The carbon cycle
A more detailed scheme of the carbon cycle is shown in Figure 16.2. Both aerobic and anaerobic reactions con-tribute to the cycle. The numbers in parentheses in the following description refer to those in Figure 16.2.
Atmospheric CO2 is fixed into organic compounds by plants, together with phototrophic and chemoau-totrophic microorganisms (1). The organic compounds thus synthesised undergo cellular respiration and CO2 is returned to the atmosphere (2). The carbon may have been passed along a food chain to consumers before this occurs. Carbon dioxide is also produced by the decomposition of dead plant, animal and microbial material by heterotrophic bacteria and fungi.
Methanogenic bacteria produce methane from organic carbon or CO2 (3, 4). This in turn is oxidised by methanotrophic bacteria; carbon may be incorporated into organic material or lost as CO2 (5, 6).