Information

46.3C: The Carbon Cycle - Biology


Carbon enters the atmosphere in the form of carbon dioxide via the carbon cycle and returns to organic carbon via photosynthesis.

Learning Objectives

  • Distinguish between the biological and biogeochemical cycles of carbon

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.

Key Terms

  • subduction: movement of one tectonic plate beneath another
  • non-renewable resource: resource, such as fossil fuel, that is either regenerated very slowly or not at all
  • autotroph: Any organism that can synthesize its food from inorganic substances, using heat or light as a source of energy
  • heterotroph: an organism that requires an external supply of energy in the form of food, as it cannot synthesize its own

The Carbon Cycle

Carbon, the second most abundant element in living organisms, is present in all organic molecules. Its role in the structure of macromolecules is of primary importance to living organisms. Carbon compounds contain especially- high forms of energy, which humans use as fuel. Since the 1800s (the beginning of the Industrial Revolution), the number of countries using massive amounts of fossil fuels increased, which raised the levels of carbon dioxide in the atmosphere. This increase in carbon dioxide has been associated with climate change and other disturbances of the earth’s ecosystems. It is a major environmental concern worldwide.

The carbon cycle is most easily studied as two interconnected sub-cycles: one dealing with rapid carbon exchange among living organisms and the other dealing with the long-term cycling of carbon through geologic processes.

The Biological Carbon Cycle

Living organisms are connected in many ways, even between ecosystems. A good example of this connection is the exchange of carbon between autotrophs and heterotrophs. Carbon dioxide is the basic building block that most autotrophs use to build multi-carbon, high-energy compounds, such as glucose. The energy harnessed from the sun is used by these organisms to form the covalent bonds that link carbon atoms together. These chemical bonds store this energy for later use in the process of respiration. Most terrestrial autotrophs obtain their carbon dioxide directly from the atmosphere, while marine autotrophs acquire it in the dissolved form (carbonic acid, H2CO3). However carbon dioxide is acquired, a by-product of the process is oxygen. The photosynthetic organisms are responsible for depositing approximately 21 percent of the oxygen content in the atmosphere that we observe today.

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). Gas exchange through the atmosphere and water is one way that the carbon cycle connects all living organisms on Earth.

The Biogeochemical Carbon Cycle

The movement of carbon through the land, water, and air is complex and, in many cases, it occurs much more slowly than the biological carbon cycle. 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, land sediments (including fossil fuels), and the earth’s interior.

As stated, the atmosphere, a major reservoir of carbon in the form of carbon dioxide, 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 location; each affects the other reciprocally. Carbon dioxide (CO2) from the atmosphere dissolves in water, combining with water molecules to form carbonic acid. It then ionizes to carbonate and bicarbonate ions.

More than 90 percent of the carbon in the ocean is found as bicarbonate ions. Some of these ions combine with seawater calcium to form calcium carbonate (CaCO3), a major component of marine organism shells. 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.

On land, carbon is stored in soil as a result of the decomposition of living organisms or the weathering of terrestrial rock and minerals. This carbon can be leached into the water reservoirs by surface runoff. Deeper underground, on land and at sea, are fossil fuels: the anaerobically-decomposed remains of plants that take millions of years to form. 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 by the eruption of volcanoes and other geothermal systems. Carbon sediments from the ocean floor are taken deep within the earth by the process of subduction: the movement of one tectonic plate beneath another. Carbon is released as carbon dioxide when a volcano erupts or from volcanic hydrothermal vents.

Carbon dioxide is also added to the atmosphere by the breeding and raising of livestock. The large numbers of land animals raised to feed the earth’s growing population results in increased carbon dioxide levels in the atmosphere due to farming practices, respiration, and methane production. This is another example of how human activity indirectly affects biogeochemical cycles in a significant way. Although much of the debate about the future effects of increasing atmospheric carbon on climate change focuses on fossils fuels, scientists take natural processes, such as volcanoes and respiration, into account as they model and predict the future impact of this increase.


46.3 Biogeochemical Cycles

By the end of this section, you will be able to do the following:

  • Discuss the biogeochemical cycles of water, carbon, nitrogen, phosphorus, and sulfur
  • Explain how human activities have impacted these cycles and the potential consequences for Earth

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 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. Geologic processes, such as weathering, erosion, water drainage, and the subduction 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 .

Water contains hydrogen and oxygen, which is essential to all living processes. The hydrosphere is the area of the Earth where water movement and storage occurs. On or beneath the surface, water occurs in liquid or solid form in rivers, lakes, oceans, groundwater, polar ice caps, and glaciers. And it occurs as water vapor in the atmosphere. Carbon is found in all organic macromolecules and is an important constituent of fossil fuels. Nitrogen is a major component of our nucleic acids and proteins and 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 is critical to the 3-D folding of proteins, such as in disulfide binding.

