CARBON CYCLE IN MICROORGANISMS

The carbon cycle in microorganisms is part of a larger cycling of carbon that occurs on the global scale. The actions of microorganisms help extract carbon from non-living sources and make the carbon available to living organisms (including themselves).

The cycling of carbon by microorganisms, including a variety of bacteria and fungi, occurs in aquatic habitats. Even relatively oxygen-free zones such as in the deep mud of lakes, ponds and other water bodies can be regions where the anaerobic conversion of carbon takes place.

Much of the carbon that enters the carbon cycle of microorganisms is carbon dioxide. This form of carbon exists as a gas in the atmosphere and can be dissolved in water. The atmospheric carbon dioxide can be converted to organic material in the process of photosynthesis. Photosynthetic algae are important microorganisms in this regard. As well, chemoautotrophs, primarily bacteria and archae are capable of carbon dioxide conversion. In both systems the carbon dioxide is converted to chains that are comprised of sugars that have the structure CH₂O.

Both types of conversion take place in the presence and the absence of oxygen. Algal involvement is an aerobic process. The conversion of carbon dioxide to sugar is an energy-requiring process that generates oxygen as a by-product. This evolution of oxygen also occurs in plants and is one of the recognized vital benefits of trees to life on Earth.

The carbon available in the carbohydrate sugar molecules is cycled further by microorganisms in a series of reactions that form the so-called tricarboxylic acid (or TCA) cycle. The breakdown of the carbohydrate serves to supply energy to the microorganism. This process is also known as respiration. In anaerobic environments, microorganisms can cycle the carbon compounds to yield energy in a process known as fermentation.

Carbon dioxide can be converted to another gas called methane (CH₄). This occurs in anaerobic environments, such as deep compacted mud, and is accomplished by bacteria known as methanogenic bacteria. The conversion, which requires hydrogen, yields water and energy for the methanogens. To complete the recycling pattern another group of methane bacteria called methane-oxidizing bacteria or methanotrophs (literally "methane eaters") can convert methane to carbon dioxide. This conversion, which is an aerobic (oxygen-requiring) process, also yields water and energy. Methanotrophs tend to live at the boundary between aerobic and anaerobic zones. There they have access to the methane produced by the anaerobic methanogenic bacteria, but also access to the oxygen needed for their conversion of the methane.

Other microorganisms are able to participate in the cycling of carbon. For example the green and purple sulfur bacteria are able to use the energy they gain from the degradation of a compound called hydrogen sulfide to degrade carbon compounds. Other bacteria such as Thiobacillus ferrooxidans uses the energy gained from the removal of an electron from iron-containing compounds to convert carbon.

The anerobic degradation of carbon is done only by microorganisms. This degradation is a collaborative effort involving numerous bacteria. Examples of the bacteria include Bacteroides succinogenes, Clostridium butyricum, and Syntrophomonas sp. This bacterial collaboration, which is termed interspecies hydrogen transfer, is responsible for the bulk of the carbon dioxide and methane that is released to the atmosphere.