CARBON CYCLE:

Carbon circulates through the carbon cycle. It shows that carbon may be pres- ent 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 HCO-3 or molecular CO2 (aq). A very large amount of carbon is present in minerals, particularly magnesium and calcium 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 hydrocarbonaceous kerogen (the organic matter in oil shale), coal, and lignite. Manufacturing processes are used to convert hydro- carbons to xenobiotic compounds with functional groups containing halogens, oxygen, nitrogen, phosphorus, or sulfur. Although a very small amount of total environmental carbon, these com- pounds are particularly significant because of their toxicological chemical effects.

An important aspect of the carbon cycle is that it is the one 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 Ot2, to regenerate carbon dioxide and to 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 increases, enabling precipitation of CaCO3, and CaCO3, MgCO3. Biogeochemical processes transform organic carbon fixed by microorganisms 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 hydro- carbons, such as those that are accidentally spilled on soil or in water. Biodegradation also acts to destroy carbon-containing compounds in hazardous wastes.

The carbon cycle. Mineral carbon is held in a reservoir of limestone, CaCO3, from which it my enter a mineral solution as dissolved hydrogen carbonate ion, HCO-3, formed when dissolved CO2 (aq) reacts with CaCO3, In the atmosphere carbon is present as carbon dioxide, CO2. Atmospheric carbon dioxide is fixed as organic matter by photosynthesis, and organic carbon is released as CO2 by microbial decay of organic matter.