystem carbon and nitrogen storage from nitrogenrich bedrock. Nature, 477(7362), 78-81. https://doi.org/10.1038/nature10386
Contributors and Attributions Modified by Kyle Whittinghill (University of Pittsburgh) and Melissa Ha from the following sources: Biogeochemical Cycles, Energy, and Energy Enters Ecosystems Through Photosynthesis from Environmental Biology by Matthew R. Fisher (licensed under CC-BY) Nitrogen Cycle from Biology by John W. Kimball (licensed under CC-BY) Soil and Sustainability and Biogeochemical Cycles and the Flow of Energy in the Earth System from Sustainability: A Comprehensive Foundation by Tom Theis and Jonathan Tomkin, Editors (licensed under CC-BY). Download for free at CNX. Cycling of Matter from AP Environmental Science by University of California College Prep, University of California (licensed under CC-BY). Download for free at CNX. Nutrient Cycles from Life Sciences Grade 10 by Siyavula (CC-BY) Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Original content by OpenStax (CC BY 4.0; Access for free at https://cnx.org/contents/b3c1e1d2-83...4e119a8aafbdd). 20.2: Biogeochemical Cycles by OpenStax, is licensed CC BY Biogeochemical Cycles and the Flow of Energy in the Earth System from Sustainability: A Comprehensive Foundation by Tom Theis and Jonathan Tomkin General Microbiology Provided by: Boundless.com. License: CC BY-SA: Attribution-ShareAlike
20.4: The Nitrogen Cycle is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts. 20.2: Biogeochemical Cycles by OpenStax is licensed CC BY 4.0.
Key Points Phosphorus, a major component of nucleic acid and phospholipids, also makes up the supportive components of our bones; it is often necessary for growth in aquatic ecosystems. Phosphates (PO43-) are sent into rivers, lakes, and the ocean by leaching and natural surface runoff. Phosphate-containing ocean sediments slowly move to land by the uplifting of areas of the earth's surface.
Phosphorus is an essential nutrient for living processes. It is a major component of nucleic acids, both DNA and RNA; of phospholipids, the major component of cell membranes; and, as calcium phosphate, makes up the supportive components of our bones. Phosphorus is often the limiting nutrient (necessary for growth) in aquatic ecosystems.
Phosphorus occurs in nature as the phosphate ion (PO43-). In addition to phosphate runoff as a result of human activity, natural
surface runoff occurs when it is leached from phosphate-containing rock by weathering, thus sending phosphates into rivers, lakes,
and the ocean. This rock has its origins in the ocean. Phosphate-containing ocean sediments form primarily from the bodies of
ocean organisms and from their excretions. However, in remote regions, volcanic ash, aerosols, and mineral dust may also be
significant phosphate sources. This sediment then is moved to land over geologic time by the uplifting of areas of the earth's
). Phosphorus is also reciprocally exchanged between phosphate dissolved in the ocean and marine
ecosystems. The movement of phosphate from the ocean to the land and through the soil is extremely slow, with the average
phosphate ion having an oceanic residence time between 20,000 and 100,000 years.
: In nature, phosphorus exists as the phosphate ion (PO43-). Weathering of rocks and volcanic activity releases
phosphate into the soil, water, and air, where it becomes available to terrestrial food webs. Phosphate enters the oceans via surface
runoff, groundwater flow, and river flow. Phosphate dissolved in ocean water cycles into marine food webs. Some phosphate from
the marine food webs falls to the ocean floor, where it forms sediment.
Marine birds play a unique role in the phosphorous cycle. These birds take up phosphorous from ocean fish. Their droppings on land (guano) contain high levels of phosphorous and are sometimes mined for commercial use. A 2020 study estimated that the ecosystem services (natural processes and products that benefit humans) provided by guano are worth $470 million per year. Weathering of rocks releases phosphates into the soil and bodies of water. Plants can assimilate phosphates in the soil and incorporate it into organic molecules, making phosphorus available to consumers in terrestrial food webs. Waste and dead organisms are decomposed by fungi and bacteria, releasing phosphates back into the soil. Some phosphate is leached from the soil, entering into rivers, lakes, and the ocean. Primary producers in aquatic food webs, such as algae and photosynthetic bacteria, assimilate phosphate, and organic phosphate is thus available to consumers in aquatic food webs. Similar to terrestrial food webs, phosphorus is reciprocally exchanged between phosphate dissolved in the ocean and organic phosphorus in marine organisms.