The cycling 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.

Link to Learning

Head to this website to learn more about biogeochemical cycles.

The Water (Hydrologic) Cycle

Water is the basis of all living processes on Earth. When examining the stores of water on Earth, 97.5 percent of it is non-potable salt water (Figure 46.12). Of the remaining water, 99 percent is locked underground as water or as ice. Thus, less than 1 percent of fresh water is easily accessible from lakes and rivers. Many living things, such as plants, animals, and fungi, are dependent on that small amount of fresh surface water, a lack of which can have massive effects on ecosystem dynamics. To be successful, organisms must adapt to fluctuating water supplies. 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.

Water cycling is extremely important to ecosystem dynamics. Water has a major influence on climate and, thus, on the environments of ecosystems. Most of the water on Earth is stored for long periods in the oceans, underground, and as ice. Figure 46.13 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.

There are various processes that occur during the cycling of water, shown in Figure 46.14. These processes include the following:

  • evaporation/sublimation
  • condensation/precipitation
  • subsurface water flow
  • surface runoff/snowmelt
  • streamflow

The water cycle is driven by the sun’s energy as it warms the oceans and other surface waters. This leads to the evaporation (water to water vapor) of liquid surface water and the sublimation (ice to water vapor) of frozen water, which deposits large amounts of water vapor into the atmosphere. Over time, this water vapor condenses into clouds as liquid or frozen droplets and is eventually followed by precipitation (rain or snow), which returns water to the Earth’s surface. Rain eventually permeates into the ground, where it may evaporate again if it is near the surface, flow beneath the surface, or be stored for long periods. More easily observed is surface runoff: the flow of fresh water either from rain or melting ice. Runoff can then make its way through streams and lakes to the oceans or flow directly to the oceans themselves.

Link to Learning

Head to this website to learn more about the world’s fresh water supply.

Rain and surface runoff are major ways in which minerals, including carbon, nitrogen, phosphorus, and sulfur, are cycled from land to water. The environmental effects of runoff will be discussed later as these cycles are described.

The Carbon Cycle

Carbon is 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.

The carbon cycle is most easily studied as two interconnected sub-cycles: 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 in Figure 46.15.

Link to Learning

Click this link to read information about the United States Carbon Cycle Science Program.

The Biological Carbon Cycle

Living organisms are connected in many ways, even between ecosystems. A good example of this connection is the exchange of carbon between autotrophs and heterotrophs within and between ecosystems by way of atmospheric carbon dioxide. Carbon dioxide is the basic building block that most autotrophs use to build multicarbon, high energy compounds, such as glucose. The energy harnessed from the sun is used by these organisms to form the covalent bonds that link carbon atoms together. These chemical bonds thereby store this energy for later use in the process of respiration. Most terrestrial autotrophs obtain their carbon dioxide directly from the atmosphere, while marine autotrophs acquire it in the dissolved form (carbonic acid, H2CO3 − ). However carbon dioxide is acquired, a by-product of the process is oxygen. The photosynthetic organisms are responsible for depositing approximately 21 percent oxygen content of the atmosphere that we observe today.

Heterotrophs and autotrophs are partners in biological carbon exchange (especially the primary consumers, largely herbivores). 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). Gas exchange through the atmosphere and water is one way that the carbon cycle connects all living organisms on Earth.

The Biogeochemical Carbon Cycle

The movement of carbon through the land, water, and air is complex, and in many cases, it occurs much more slowly geologically than as seen between living organisms. 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, land sediments (including fossil fuels), and the Earth’s interior.

As stated, the atmosphere is a major reservoir of carbon in the form of carbon dioxide and 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 location, and each one affects the other reciprocally. Carbon dioxide (CO2) from the atmosphere dissolves in water and combines with water molecules to form carbonic acid, and then it ionizes to carbonate and bicarbonate ions (Figure 46.16)

The equilibrium coefficients are such that more than 90 percent of the carbon in the ocean is found as bicarbonate ions. Some of these ions combine with seawater calcium to form calcium carbonate (CaCO3), a major component of marine organism shells. 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.

On land, carbon is stored in soil as a result of the decomposition of living organisms (by decomposers) or from weathering of terrestrial rock and minerals. This carbon can be leached into the water reservoirs by surface runoff. Deeper underground, on land and at sea, are fossil fuels: the anaerobically decomposed remains of plants that take millions of years to form. Fossil fuels are considered a nonrenewable resource because their use far exceeds their rate of formation. A nonrenewable resource , such as fossil fuel, 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 the Earth by the process of subduction : the movement of one tectonic plate beneath another. Carbon is released as carbon dioxide when a volcano erupts or from volcanic hydrothermal vents.

