Co

Methanogenesis

Methane oxidation

Figure 28.19 The Basic Carbon Cycle in the Environment. Carbon fixation can occur through the activities of photoautotrophic and chemoautotrophic microorganisms. Methane can be produced from inorganic substrates (CO2 + H2) or from organic matter. Carbon monoxide (CO)—produced by sources such as automobiles and industry—is returned to the carbon cycle by CO-oxidizing bacteria. Aerobic processes are noted with blue arrows, and anaerobic processes are shown with red arrows. Reverse methanogenesis will be discussed in chapter 29.

present in an integrated basic carbon cycle are shown in figure 28.19. Reductants (e.g., hydrogen, which is a strong reductant) and oxidants (e.g., O2) influence the course of biological and chemical reactions involving carbon. Hydrogen can be produced during organic matter degradation especially under anaerobic conditions when fermentation occurs. If hydrogen and methane are generated, they can move upward from anaerobic to aerobic areas. This creates an opportunity for aerobic hydrogen and methane oxidizers to function.

Methane levels in the atmosphere have been increasing approximately 1% per year, from 0.7 to 1.6 to 1.7 ppm (volume) in the last

300 years. This methane is derived from a variety of sources. If an aerobic water column is above the anaerobic zone where the methanogens are located, the methane can be oxidized before it reaches the atmosphere. In many situations, such as in rice paddies without an overlying aerobic water zone, the methane will be released directly to the atmosphere, thus contributing to global atmospheric methane increases. Rice paddies, ruminants, coal mines, sewage treatment plants, landfills, and marshes are important sources of methane. Anaerobic microorganisms such as Methanobrevibacter in the guts of termites also can contribute to methane production. Physiology of aerobic hydrogen and methane utilizers (pp. 193, 502-3)

Table 28.5

Complex Organic Substrate Characteristics That Influence Decomposition and Degradability

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