The movement of phosphorus from rock to living organisms is normally a very slow process, but some human activities speed up the process. Phosphate-bearing rock is often mined for use in the manufacture of fertilizers and detergents. This commercial production greatly accelerates the phosphorous cycle. In addition, runoff from agricultural land and the release of sewage into water systems can cause a local overload of phosphate. The increased availability of phosphate can cause overgrowth of algae. This reduces the oxygen level, causing eutrophication and the destruction of other aquatic species.
The processes of plant production and decomposition are important for biogeochemical cycling of phosphorus within terrestrial and
aquatic ecosystems, just as they are for the nitrogen and carbon cycles. Figure
nitrogen) cycling within a wetland controlled by plant productivity and decomposition.
: A simplified diagram of phosphorus and nitrogen cycles in a wetland. Source: Kadlec and Knight (1996), CC BY-SA 4.0, via Wikimedia Commons.
Contributors and Attributions Modified by Kyle Whittinghill (University of Pittsburgh) and Melissa Ha from the following sources: Biogeochemical Cycles, Energy, and Energy Enters Ecosystems Through Photosynthesis from Environmental Biology by Matthew R. Fisher (licensed under CC-BY) Energy Flow through Ecosystems from General Biology by OpenStax (licensed under CC-BY) Soil and Sustainability and Biogeochemical Cycles and the Flow of Energy in the Earth System from Sustainability: A Comprehensive Foundation by Tom Theis and Jonathan Tomkin, Editors (licensed under CC-BY). Download for free at CNX. Cycling of Matter from AP Environmental Science by University of California College Prep, University of California (licensed under CC-BY). Download for free at CNX. Nutrient Cycles from Life Sciences Grade 10 by Siyavula (CC-BY) Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Original content by OpenStax (CC BY 4.0; Access for free at https://cnx.org/contents/b3c1e1d2-83...4e119a8aafbdd). 20.2: Biogeochemical Cycles by OpenStax, is licensed CC BY General Microbiology Provided by: Boundless.com. License: CC BY-SA: Attribution-ShareAlike
20.5: The Phosphorus Cycle is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
Key Points Excess phosphorus and nitrogen in the ecosystem leads to the death of many organisms, causing dead zones. Dead zones are caused by by eutrophication, oil spills, dumping of toxic chemicals, and other human activities.
Key Terms eutrophication: process whereby excess levels of nitrogen or phosphorus cause excessive growth of microorganisms, depleting dissolved oxygen levels and kill ecosystem fauna dead zone: an area within a freshwater or marine ecosystem where large areas are depleted of their normal flora and fauna; caused by excessive nutrient pollution
Eutrophication and Dead Zones Nutrients in soil and water are generally beneficial when they exist at naturally occurring levels. Nitrogen fertilizers have been applied to farm fields for decades in order to maximize production of agricultural lands. However, an unintended consequence is that the same nutrients can be detrimental to aquatic ecosystems when introduced excessively for agricultural or other purposes. Nitrogen (N) and Phosphorus (P) are introduced by fertilizers that are used intensively in agriculture, as well as golf courses and some lawns and gardens. Farm animal waste and sewage also provide large amounts of reactive N and P. Phosphorus was formerly used heavily as an additive in laundry and dishwater detergents, but since the 1970's it has been phased out in both through a combination of state and federal regulations. Overall, our modern society has altered the global N and P cycles such that there is an overabundance in many settings.
Excessive nutrients (not utilized) are often washed into drainage ways, streams, and rivers during rainfall and storm events.
Eutrophication occurs when excess phosphorus and nitrogen from fertilizer runoff or sewage causes excessive growth of algae.