Humans contribute to atmospheric carbon by the burning of fossil fuels and other materials. Since the Industrial Revolution, humans have significantly increased the release of carbon and carbon compounds, which has in turn affected the climate and overall environment.

Animal husbandry by humans also increases atmospheric carbon. The large numbers of land animals raised to feed the Earth’s growing population results in increased carbon dioxide levels in the atmosphere due to farming practices and respiration and methane production. This is another example of how human activity indirectly affects biogeochemical cycles in a significant way. Although much of the debate about the future effects of increasing atmospheric carbon on climate change focuses on fossils fuels, scientists take natural processes, such as volcanoes and respiration, into account as they model and predict the future impact of this increase.

The Nitrogen Cycle

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 via free-living and symbiotic bacteria, which incorporate nitrogen into their macromolecules through nitrogen fixation (conversion of N2). Cyanobacteria live in most aquatic ecosystems where sunlight is present they play a key role in nitrogen fixation. Cyanobacteria are able to use inorganic sources of nitrogen to “fix” nitrogen. Rhizobium bacteria live symbiotically in the root nodules of legumes (such as peas, beans, and peanuts) and provide them with the organic nitrogen they need. (For example, gardeners often grow peas both for their produce and to naturally add nitrogen to the soil. This practice goes back to ancient times, even if the science has only been recently understood.) Free-living bacteria, such as Azotobacter, are also important nitrogen fixers.

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. As shown in Figure 46.17, the nitrogen that enters living systems by nitrogen fixation is successively 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 process converts nitrogenous waste from living animals or from the remains of dead animals into ammonium (NH4 + ) by certain bacteria and fungi. Second, the ammonium is converted to nitrites (NO2 − ) by nitrifying bacteria, such as Nitrosomonas, through nitrification. Subsequently, nitrites are converted to nitrates (NO3 − ) by similar organisms. Third, the process of denitrification occurs, whereby bacteria, such as Pseudomonas and Clostridium, convert the nitrates into nitrogen gas, allowing it to reenter the atmosphere.

Visual Connection

Which of the following statements about the nitrogen cycle is false?

  1. Ammonification converts organic nitrogenous matter from living organisms into ammonium (NH4 + ).
  2. Denitrification by bacteria converts nitrates (NO3 − ) to nitrogen gas (N2).
  3. Nitrification by bacteria converts nitrates (NO3 − ) to nitrites (NO2 − ).
  4. Nitrogen fixing bacteria convert nitrogen gas (N2) into organic compounds.

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 in agriculture, which are then washed into lakes, streams, and rivers by surface runoff. Atmospheric nitrogen is associated with several effects on Earth’s ecosystems including the production of acid rain (as nitric acid, HNO3) and greenhouse gas (as nitrous oxide, N2O) potentially causing climate change. A major effect from fertilizer runoff is saltwater and freshwater eutrophication , a process whereby nutrient runoff causes the excess growth of microorganisms, depleting dissolved oxygen levels and killing ecosystem fauna.

A similar process occurs in the marine nitrogen cycle, where the ammonification, nitrification, and denitrification processes are performed by marine bacteria. Some of this nitrogen falls to the ocean floor as sediment, which can then be moved to land in geologic time by uplift of the Earth’s surface 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. 3

The Phosphorus Cycle

Phosphorus is an essential nutrient for living processes it is a major component of nucleic acid and phospholipids, and, as calcium phosphate, makes up the supportive components of our bones. Phosphorus is often the limiting nutrient (necessary for growth) in aquatic ecosystems (Figure 46.18).

Phosphorus occurs in nature as the phosphate ion (PO4 3− ). 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 surface.

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.

As discussed in Chapter 44, excess phosphorus and nitrogen that enters these ecosystems from fertilizer runoff and from sewage causes excessive growth of microorganisms and depletes the dissolved oxygen, which leads to the death of many ecosystem fauna, such as shellfish and finfish. This process is responsible for dead zones in lakes and at the mouths of many major rivers (Figure 46.19).

As discussed earlier, a dead zone is an area within a freshwater or marine ecosystem where large areas are depleted of their normal flora and fauna these zones can be caused by eutrophication, oil spills, dumping of toxic chemicals, and other human activities. The number of dead zones has been increasing for several years, and more than 400 of these zones were present as of 2008. One of the worst dead zones is off the coast of the United States in the Gulf of Mexico, where fertilizer runoff from the Mississippi River basin has created a dead zone of over 8463 square miles. Phosphate and nitrate runoff from fertilizers also negatively affect several lake and bay ecosystems including the Chesapeake Bay in the eastern United States.

Everyday Connection

Chesapeake Bay

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. Several species 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, and everyday homeowners.

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 overharvesting. 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 nonprofit 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 interstate 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.

