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The carbon cycle is a fundamental Earth process involving the transfer of carbon among the biosphere, lithosphere, atmosphere, and hydrosphere. It plays a critical role in regulating the planet’s climate and supporting life by cycling carbon through various chemical forms and reservoirs. Carbon primarily circulates as carbon dioxide (CO₂), representing its oxidized form, while reduced forms such as methane (CH₄) and organic compounds also play essential roles.
Microbial activity is central to the functioning of the carbon cycle. Photosynthetic microorganisms, particularly cyanobacteria, absorb atmospheric CO₂ and convert it into organic carbon via oxygenic photosynthesis. These microbes are primary producers in many ecosystems, forming the base of the food web. On the other hand, some microorganisms use anoxygenic photosynthesis or chemolithoautotrophy to fix carbon under anaerobic or light-limited conditions. For example, purple sulfur bacteria and green sulfur bacteria conduct anoxygenic photosynthesis using hydrogen sulfide instead of water, releasing elemental sulfur rather than oxygen.
Heterotrophic microbes decompose organic matter through aerobic or anaerobic respiration, releasing CO₂ back into the environment. In anoxic habitats, such as wetlands, rice paddies, or the digestive tracts of ruminants, methanogens—a group of archaea—convert organic substrates into methane. This CH₄ is either released into the atmosphere or consumed by methanotrophs, which oxidize methane back to CO₂ under aerobic conditions. Anaerobic oxidation of methane (AOM), often facilitated by consortia of archaea and bacteria using nitrate, sulfate, or metal oxides, also returns CH₄ to CO₂.
The carbon and nitrogen cycles are interconnected through microbial processes. For instance, the ammonium produced during decomposition can be oxidized to nitrate via nitrification. This nitrate serves as a nitrogen source for plants and microbes, while the simultaneous release of CO₂ reinforces the cycling of essential elements through the biosphere.