Algal blooms that block light and therefore kill aquatic plants in rivers, lakes, and seas. The subsequent death and decay of these
organisms depletes dissolved oxygen, which leads to the death of aquatic organisms such as shellfish and fish. This process is
responsible for dead zones, large areas in lakes and oceans near the mouths of rivers that are periodically depleted of their normal
flora and fauna, and for massive fish kills, which often occur during the summer months (figure
dead zones worldwide. Phosphate and nitrate runoff from fertilizers also negatively affect several lake and bay ecosystems
including the Chesapeake Bay in the eastern United States.
One of the worst dead zones is off the coast of the United States in the Gulf of Mexico (figure
Mississippi River basin created a dead zone, which reached its peak size of 8,776 square miles in 2017. The Mississippi River
dumps high-nutrient runoff from its drainage basin that includes vast agricultural lands in the American Midwest. Increased algal
growth produced by these nutrients has affected important shrimp fishing grounds in the Gulf. The primary source of the nutrients
is the heavily tile-drained areas of farmland in the Midwest corn and soybean belt (SW Minnesota, N Iowa, NE Illinois, N Indiana
and NW Ohio). Improved soil drainage systems over the past century or more have allowed for effective transport of nitrate
compounds as stormwater runoff into drainage basins (Ohio River, Wabash River, Illinois River, Missouri River, etc.) that feed into
the Mississippi River. Prior to drainage, these wetlands would also have removed nutrients through denitrification and other
biogeochemical processes. In other words, the same drainage tiles that allow for the agricultural benefit of having rich
bottomland/wetland soils in production, have the disadvantage of increased and more rapid movements of nitrate solutes to the
Gulf of Mexico. Such large-scale problems, across state governmental boundaries, can only be fully addressed in the future with a
national system of incentives, regulations, or laws.
: Dead zones occur when phosphorus and nitrogen from fertilizers cause excessive growth of microorganisms, which
depletes oxygen and kills fauna. This map shows dead zones around the world in 2008. Worldwide, large dead zones are found in
coastal areas of high population density (credit: NASA Earth Observatory).
In addition to fertilizers, nitrogen inputs to watersheds can also include atmospheric deposition, livestock waste, and sewage, but
nitrogen fertilizers comprise a significant majority of the input to monitored streams, particularly in springtime when much
fertilizer is applied. Possible solutions to this problem include encouraging farmers to apply a more limited quantity of fertilizer in
the spring (only as much as necessary), rather than in the fall, to allow for considerably less time for stormwater or meltwater
runoff. Other solutions include maintaining cover crops, or restoring wetlands in key locations to contain nitrate losses. An overall
strategy that limits the excess capacity of nutrients can simultaneously benefit farmers (by limiting cost), the ecology of stream
watersheds and coastal ecosystems (also locally stressed by oil spills and other pollution). Over the long term, more efforts will
need to be made in the Mississippi River Basin, and globally in similarly stressed agricultural or urban watersheds (figure
to improve the health and sustainability of our soil, land, and aquatic ecosystems.
The Chesapeake Bay has long been valued as one of the most scenic areas on Earth; it is now in distress and is recognized as a
declining ecosystem. In the 1970s, the Chesapeake Bay was one of the first ecosystems to have identified dead zones, which
continue to kill many fish and bottom-dwelling species, such as clams, oysters, and worms (Figure
have declined in the Chesapeake Bay due to surface water runoff containing excess nutrients from artificial fertilizer used on
land. The source of the fertilizers (with high nitrogen and phosphate content) is not limited to agricultural practices. There are
many nearby urban areas and more than 150 rivers and streams empty into the bay that are carrying fertilizer runoff from lawns
and gardens. Thus, the decline of the Chesapeake Bay is a complex issue and requires the cooperation of industry, agriculture,
: This (a) satellite image shows the Chesapeake Bay, an ecosystem affected by phosphate and nitrate runoff. A (b)
member of the Army Corps of Engineers holds a clump of oysters being used as a part of the oyster restoration effort in the
bay (credit a: modification of work by NASA/MODIS; credit b: modification of work by U.S. Army).
