The Carbon Cycle


There is a relationship between photosynthesis and cellular respiration. Cellular respiration releases carbon dioxide, which is used in photosynthesis. Photosynthesis in turn releases oxygen used in respiration. Animals and other heterotrophs depend on green organisms for organic food, energy, and oxygen. In the carbon cycle, organisms exchange carbon dioxide with the atmosphere. On land, plants take up carbon dioxide via photosynthesis and incorporate it into food used by themselves and heterotrophs. When organisms respire, some of this carbon is returned to the atmosphere in the molecules of carbon dioxide. In aquatic ecosystems, carbon dioxide from air combines with water to give carbonic acid, which breaks down to bicarbonate ions. Bicarbonate ions are a source of carbon for algae. When aquatic organisms respire, they release carbon dioxide that becomes bicarbonate (HCO3). The amount of bicarbonate in water is in equilibrium with amount of carbon dioxide in air.

Living and dead organisms are reservoirs of carbon in carbon cycle. More than 800 billion tons of carbon are in the world's biota, mainly in cells of trees. An additional 1,000 to 3,000 billion tons in plant and animal remains in the soil. Fossil fuels, such as coal, petroleum, and natural gas, were formed during various times of the geologic past when exceptional amount of organic matter were rapidly buried in an environment that locally lacked biologic activity. Inorganic calcium carbonate (CaCO3) accumulates in limestone and the calcite of shells.

Human burning of fossil fuels and wood has increased the amount of carbon dioxide released into atmosphere to 42 billion metric tons in only 22 years. Since human activity in 22 years probably released 78 billion metric tons, 36 billion metric tons were probably absorbed in oceans. Increased carbon dioxide levels may increase the greenhouse effect, where such gases allow the Sun's radiant energy to pass through to Earth where it is absorbed and reradiated as heat. Instead of radiating fromthe earth back into space, this heat is then trapped on Earth, perhaps causing or contributing to global warming.

Excess output occurs when the biomass is suddenly released, such as in slash and burn agriculture. Excess input commonly occurs from agricultural increase in organic fertilizer and other nutrients into an ecosystem. Generally speaking, the faster matter cycles, the faster it can recover from human intervention.

Recently, speculation has centered on the interconnection of biogeochemical cycles and their possible roles in a "superorganism" that James Lovelock has called Gaia, after the ancient Greek Earth goddess. While an intriguing notion, the degree of organization of these cycles is not as complex or integrated as the cells in a body, and Gaia remains controversial hypothesis.



The Nitrogen Cycle

Atmospheric nitrogen gas (N2) is unavailable to plants. Plants therefore depend on various types of nitrogen-fixing bacteria to take up nitrogen gas and make it available to them as some form of organic nitrogen. Nitrogen fixation occurs when nitrogen gas is chemically reduced and nitrogen is added to organic compounds. Atmospheric nitrogen is converted to ammonium (NH4+) by some cyanobacteria in aquatic ecosystems and by nitrogen-fixing bacteria in the nodules on roots of legume (beans, peas, clover, etc.) plants in terrestrial ecosystems. Plants take up both NH4+ and nitrate (NO3-) from soil. The nitrate (NO3-) is enzymatically reduced to ammonium (NH4+) and used in the production of both amino acids and nucleic acids.

Nitrification is the inorganic production of nitrates. Nitrogen gas (N2) is converted to nitrate (NO3-) by cosmic radiation, meteor trails, and lightning in the atmosphere. Human technology can now manufacture nitrates for use in fertilizers. In soil, bacteria convert ammonium (NH4+) to nitrate (NO3-) by a two-step process. Nitrite-producing bacteria (e.g. Nitrosomonas) convert ammonia to nitrite (NO2-). Next, nitrate-producing bacteria (e.g. Nitrobacter) convert nitrite to nitrate. These two groups of bacteria are called nitrifying bacteria.


Denitrification is conversion of nitrate to nitrous oxide and nitrogen gas back to atmosphere. This is done by denitrifying bacteria in both aquatic and terrestrial ecosystems. The process of denitrification almost, but not completely, counterbalances nitrogen fixation.



Development of a root nodule, a place in the roots of certain plants, most notably legumes (the pea family), where bacteria live symbiotically with the plant. This is one route by which inorganic nitrogen (N2) is converted into organic nitrogen that can then be used by other organisms. Images from W.H. Freeman and Sinauer Associates, used by



Taken from On line biology book