OpenStax Biology 2e
Carbon is one of the most important macronutrients, and prokaryotes play an important role in the carbon cycle. The carbon cycle traces the movement of carbon from inorganic to organic compounds and back again. Carbon is cycled through Earth’s major reservoirs: land, the atmosphere, aquatic environments, sediments and rocks, and biomass. In a way, the carbon cycle echoes the role of the “four elements” first proposed by the ancient Greek philosopher, Empedocles: fire, water, earth, and air. Carbon dioxide is removed from the atmosphere by land plants and marine prokaryotes, and is returned to the atmosphere via the respiration of chemoorganotrophic organisms, including prokaryotes, fungi, and animals. Although the largest carbon reservoir in terrestrial ecosystems is in rocks and sediments, that carbon is not readily available.
Participants in the carbon cycle are roughly divided among producers, consumers, and decomposers of organic carbon compounds. The primary producers of organic carbon compounds from CO2 are land plants and photosynthetic bacteria. A large amount of available carbon is found in living land plants. A related source of carbon compounds is humus, which is a mixture of organic materials from dead plants and prokaryotes that have resisted decomposition. (The term “humus,” by the way, is the root of the word “human.”) Consumers such as animals and other heterotrophs use organic compounds generated by producers and release carbon dioxide to the atmosphere. Other bacteria and fungi, collectively called decomposers, carry out the breakdown (decomposition) of plants and animals and their organic compounds. Most carbon dioxide in the atmosphere is derived from the respiration of microorganisms that decompose dead animals, plants, and humus.
In aqueous environments and their anoxic sediments, there is another carbon cycle taking place. In this case, the cycle is based on one-carbon compounds. In anoxic sediments, prokaryotes, mostly archaea, produce methane (CH4). This methane moves into the zone above the sediment, which is richer in oxygen and supports bacteria called methane oxidizers that oxidize methane to carbon dioxide, which then returns to the atmosphere.
Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: https://openstax.org/details/books/biology-2e