Of particular interest to conservationists is the oyster population; it is estimated that more than 200,000 acres of oyster reefs existed in the bay in the 1700s, but that number has now declined to only 36,000 acres. Oyster harvesting was once a major industry for Chesapeake Bay, but it declined 88 percent between 1982 and 2007. This decline was due not only to fertilizer runoff and dead zones but also to overexploitation. Oysters require a certain minimum population density because they must be in close proximity to reproduce. Human activity has altered the oyster population and locations, greatly disrupting the ecosystem.
The restoration of the oyster population in the Chesapeake Bay has been ongoing for several years with mixed success. Not only do many people find oysters good to eat, but they also clean up the bay. Oysters are filter feeders, and as they eat, they clean the water around them. In the 1700s, it was estimated that it took only a few days for the oyster population to filter the entire volume of the bay. Today, with changed water conditions, it is estimated that the present population would take nearly a year to do the same job.
Restoration efforts have been ongoing for several years by non-profit organizations, such as the Chesapeake Bay Foundation. The restoration goal is to find a way to increase population density so the oysters can reproduce more efficiently. Many disease-resistant varieties (developed at the Virginia Institute of Marine Science for the College of William and Mary) are now available and have been used in the construction of experimental oyster reefs. Efforts to clean and restore the bay by Virginia and Delaware have been hampered because much of the pollution entering the bay comes from other states, which stresses the need for inter-state cooperation to gain successful restoration.
The new, hearty oyster strains have also spawned a new and economically viable industry--oyster aquaculture--which not only supplies oysters for food and profit, but also has the added benefit of cleaning the bay.
Suggested Supplementary Reading Bruckner, M. 2018. The Gulf of Mexico Dead Zone. [Website]
References Cell Press. (2020, August 6). Researchers hope to save seabirds by calculating the value of their excrement. Retrieved August 7, 2020 from ScienceDaily.
Contributors and Attributions Modified by Kyle Whittinghill (University of Pittsburgh) and Melissa Ha from the following sources: Biogeochemical Cycles, Energy, and Energy Enters Ecosystems Through Photosynthesis from Environmental Biology by Matthew R. Fisher (licensed under CC-BY) Carbon Cycle and Nitrogen Cycle from Biology by John W. Kimball (licensed under CC-BY) Energy Flow through Ecosystems from General Biology by OpenStax (licensed under CC-BY) Soil and Sustainability and Biogeochemical Cycles and the Flow of Energy in the Earth System from Sustainability: A Comprehensive Foundation by Tom Theis and Jonathan Tomkin, Editors (licensed under CC-BY). Download for free at CNX. Cycling of Matter from AP Environmental Science by University of California College Prep, University of California (licensed under CC-BY). Download for free at CNX. Nutrient Cycles from Life Sciences Grade 10 by Siyavula (CC-BY) Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Original content by OpenStax (CC BY 4.0; Access for free at https://cnx.org/contents/b3c1e1d2-83...4e119a8aafbdd). 20.2: Biogeochemical Cycles by OpenStax, is licensed CC BY 20.6: Eutrophication and Dead Zones is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.
Key Points Sulfur is an essential element for the macromolecules of living things since it determines the 3-D folding patterns of proteins. Chemosynthesis using sulfur is way some autotrophs create sugars. On land, sulfur enters the atmosphere via acid rain, fallout, the weathering of rocks, decomposition of organic materials, and geothermal vents. Sulfur enters the ocean via runoff, fallout, and underwater geothermal vents; some marine ecosystems also rely on chemoautotrophs as a sulfur source. The burning of fossil fuels increases the amount of sulfide in the atmosphere and causes acid rain. Acid rain is corrosive rain that causes damage to aquatic ecosystems by lowering the pH of lakes, killing many of the resident fauna; it also degrades buildings and human-made structures.
