Carbon it may occur as combustion, such as when

Carbon is
circulated through the carbon cycle. This cycle shows that carbon may be
present as gaseous atmospheric CO2, constituting a relatively small but highly
significant portion of global carbon. Some of the carbon is dissolved in
surface water and groundwater as HCO3  or
molecular CO2(aq). A very large amount of carbon is present in minerals,
particularly calcium and magnesium carbonates such as CaCO3. Photosynthesis
fixes inorganic C as biological carbon, represented as {CH2O}, which is a constituent
of all life molecules. Another fraction of carbon is fixed as petroleum and
natural gas, with a much larger amount as hydro carbonaceous kerogen (the
organic matter in oil shale), coal, and lignite, represented as CxH2x.  Manufacturing processes are used to convert
hydrocarbons to xenobiotic compounds with functional groups containing
halogens, oxygen, nitrogen, phosphorus, or sulphur. Though a very small amount
of total environmental carbon, these compounds are particularly significant
because of their toxicological chemical effects. An important aspect of the
carbon cycle is that it is the cycle by which solar energy is transferred to
biological systems and ultimately to the geosphere and anthrosphere as fossil
carbon and fossil fuels. Organic, or biological, carbon, {CH2O}, is contained
in energy-rich molecules that can react biochemically with molecular oxygen,
O2, to regenerate carbon dioxide and produce energy. This can occur
biochemically in an organism through aerobic respiration or it may occur as
combustion, such as when wood or fossil fuels are burned. Microorganisms are
strongly involved in the carbon cycle, mediating crucial biochemical reactions
discussed later in this section. Photosynthetic algae are the predominant
carbon-fixing agents in water; as they consume CO2 to produce biomass the pH of
the water is raised enabling precipitation of CaCO3 and CaCO3•MgCO3. Organic
carbon fixed by microorganisms is transformed by biogeochemical processes to
fossil petroleum, kerogen, coal, and lignite. Microorganisms degrade organic
carbon from biomass, petroleum, and xenobiotic sources, ultimately returning it
to the atmosphere as CO2. Hydrocarbons such as those in crude oil and some
synthetic hydrocarbons are degraded by microorganisms. This is an important mechanism
for eliminating pollutant hydrocarbons, such as those that are accidentally spilled
on soil or in water. Biodegradation can also be used to treat carbon-containing
compounds in hazardous wastes