The Sulfur Cycle

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 shown in Figure 46.21, sulfur cycles between the oceans, land, and atmosphere. Atmospheric sulfur is found in the form of sulfur dioxide (SO2) and enters the atmosphere in three ways: from the decomposition of organic molecules, from volcanic activity and geothermal vents, and from the burning of fossil fuels by humans.

On land, sulfur is deposited in four major ways: precipitation, direct fallout from the atmosphere, rock weathering, and geothermal vents (Figure 46.21). Atmospheric sulfur is found in the form of sulfur dioxide (SO2), and as rain falls through the atmosphere, sulfur is dissolved in the form of weak sulfurous acid (H2SO3). Sulfur can also fall directly from the atmosphere in a process called fallout . Also, the weathering of sulfur-containing rocks releases sulfur into the soil. These rocks originate from ocean sediments that are moved to land by the geologic uplifting of ocean sediments. Terrestrial ecosystems can then make use of these soil sulfates ( SO 4 − SO 4 − ), and upon the death and decomposition of these organisms, release the sulfur back into the atmosphere as hydrogen sulfide (H2S) gas.

Sulfur enters the ocean via runoff from land, from atmospheric fallout, and from underwater geothermal vents. Some ecosystems (Figure 46.9) rely on chemoautotrophs using sulfur as a biological energy source. 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. The burning of large quantities of fossil fuels, especially from coal, releases larger amounts of hydrogen sulfide gas into the atmosphere. Acid rain is caused by rainwater falling to the ground through this sulfur dioxide gas, turning it into weak sulfuric acid. Acid rain damages the natural environment by lowering the pH of lakes, which kills many of the resident fauna it 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.


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The Fast Carbon Cycle

The time it takes carbon to move through the fast carbon cycle is measured in a lifespan. The fast carbon cycle is largely the movement of carbon through life forms on Earth, or the biosphere. Between 10 15 and 10 17 grams (1,000 to 100,000 million metric tons) of carbon move through the fast carbon cycle every year.

Carbon plays an essential role in biology because of its ability to form many bonds&mdashup to four per atom&mdashin a seemingly endless variety of complex organic molecules. Many organic molecules contain carbon atoms that have formed strong bonds to other carbon atoms, combining into long chains and rings. Such carbon chains and rings are the basis of living cells. For instance, DNA is made of two intertwined molecules built around a carbon chain.

The bonds in the long carbon chains contain a lot of energy. When the chains break apart, the stored energy is released. This energy makes carbon molecules an excellent source of fuel for all living things.

During photosynthesis, plants absorb carbon dioxide and sunlight to create fuel&mdashglucose and other sugars&mdashfor building plant structures. This process forms the foundation of the fast (biological) carbon cycle. (Illustration adapted from P.J. Sellers et al., 1992.)

Plants and phytoplankton are the main components of the fast carbon cycle. Phytoplankton (microscopic organisms in the ocean) and plants take carbon dioxide from the atmosphere by absorbing it into their cells. Using energy from the Sun, both plants and plankton combine carbon dioxide (CO2) and water to form sugar (CH2O) and oxygen. The chemical reaction looks like this:

Four things can happen to move carbon from a plant and return it to the atmosphere, but all involve the same chemical reaction. Plants break down the sugar to get the energy they need to grow. Animals (including people) eat the plants or plankton, and break down the plant sugar to get energy. Plants and plankton die and decay (are eaten by bacteria) at the end of the growing season. Or fire consumes plants. In each case, oxygen combines with sugar to release water, carbon dioxide, and energy. The basic chemical reaction looks like this:

In all four processes, the carbon dioxide released in the reaction usually ends up in the atmosphere. The fast carbon cycle is so tightly tied to plant life that the growing season can be seen by the way carbon dioxide fluctuates in the atmosphere. In the Northern Hemisphere winter, when few land plants are growing and many are decaying, atmospheric carbon dioxide concentrations climb. During the spring, when plants begin growing again, concentrations drop. It is as if the Earth is breathing.

The ebb and flow of the fast carbon cycle is visible in the changing seasons. As the large land masses of Northern Hemisphere green in the spring and summer, they draw carbon out of the atmosphere. This graph shows the difference in carbon dioxide levels from the previous month, with the long-term trend removed.

This cycle peaks in August, with about 2 parts per million of carbon dioxide drawn out of the atmosphere. In the fall and winter, as vegetation dies back in the northern hemisphere, decomposition and respiration returns carbon dioxide to the atmosphere.

These maps show net primary productivity (the amount of carbon consumed by plants) on land (green) and in the oceans (blue) during August and December, 2010. In August, the green areas of North America, Europe, and Asia represent plants using carbon from the atmosphere to grow. In December, net primary productivity at high latitudes is negative, which outweighs the seasonal increase in vegetation in the southern hemisphere. As a result, the amount of carbon dioxide in the atmosphere increases.