Sulfur is an essential element for the macromolecules of living things. As a part of the amino acid cysteine, it is involved in the
formation of disulfide bonds within proteins, which help to determine their 3-D folding patterns and, hence, their functions. As
, sulfur cycles between the oceans, land, and atmosphere. Atmospheric sulfur is found in the form of sulfur
dioxide (SO2), which enters the atmosphere in three ways: first, from the decomposition of organic molecules; second, from
volcanic activity and geothermal vents; and, third, from the burning of fossil fuels by humans.
: Sulfur dioxide from the atmosphere becomes available to terrestrial and marine ecosystems when it is dissolved in
precipitation as weak sulfuric acid or when it falls directly to the earth as fallout. Weathering of rocks also makes sulfates available
to terrestrial ecosystems. Decomposition of living organisms returns sulfates to the ocean, soil, and atmosphere.
On land, sulfur is deposited in five major ways: precipitation, direct fallout from the atmosphere, rock weathering, decomposition
of organic materials, and geothermal vents (Figure
). As rain falls through the atmosphere, sulfur dioxide (SO2) in the
atmosphere is dissolved in the form of weak sulfuric acid (H2SO4), creating acid rain. Sulfur can also fall directly from the
atmosphere in a process called fallout. The weathering of sulfur-containing rocks also releases sulfur into the soil. These rocks
originate from ocean sediments that are moved to land by the geologic uplift. Terrestrial ecosystems can then make use of these soil sulfates (SO42-), which enter the food web by being taken up by plant roots. Upon the death and decomposition of these organisms,
sulfur is released back into the atmosphere as hydrogen sulfide (H2S) gas.
: At this sulfur vent in Lassen Volcanic National Park in northeastern California, the yellowish sulfur deposits are
Sulfur enters the ocean via runoff from land, fallout, and underwater geothermal vents. Some ecosystems rely on microorganisms using sulfur as a biological energy source (chemoautotrophs) in contrast to ecosystems with photosynthetic producers. This sulfur then supports marine ecosystems in the form of sulfates.
Human activities have played a major role in altering the balance of the global sulfur cycle (Figure
quantities of fossil fuels, especially from coal, releases sulfur dioxide, which reacts with the atmosphere to form sulfuric acid. Like
nitric acid, sulfuric acid contributes to acid deposition. Acid deposition (sometimes referred to simply as acid rain) damages the
natural environment by lowering the pH of lakes, thus killing many of the resident plants and animals. Acid deposition also affects
the man-made environment through the chemical degradation of buildings. For example, many marble monuments, such as the
Lincoln Memorial in Washington, DC, have suffered significant damage from acid rain over the years. These examples show the
wide-ranging effects of human activities on our environment and the challenges that remain for our future.
: Schematic figure of the Sulfur cycle including human impacts from mining and burning fossil fuels. Source: Hanna
K. Lappalainen, Veli-Matti Kerminen, Tuukka Petäjä, Theo Kurten, Aleksander Baklanov, Anatoly Shvidenko, Jaana Bäck, Timo
Vihma, Pavel Alekseychik, Meinrat O. Andreae, Stephen R. Arnold, Mikhail Arshinov, Eija Asmi, Boris Belan, Leonid Bobylev,
Sergey Chalov, Yafang Cheng, Natalia Chubarova, Gerrit de Leeuw, Aijun Ding, Sergey Dobrolyubov, Sergei Dubtsov, Egor
Dyukarev, Nikolai Elansky, Kostas Eleftheriadis, Igor Esau, Nikolay Filatov, Mikhail Flint, Congbin Fu, Olga Glezer, Aleksander
Gliko, Martin Heimann, Albert A. M. Holtslag, Urmas Hõrrak, Juha Janhunen, Sirkku Juhola, Leena Järvi, Heikki Järvinen, Anna
Kanukhina, Pavel Konstantinov, Vladimir Kotlyakov, Antti-Jussi Kieloaho, Alexander S. Komarov, Joni Kujansuu, Ilmo Kukkonen,
Ella-Maria Duplissy, Ari Laaksonen, Tuomas Laurila, Heikki Lihavainen, Alexander Lisitzin, Alexsander Mahura, Alexander
Makshtas, Evgeny Mareev, Stephany Mazon, Dmitry Matishov, Vladimir Melnikov, Eugene Mikhailov, Dmitri Moisseev, Robert
Nigmatulin, Steffen M. Noe, Anne Ojala, Mari Pihlatie, Olga Popovicheva, Jukka Pumpanen, Tatjana Regerand, Irina Repina,
Aleksei Shcherbinin, Vladimir Shevchenko, Mikko Sipilä, Andrey Skorokhod, Dominick V. Spracklen, Hang Su, Dmitry A. Subetto,
Junying Sun, Arkady Y. Terzhevik, Yuri Timofeyev, Yuliya Troitskaya, Veli-Pekka Tynkkynen, Viacheslav I. Kharuk, Nina Zaytseva,
Jiahua Zhang, Yrjö Viisanen, Timo Vesala, Pertti Hari, Hans Christen Hansson, Gennady G. Matvienko, Nikolai S. Kasimov,
Huadong Guo, Valery Bondur, Sergej Zilitinkevich, Markku Kulmala, CC BY 3.0, via Wikimedia Commons.
