Microorganisms in the Carbon Cycle

Carbon is constantly recycled between the atmosphere, living organisms, soil and oceans. Microorganisms (bacteria and fungi) are essential to this cycle because they release the carbon locked up in dead organic matter.Saprobionts (decomposers) feed on dead organisms, faeces and urine. They secrete enzymes onto the material and absorb the soluble products of this extracellular digestion. Some of the absorbed glucose is used in respiration, releasing carbon dioxide into the air or soil water. Without decomposers, carbon would stay trapped in dead bodies and waste, and less CO₂ would be available for photosynthesis.In anaerobic conditions, such as waterlogged soils, marshes, landfill sites and the guts of ruminants, methanogenic microorganisms (archaea) break down organic matter and release carbon as methane (CH₄) instead of CO₂. Methane is a powerful greenhouse gas, so these microorganisms affect the climate as well as the cycle.Microorganisms also remove carbon from the atmosphere. Photosynthetic microorganisms such as cyanobacteria and algae in the phytoplankton fix CO₂ into organic compounds. In the oceans they carry out about half of all the photosynthesis on Earth, and they are the base of marine food chains.So microorganisms act at both ends of the cycle: photosynthetic ones fix carbon, and saprobionts and methanogens return it to the atmosphere. The balance between these processes helps decide how much carbon is stored in ecosystems and how much is in the air.