(Graph by Marit Jentoft-Nilsen and Robert Simmon, using data from the NOAA Earth System Research Laboratory. Maps by Robert Simmon and Reto Stöckli, using MODIS data.)


46.3C: The Carbon Cycle - Biology

Through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere in the slow carbon cycle. On average, 10 13 to 10 14 grams (10&ndash100 million metric tons) of carbon move through the slow carbon cycle every year. In comparison, human emissions of carbon to the atmosphere are on the order of 10 15 grams, whereas the fast carbon cycle moves 10 16 to 10 17 grams of carbon per year.

The movement of carbon from the atmosphere to the lithosphere (rocks) begins with rain. Atmospheric carbon combines with water to form a weak acid&mdashcarbonic acid&mdashthat falls to the surface in rain. The acid dissolves rocks&mdasha process called chemical weathering&mdashand releases calcium, magnesium, potassium, or sodium ions. Rivers carry the ions to the ocean.

Rivers carry calcium ions&mdashthe result of chemical weathering of rocks&mdashinto the ocean, where they react with carbonate dissolved in the water. The product of that reaction, calcium carbonate, is then deposited onto the ocean floor, where it becomes limestone. (Photograph ©2009 Greg Carley.)

In the ocean, the calcium ions combine with bicarbonate ions to form calcium carbonate, the active ingredient in antacids and the chalky white substance that dries on your faucet if you live in an area with hard water. In the modern ocean, most of the calcium carbonate is made by shell-building (calcifying) organisms (such as corals) and plankton (like coccolithophores and foraminifera). After the organisms die, they sink to the seafloor. Over time, layers of shells and sediment are cemented together and turn to rock, storing the carbon in stone&mdashlimestone and its derivatives.

Limestone, or its metamorphic cousin, marble, is rock made primarily of calcium carbonate. These rock types are often formed from the bodies of marine plants and animals, and their shells and skeletons can be preserved as fossils. Carbon locked up in limestone can be stored for millions&mdashor even hundreds of millions&mdashof years. (Photograph ©2008 Rookuzz (Hmm).)

Only 80 percent of carbon-containing rock is currently made this way. The remaining 20 percent contain carbon from living things (organic carbon) that have been embedded in layers of mud. Heat and pressure compress the mud and carbon over millions of years, forming sedimentary rock such as shale. In special cases, when dead plant matter builds up faster than it can decay, layers of organic carbon become oil, coal, or natural gas instead of sedimentary rock like shale.

This coal seam in Scotland was originally a layer of sediment, rich in organic carbon. The sedimentary layer was eventually buried deep underground, and the heat and pressure transformed it into coal. Coal and other fossil fuels are a convenient source of energy, but when they are burned, the stored carbon is released into the atmosphere. This alters the balance of the carbon cycle, and is changing Earth&rsquos climate. (Photograph ©2010 Sandchem.)

The slow cycle returns carbon to the atmosphere through volcanoes. Earth&rsquos land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks beneath the other, and the rock it carries melts under the extreme heat and pressure. The heated rock recombines into silicate minerals, releasing carbon dioxide.

When volcanoes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to begin the cycle again. At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. For comparison, humans emit about 30 billion tons of carbon dioxide per year&mdash100&ndash300 times more than volcanoes&mdashby burning fossil fuels.

Chemistry regulates this dance between ocean, land, and atmosphere. If carbon dioxide rises in the atmosphere because of an increase in volcanic activity, for example, temperatures rise, leading to more rain, which dissolves more rock, creating more ions that will eventually deposit more carbon on the ocean floor. It takes a few hundred thousand years to rebalance the slow carbon cycle through chemical weathering.

Carbon stored in rocks is naturally returned to the atmosphere by volcanoes. In this photograph, Russia&rsquos Kizimen Volcano vents ash and volcanic gases in January 2011. Kizimen is located on the Kamchatka Peninsula, where the Pacific Plate is subducting beneath Asia. (Photograph ©2011 Artyom Bezotechestvo/Photo Kamchatka.)

However, the slow carbon cycle also contains a slightly faster component: the ocean. At the surface, where air meets water, carbon dioxide gas dissolves in and ventilates out of the ocean in a steady exchange with the atmosphere. Once in the ocean, carbon dioxide gas reacts with water molecules to release hydrogen, making the ocean more acidic. The hydrogen reacts with carbonate from rock weathering to produce bicarbonate ions.

