Carbon it may occur as combustion, such as when

Carbon iscirculated through the carbon cycle.

This cycle shows that carbon may bepresent as gaseous atmospheric CO2, constituting a relatively small but highlysignificant portion of global carbon. Some of the carbon is dissolved insurface water and groundwater as HCO3  ormolecular CO2(aq). A very large amount of carbon is present in minerals,particularly calcium and magnesium carbonates such as CaCO3. Photosynthesisfixes inorganic C as biological carbon, represented as {CH2O}, which is a constituentof all life molecules. Another fraction of carbon is fixed as petroleum andnatural gas, with a much larger amount as hydro carbonaceous kerogen (theorganic matter in oil shale), coal, and lignite, represented as CxH2x.  Manufacturing processes are used to converthydrocarbons to xenobiotic compounds with functional groups containinghalogens, oxygen, nitrogen, phosphorus, or sulphur.

Though a very small amountof total environmental carbon, these compounds are particularly significantbecause of their toxicological chemical effects. An important aspect of thecarbon cycle is that it is the cycle by which solar energy is transferred tobiological systems and ultimately to the geosphere and anthrosphere as fossilcarbon and fossil fuels. Organic, or biological, carbon, {CH2O}, is containedin energy-rich molecules that can react biochemically with molecular oxygen,O2, to regenerate carbon dioxide and produce energy. This can occurbiochemically in an organism through aerobic respiration or it may occur ascombustion, such as when wood or fossil fuels are burned. Microorganisms arestrongly involved in the carbon cycle, mediating crucial biochemical reactionsdiscussed later in this section. Photosynthetic algae are the predominantcarbon-fixing agents in water; as they consume CO2 to produce biomass the pH ofthe water is raised enabling precipitation of CaCO3 and CaCO3•MgCO3. Organiccarbon fixed by microorganisms is transformed by biogeochemical processes tofossil petroleum, kerogen, coal, and lignite. Microorganisms degrade organiccarbon from biomass, petroleum, and xenobiotic sources, ultimately returning itto the atmosphere as CO2.

Hydrocarbons such as those in crude oil and somesynthetic hydrocarbons are degraded by microorganisms. This is an important mechanismfor eliminating pollutant hydrocarbons, such as those that are accidentally spilledon soil or in water. Biodegradation can also be used to treat carbon-containingcompounds in hazardous wastes