THE CARBON CYCLE

The carbon cycle is based on carbon dioxide, which makes up only about 0.03% of the atmosphere. The worldwide synthesis of organic compounds from carbon dioxide and water results in the fixation of about 10% of the roughly 700 billion metric tons of carbon dioxide in the atmosphere each year. This enormous amount of biological activity takes place as a result of the combined activities of photosynthetic bacteria, algae, and plants. All heterotrophic organisms-including the nonphotosyn-thetic bacteria and protists, the fungi, the animals, and a relatively few plants, such as dodder, that have lost the ability to photosynthesize-obtain their carbon indirectly, from the organisms that fix it. When their bodies decompose, organisms release carbon dioxide to the atmosphere again. Once there, it can be rein-corporated into the bodies of other organisms.
About 10% of the estimated 700 billion metric tons of carbon dioxide in the atmosphere is fixed annually by the process of photosynthesis.
Most of the organic compounds that are formed as a result of carbon dioxide fix-' ation in the bodies of photosynthetic organisms are ultimately broken down and released back into the atmosphere or water. Certain carbon-containing compounds, such as cellulose, are more resistant to breakdown than others, but certain bacteria and fungi, as well as a few kinds of insects, are able to accomplish this feat. Some cellulose, however, accumulates as undecomposed organic matter, such as peat. The carbon in this cellulose may eventually be incorporated into fossil fuels, such as oil or coal,
In addition to the roughly 700 billion metric tons of carbon dioxide in the atmosphere, approximately 1000 billion metric tons are dissolved in the ocean; more than half of this quantity is in the upper layers, where photosynthesis takes place. The fossil fuels, primarily oil and coal, contain more than 5000 billion additional metric ions of carbon, and between 600 and 1000 billion metric tons are locked up in living organisms at any one time. In global terms, photosynthesis and respiration are approximately balanced, but the balance has been shifted recently because of our consumption of fossil fuels. The release of the carbon in coal, oil, and natural gas as carbon dioxide, a process that is proceeding rapidly as a result of combustion of fuels by bu-mans, currently appears to be changing global climates, and may do so even more rapidly in the future.