Before the industrial age, the ocean vented carbon dioxide to the atmosphere in balance with the carbon the ocean received during rock weathering. However, since carbon concentrations in the atmosphere have increased, the ocean now takes more carbon from the atmosphere than it releases. Over millennia, the ocean will absorb up to 85 percent of the extra carbon people have put into the atmosphere by burning fossil fuels, but the process is slow because it is tied to the movement of water from the ocean&rsquos surface to its depths.

In the meantime, winds, currents, and temperature control the rate at which the ocean takes carbon dioxide from the atmosphere. (See The Ocean&rsquos Carbon Balance on the Earth Observatory.) It is likely that changes in ocean temperatures and currents helped remove carbon from and then restore carbon to the atmosphere over the few thousand years in which the ice ages began and ended.


The Carbon Cycle

The carbon cycle describes how carbon transfers between different reservoirs located on Earth. This cycle is important for maintaining a stable climate and carbon balance on Earth.

Biology, Conservation, Earth Science

Quinault River Rainforest

Full of living entities, and the formerly living, the temperate rainforest at the Quinault River in Olympic Peninsula, Washington, and places like it are rich reservoirs of carbon.

Carbon is an essential element for all life forms on Earth. Whether these life forms take in carbon to help manufacture food or release carbon as part of respiration, the intake and output of carbon is a component of all plant and animal life.

Carbon is in a constant state of movement from place to place. It is stored in what are known as reservoirs, and it moves between these reservoirs through a variety of processes, including photosynthesis, burning fossil fuels, and simply releasing breath from the lungs. The movement of carbon from reservoir to reservoir is known as the carbon cycle.

Carbon can be stored in a variety of reservoirs, including plants and animals, which is why they are considered carbon life forms. Carbon is used by plants to build leaves and stems, which are then digested by animals and used for cellular growth. In the atmosphere, carbon is stored in the form of gases, such as carbon dioxide. It is also stored in oceans, captured by many types of marine organisms. Some organisms, such as clams or coral, use the carbon to form shells and skeletons. Most of the carbon on the planet is contained within rocks, minerals, and other sediment buried beneath the surface of the planet.

Because Earth is a closed system, the amount of carbon on the planet never changes. However, the amount of carbon in a specific reservoir can change over time as carbon moves from one reservoir to another. For example, some carbon in the atmosphere might be captured by plants to make food during photosynthesis. This carbon can then be ingested and stored in animals that eat the plants. When the animals die, they decompose, and their remains become sediment, trapping the stored carbon in layers that eventually turn into rock or minerals. Some of this sediment might form fossil fuels, such as coal, oil, or natural gas, which release carbon back into the atmosphere when the fuel is burned.

The carbon cycle is vital to life on Earth. Nature tends to keep carbon levels balanced, meaning that the amount of carbon naturally released from reservoirs is equal to the amount that is naturally absorbed by reservoirs. Maintaining this carbon balance allows the planet to remain hospitable for life. Scientists believe that humans have upset this balance by burning fossil fuels, which has added more carbon to the atmosphere than usual and led to climate change and global warming.

Full of living entities, and the formerly living, the temperate rainforest at the Quinault River in Olympic Peninsula, Washington, and places like it are rich reservoirs of carbon.


The ocean and carbon

The ocean plays an important part in the carbon cycle. Overall, the ocean is called a carbon ‘sink’ because it takes up more carbon from the atmosphere than it gives up.

Carbon dioxide from the atmosphere dissolves in the surface waters of the ocean. Some of the carbon dioxide stays as dissolved gas, but much of it gets turned into other things. Photosynthesis by tiny marine plants (phytoplankton) in the sunlit surface waters turns the carbon into organic matter. Many organisms use carbon to make calcium carbonate, a building material of shells and skeletons. Other chemical processes create calcium carbonate in the water. The using up of carbon by biological and chemical processes allows more carbon dioxide to enter the water from the atmosphere.


CAREER CONNECTION

Chesapeake Bay

Figure 7: 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)

The Chesapeake Bay (Figure 7a) is one of the most scenic areas on Earth it is now in distress and is recognized as a case study of a declining ecosystem. In the 1970s, the Chesapeake Bay was one of the first aquatic ecosystems to have identified dead zones, which continue to kill many fish and bottom-dwelling species such as clams, oysters, and worms. Several species have declined in the Chesapeake Bay because surface water runoff contains excess nutrients from artificial fertilizer use 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, and individual homeowners.

Of particular interest to conservationists is the oyster population (Figure 7b) 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 caused not only by fertilizer runoff and dead zones, but also because of overharvesting. 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, thus 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 the oysters also clean up the bay. They are filter feeders, and as they eat, they clean the water around them. Filter feeders eat by pumping a continuous stream of water over finely divided appendages (gills in the case of oysters) and capturing prokaryotes, plankton, and fine organic particles in their mucus. 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 the 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 by Virginia and Delaware to clean and restore the bay have been hampered because much of the pollution entering the bay comes from other states, which emphasizes the need for interstate 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.