Chemosynthesis Why do bacteria that live deep below the ocean's surface rely on chemical compounds instead of sunlight for energy to make food?
Most autotrophs make food by photosynthesis, but this isn't the only way that autotrophs produce food. Some bacteria make food by another process, which uses chemical energy instead of light energy. This process is called chemosynthesis. In chemosynthesis, one or more carbon molecules (usually carbon dioxide or methane, CH4) and nutrients is converted into organic matter, using the oxidation of inorganic molecules (such as hydrogen gas, hydrogen sulfide (H2S) or ammonia (NH3)) or methane as a source of energy, rather than sunlight. In hydrogen sulfide chemosynthesis, in the presence of carbon dioxide and oxygen, carbohydrates (CH2O) can be produced:
Many organisms that use chemosynthesis are extremophiles, living in harsh conditions, such as in the absence of sunlight and a
wide range of water temperatures, some approaching the boiling point. Some chemosynthetic bacteria live around deep-ocean vents
known as "black smokers." Compounds such as hydrogen sulfide, which flow out of the vents from Earth's interior, are used by the
bacteria for energy to make food. These organisms are known as chemoautotrophs. Many chemosynthetic microorganisms are
consumed by other organisms in the ocean, and symbiotic associations between these organisms and respiring heterotrophs are
quite common. Consumers that depend on these bacteria to produce food for them include giant tubeworms (Figure
: Tubeworms deep in the Galapagos Rift get their energy from chemosynthetic bacteria. Tubeworms have no mouth,
eyes or stomach. Their survival depends on a symbiotic relationship with the billions of bacteria that live inside them. These
bacteria convert the chemicals that shoot out of the hydrothermal vents into food for the worm.
: Is it possible to live in temperatures over 175°F? It is if you're a Pompeii worm. The Pompeii worm, the most heat-
tolerant animal on Earth, lives in the deep ocean at super-heated hydrothermal vents. Covering this deep-sea worm's back is a
fleece of bacteria. These microbes contain all the genes necessary for life in extreme environments.
Contributors and Attributions This section was modified from the original by Kyle Whittinghill Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Original content by OpenStax (CC BY 4.0; Access for free at https://cnx.org/contents/b3c1e1d2-83...4e119a8aafbdd). 20.2: Biogeochemical Cycles by OpenStax, is licensed CC BY 2.24: Chemosynthesis by CK-12: Biology Concepts, is licensed CC BY-NC General Microbiology Provided by: Boundless.com. License: CC BY-SA: Attribution-ShareAlike Atmosphere-Biosphere-Hydrosphere-Lithosphere. Provided by: Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...ithosphere.png. License: CC BY: Attribution
20.7: The Sulfur Cycle is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts. 20.2: Biogeochemical Cycles by OpenStax is licensed CC BY 4.0. 2.24: Chemosynthesis by CK12 is licensed CK-12. Original source: http://www.ck12.org/book/CK-12-Biology-Concepts.