It is important to note that although the following cycles are explained individually they all work together.
Carbon Cycle
Carbon is sixth element on the periodic table. In its pure form, carbon can be found in nature as diamonds, graphite, and coal, depending on its crystalline structure which is dependent on its cooling rate. Carbon is the basis for all organic life and is found in tissues, bones, carbohydrates, lipids, and proteins
5
.
Carbon exists in all spheres; the atmosphere, the lithosphere, the hydrosphere, and the biosphere. Carbon has its longest residence time in the lithosphere, so the rocks would be considered a primary sink for carbon. Second to the lithosphere, carbon has a long residence time in the oceans, its second largest sink
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.
Carbon in its inorganic form, carbon dioxide (CO
2
), exists in the atmosphere. Although it is less than a tenth of a percent of total atmospheric gases, it is a primary greenhouse gas and even small increases in its amounts cause a positive feedback, increasing the warming potential of the atmosphere
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.
Most carbon dioxide is removed from the atmosphere by photoautotrophic organisms, such as marine organisms like Prochlorococcus
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, a type of cyanobacteria, and higher order plants, for photosynthesis. During photosynthesis, phototrophs that are also autotrophs and able to make their own biomass from CO
2,
they combine six carbon dioxide molecules and 6 water molecules, in the presence of light, to form one carbohydrate and six oxygen molecules (6CO
2
+ 6H
2
O → C
6
H
12
O
6
+ 6O
2
). The carbohydrate that is produced is used by other organisms higher on the food chain, to make ATP, the energy used by living organisms. From this point, the carbon cycle can go one of two ways, either the plant is consumed by a primary consumer or it dies
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.
When a plant is consumed by a primary consumer, the carbohydrates are used as fuel for that organism. One process by which an organism uses this energy is aerobic respiration. The chemical reaction involves one carbohydrate and six oxygen molecules to produce six carbon dioxide molecules, six water molecules and energy (C
6
H
12
O
6
+ 6O
2
→ 6CO
2
+ 6H
2
O + 38ATP). It is just the opposite of photosynthesis
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. The energy obtained is used to build tissue and do work. From here the cycle can repeat itself, from a primary consumer to a secondary consumer, then to a tertiary consumer, and then to decomposition.
Decomposition begins once an organism has died. The remaining biomass is degraded through anaerobic, aerobic, and fermentation processes initiated by various microorganisms. This process is carried out by the microorganisms oxidizing organic carbon to create carbon dioxide and methane. Methane is also produced by microorganisms that reduce CO
2
using hydrogen. Carbon dioxide and methane are then released into the environment. Some of this is stored in the lithosphere and the rest released into the atmosphere as CO
2
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.
Microorganisms complete the cycle. Without them the carbon stored in the biomass that winds up in the various reservoirs would stay there and the CO
2
in the atmosphere would eventually be depleted, halting photosynthesis and eventually all organisms that depend on energy from the Sun and photosynthetic primary producers would die. However, it is important to note that there exist many organisms that live beneath the surface and receive their energy from chemicals like sulfur from hydrothermal vents that would carry on.
Nitrogen Cycle
Nitrogen is the seventh element on the periodic table. It exists in its elemental state in the atmosphere in the form of N
2
gas, dinitrogen. Nitrogen is an essential part of DNA and RNA and therefore needed by all living organisms. Approximately 78 percent of the atmosphere is nitrogen and although organisms breathe in this gas they cannot use it in this form. It must be fixed into NH
4
+
, NO
3
- -
, and organic nitrogen. Microorganisms are responsible for the majority of biological fixation of nitrogen
5
. Two organisms capable of fixing nitrogen are cyanobacteria and Trichodesmium. Nitrogen can also be fixed by lightening, by a few plants living in symbiotic relationships with microbes
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, and synthetically by humans.