The Carbon Cycle

After completing this section, you should be able to discuss Earth's carbon cycle, including the primary reservoirs and anthropogenic transfer mechanisms. You need not remember specific transfer rates or reservoir "sizes", but you should be able to identify the largest reservoirs and transfer mechanisms, as well as describe the consequences of the unbalanced, anthropogenic portions of the carbon cycle.

Concentrations of atmospheric carbon dioxide are increasing, largely because of the burning of fossil fuels. But, are trends in atmospheric carbon dioxide concentrations that straightforward? If you refer to the data from the Mauna Loa Observatory, you can see that there's a clear increase since the late 1950s, but there's also a yearly cycle that's apparent (note the regular ups and downs in the red trace). Carbon dioxide concentrations vary throughout the year because of plant photosynthesis. During warmer months, when plants are more actively growing, the process of photosynthesis uses carbon dioxide, which removes it from the air. During colder months, with less plant growth, atmospheric carbon dioxide concentrations increase again because less is being consumed by photosynthesis.

So, not all carbon dioxide that human activities have added to the atmosphere stays in the atmosphere (and not all carbon dioxide in the atmosphere comes from anthropogenic sources). As it turns out, Earth has a carbon cycle, which contains several carbon "reservoirs" (places that retain carbon), and carbon continuously gets exchanged between the earth and the atmosphere. But, the carbon cycle deals with more than just anthropogenic emissions and plant growth.

For starters, the earth-atmosphere system has a "carbon budget" of sorts, which, ideally, would be approximately balanced (exchanges of carbon between the earth and atmosphere would be equal). Historically, we know that the cycle hasn't been perfectly balanced at all times, because concentrations of atmospheric carbon dioxide have varied (historical concentrations have ups and downs). Still, over the long haul, the "ups" have been offset by the "downs" because of the earth-atmosphere system always seeking to balance the cycle. But, since the dawn of the industrial age, that balance has changed.

The primary reservoirs of carbon dioxide are the oceans, the terrestrial surface (primarily in plants and soil), and geological reserves of fossil fuels. The atmosphere is a carbon reservoir, too, but as you can see from the schematic of the carbon cycle below, the atmosphere contains a tiny fraction of the carbon (in the form of carbon dioxide) compared to the oceans and geological reserves.

The oceans are, by far, the largest reservoir of carbon, followed by geological reserves of fossil fuels, the terrestrial surface (plans and soil), and the atmosphere. But, carbon moves naturally between the earth and atmosphere continuously. For example, volcanoes and other geologic activity emit carbon dioxide into the atmosphere. On the other hand, the weathering of some rocks results in chemical reactions with atmospheric carbon dioxide that removes it from the atmosphere. Plant photosynthesis also removes carbon dioxide from the atmosphere and returns it to the terrestrial surface. Note in the diagram above that the natural exchanges (marked by purple arrows) between the atmosphere, ocean, and terrestrial surface are balanced (emissions into the atmosphere are balanced by transfer back to the ocean and terrestrial surface).

Geological reserves were largely left out of the cycle until industrialization resulted in the large-scale recovery and burning of carbon-based fossil fuels, which creates carbon dioxide as a byproduct. The transfer of carbon dioxide into the atmosphere from the burning of fossil fuels is actually much smaller than that which naturally occurs from the ocean and terrestrial surface, but it's an unbalanced part of the cycle.

Deforestation also adds carbon dioxide to the atmosphere, because wood is roughly 50 percent carbon. So, when forests are cleared, much of that carbon eventually makes its way into the atmosphere. This process is exacerbated when deforestation occurs via burning. While the amount of carbon dioxide added to the atmosphere each year from deforestation is somewhat uncertain (that's why a range of 1 to 2 billion kilograms per year is shown in the diagram), deforestation on a global scale may be responsible for more than a quarter of anthropogenic emissions, and it's also an unbalanced part of the cycle. So, deforestation has some global climate impacts, too, in addition to the local ones we discussed previously.

The important thing to take away from this discussion is that the anthropogenic transfers of carbon dioxide to the atmosphere (via fossil fuels and deforestation) are unbalanced parts of the cycle. No mechanisms perfectly balance them and transfer equal amounts of carbon dioxide back into the oceans and terrestrial surface. So, while the anthropogenic additions of carbon dioxide to the atmosphere are small compared to natural ones (refer to the carbon cycle diagram above), since they're unbalanced, the anthropogenic contributions gradually add up over time, which is why carbon dioxide concentrations in the atmosphere have increased more than 40 percent since pre-industrial days, and more than 25 percent just since the late 1950s.

