All living things are made of molecules that contain carbon. Carbon is found in gas, liquid, and solid forms. It is in the atmosphere as inorganic carbon dioxide, organic methane, and volatile organic compounds. It is in the carbohydrates, nucleic acids, fats, and proteins that make up all living things. It is a major component of the decomposing organic matter in soil. The bulk of carbon, though, is found inorganically stored in rocks that make up the earth itself.
The carbon cycle can consists of slow and fast phases operating in tandem. In the slow phase, carbon moves from the atmosphere into the ocean, and then into the crust of the earth. The slow phase returns carbon to the atmosphere via tectonic movement. Water in the atmosphere combines with carbon dioxide to form carbonic acid. When it rains, this weak acid slowly weathers rocks, releasing inorganic ions that plants use as nutrients. Some of those ions make their way to the oceans in runoff, where they are used by organisms to build shells made of calcium carbonate. These shells fall to the bottom of the ocean when they die, and over geologic timescales they accumulate and are compressed, forming carbon-containing limestone. Eventually, through the movement of tectonic plates, this stone is brought back to the surface of the earth, or it violently comes back into the atmosphere as carbon dioxide in volcanic eruptions.
In the fast phase of the carbon cycle, carbon from the atmosphere is cycled through biological reservoirs by photosynthesis and cellular respiration. Microbes are a crucial participant in the carbon cycle. Cyanobacteria and microscopic algae in the oceans are responsible for about half of the carbon that is fixed through photosynthesis globally. That carbon becomes part of their bodies and flows through food chains as organisms are consumed by one another. Microbes perform all of the carbon fixation that occurs through pathways other than photosynthesis, such as chemosynthesis performed by methanogenic archaea. The carbon fixed in photosynthesis and chemosynthesis is returned when autotrophs are consumed. The consumer uses breaks down the organic molecules of the autotroph to obtain energy and release carbon dioxide as a byproduct of cellular respiration. Glucose is used to create ATP (the energy molecule of the cell), and carbon dioxide is returned to the atmosphere. When organisms die, the organic molecules of their bodies are broken down by decomposers and converted to energy through cellular respiration. Microbes are the major decomposers on land and in the water. By decomposing dead organisms, microbes recycle nutrients and carbon.Dead organisms may also return carbon to the slow phase of the carbon cycle. Over geologic time, pieces of dead organisms accumulate on the ocean floor, where they are compressed and transformed into fossil carbon, such as oil that only slowly returns to the surface of the earth to reenter the fast phase. These fossil carbon sources have become more meaningful to the fast phase of the carbon cycle since the beginning of the industrial revolution. Human burning of fossil fuels transfers carbon from the slow phase to the fast phase by converting it from a long-term storage pool to the fast-phase pool in the atmosphere.