The first step in making nitrogen available for biological use is nitrogen fixation. Nitrogen fixation occurs when hydrogen is added to nitrogen to form NH
3
, ammonia. This process is accomplished by a small fraction of bacteria that complete the process in both aerobic (Azotobacter) and anaerobic (Clostridium) environments . These bacteria live in both terrestrial (in symbiosis with plants) and aquatic environments . In the soil, bacteria can fix nitrogen on their own, yielding small amounts of NH
3
, or in conjunction with plant roots, yielding much larger amounts of NH
3
. Nitrogen fixation is an energy intensive activity, so the bacteria that work with the plants make a better living. Rhizobium is one such group of bacteria that exists in a mutualistic relationship with legumes such as soybeans
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.
Once in the form of ammonia plants can use the nitrogen. Once the plant dies the bacteria then convert NH
3
into ammonium, NH
4
+
, through a process called ammonia assimilation. The ammonium made is also available for plant use. Some of it is oxidized to nitrate, NO
3
- -
, through a process called nitrification
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.
The next step in the process is to reduce the nitrate back into N
2
, dinitrogen. This process is called denitrification
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.
If the cycle just included plants, it would be complete, but many other organisms require nitrogen for life. Consumers, receive their nitrogen from eating microbial producers and plants. The nitrogen is either expelled through waste processes or returned to the soil to decompose. At this point the denitrification would begin
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.
Phosphorus Cycle
Phosphorus, the fifteenth element is another vital part of biological systems. Phosphorus is used in the formation of DNA, RNA, ADP, and ATP and occurs primarily as phosphate (PO
4
3 - -
). Unlike the other elements phosphorus does not cycle through the atmosphere; however trace particles can be found in the air
5
. The primary sink for phosphorus is the lithosphere in the mineral apatite (Ca
5
(PO
4
)
3
OH). Apatite is the mineral found in our bones and teeth
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.
The phosphorus that is trapped in the rocks can only be released (naturally) through weathering. Since weathering is a slow process, the amount of phosphorus available to plants is limited. Primary, secondary, and tertiary consumers, receive their phosphorus through the consumption of plants and lower level consumers. Either through waste or death and decay of an organism, organic phosphorus in biomass is returned to the soil where bacteria can turn it back into inorganic phosphorus
5
.
Sulfur Cycle
Sulfur is the sixteenth element. It plays a vital role in the proper functioning of some amino acids, hormones, vitamins, and coenzymes. Sulfur cycles through all spheres. It's largest sinks are the lithosphere and the oceans, respectively
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.
Sulfur in the form of sulfur dioxide (SO
2
) and hydrogen sulfide (H
2
S) is released into the atmosphere by both natural and anthropogenic processes. Naturally, SO
2
and H
2
S are released into the atmosphere through volcanic eruptions and bacterial activity. Humans emit these compounds through the combustion of fossil fuels. These compounds are released from the atmosphere by rain, where they then fall to the land or the oceans. In the oceans H
2
S dissolves, bonds with iron and is precipitated out as pyrite (FeS
2
). The SO
2,
dissolves, bonds with calcium and is precipitated out as gypsum (CaSO
4
). On land these compounds find their way into the soil. Plants and microorganisms are able to use inorganic sulfur in the form of sulfate and reduce it internally in a process called assimilatory sulfate reduction. While inside the cell the sulfate is reduced to sulfite and then sulfide. The sulfide is then used to form the amino acid cysteine. The sulfur is eventually released from its organic form by another amino acid in a process called sulfur mineralization
. With the help of microbes sulfur eventually winds up back in the rock as metal compounds or in the atmosphere as gaseous compounds where the cycle begins again
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.
In oxygenated environments, a group of chemolithoautotrophic bacteria, like Acidithiobacillus (this is the classic acid mine drainage microbe that oxidizes sulfur (and Fe
2 +
) in pyrite at low pH) and Sulfobacillus oxidizes sulfide to sulfate (SO
4
2 - -
) at low . They use the energy produced by oxidizing sulfur to fix CO
2
. In environments devoid of oxygen, but rich in light, photoautotrophic bacteria, like Chlorobium and Chromatium, more commonly known as green and purple sulfur bacteria respectively, are able to oxidize H
2
S to SO
4
2 - -
. Their environments include mud and shallow water. In these oxygen poor environments these microbes are able to use light and sulfur to fix carbon. They can be identified by their green or purplish color. Environments that contain these microbes, like marshes, are easy to identify by their black sediment and pungent rotten egg smell
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.