However, the earth-atmosphere system is very dynamic, and as the earth has warmed and atmospheric carbon dioxide has increased, the rate of natural processes that remove carbon dioxide from the atmosphere has also increased, which has had the overall effect of removing some anthropogenic carbon dioxide from the atmosphere. It turns out that roughly half of the carbon dioxide that humans have emitted into the atmosphere has been returned to the oceans and terrestrial surface by natural processes. In other words, nature is doing its very best to seek balance and offset the increasing carbon dioxide concentrations in the atmosphere from human activity. But, these natural removal processes haven't been able to keep up with the rate of anthropogenic emissions, and show no signs of being able to in the future. As long as more carbon dioxide is being emitted into the atmosphere than is being removed, atmospheric concentrations of carbon dioxide will continue to increase, just as your bank account balance grows if you deposit more money than you withdraw over a period of time.

Summary

  • Carbon is stored in four main reservoirs -- oceans (the largest reservoir), geological reserves of fossil fuels, the terrestrial surface (plants and soil, mainly), and the atmosphere.
  • Natural processes result in a continuous exchange of carbon between the atmosphere, oceans, and terrestrial surface, which ideally is approximately balanced.
  • Fossil fuel use and deforestation represent unbalanced additions to atmospheric carbon dioxide. Only about half of anthropogenic carbon dioxide in the atmosphere has been removed and returned to oceans and terrestrial surface by natural processes.
  • As long as more carbon dioxide is being emitted into the atmosphere than is being removed, atmospheric concentrations of carbon dioxide will continue to increase.

The end result of the increase in atmospheric carbon dioxide (and other greenhouse gases) is a strengthening greenhouse effect that gradually warms the planet. But, the observed warming trend since the late 1800s has hardly been as smooth and consistent as the increase in greenhouse gas concentrations. Up next, we'll take a closer look at the how scientists take Earth's temperature, and dial in on the details of the observed warming trends.


46.3C: The Carbon Cycle - Biology

Carbon is the fourth most abundant element in the Universe, after hydrogen, helium, and oxygen. On Earth, carbon cycles through the land, ocean, atmosphere, and the Earth's interior in a major biogeochemical cycle (the circulation of chemical components through the biosphere from or to the lithosphere, atmosphere, and hydrosphere). The global carbon cycle can be divided into two categories: the geological, which operates over large time scales (millions of years), and the biological/physical, which operates at shorter time scales (days to thousands of years).

Photosynthesis traps carbon dioxide from the atmosphere to produce glucose and it stores energy. Glucose, of course, is used to make other organic molecules and is used as a source of energy in respiration.

In respiration and in the oxidative decomposition of plant materials, the carbon in organic molecules is converted to CO2. Only a very small percentage of the organic carbon is sequestered in sediments.

The biological carbon cycle is not only faster than the geological carbon cycle. The amount of carbon taken up by photosynthesis and released back to the atmosphere by respiration each year is 1,000 times greater than the amount of carbon that moves through the geological cycle on an annual basis.

The biological carbon cycle plays a role in the long-term, geological cycling of carbon. The presence of land vegetation enhances the weathering of soil, leading to the uptake of carbon dioxide from the atmosphere. In the oceans, some of the carbon taken up by phytoplankton is used to make shells of calcium carbonate that settle to the bottom after the organisms die to form sediments. Marine animals, such as corals, also use dissolved carbon dioxide in biomineralization.

During the daytime in the growing season, leaves absorb sunlight and take up carbon dioxide from the atmosphere. Plants, animals and soil microbes consume the carbon in organic matter and return carbon dioxide to the atmosphere.

When conditions are too cold or too dry, photosynthesis and respiration cease along with the movement of carbon between the atmosphere and the land surface.

The amounts of carbon that move from the atmosphere through photosynthesis, respiration, and back to the atmosphere are large and produce oscillations in atmospheric carbon dioxide concentrations.

Significant amounts of carbon are stored in the biomass of forests and in the soil. Terrestrial sources release the stored carbon when forests are cleared for agriculture. Organisms in the ocean consume and release large quantities of carbon dioxide but ocean biological carbon cycles are faster than terrestrial cycles. There is virtually no storage of carbon as as biomass. Photosynthetic plankton are consumed by zooplankton within days to weeks.

Carbon dioxide exchange in the oceans is controlled by sea surface temperatures, circulating currents, and by the biological processes of photosynthesis and respiration. Carbon dioxide solvation is temperature dependent. Cold ocean temperatures favor the uptake of carbon dioxide from the atmosphere while warm temperatures can cause the ocean surface to release carbon dioxide. Cold, downward moving currents such as those that occur over the North Atlantic absorb carbon dioxide and transfer it to the deep ocean. Upward moving currents such as those in the tropics bring carbon dioxide up from depth and release it to the atmosphere.