Model Of Interconnected Cycles Of Electrons

This system of flowing e~ is like an electrical system with two cycles, an anoxy-genic (not O2-producing) cycle and an oxygenic (O2-producing) cycle, and four circuits, two phototrophic, one chemoorganotrophic, and one chemolithotrophic, hooked in parallel (Fig. 9.7). The microbial component consists of four groups of organisms described using the nutritional classification in Table 9.1. Three groups of organisms are responsible for C addition to the pedosphere, and only one (chemoorganotrophs) for its removal. The two cycles are distinguished on the basis of the photosynthetic mechanisms: anoxygenic or oxygenic. The chemoorgan-otrophic circuit unites the anoxygenic cycle with the oxygenic cycle.

CH2O

Anoxygenic

Oxygenic

FIGURE 9.7 Schematic outline of electron flow through oxygenic and anoxygenic cycles starting with anoxygenic (not oxygen-producing) on the left and oxygenic photosynthesis (oxygen-producing) on the right. Anoxygenic photosynthesis (photolithotrophs) generates oxidized S species from reduced S under anaerobic conditions, thereby maintaining anaerobic conditions. Oxygenic photosynthesis (photoaquatrophs) generates oxygen from water, thereby producing aerobic conditions. Both yield CH2O for biomass production. In contrast, however, photolithotrophs serve the mineral-oxidizing function of chemolithotrophs in oxygenic cycles, but do not produce oxygen. Chemo-organotrophs unite both sets of cycles by oxidizing reduced C aerobically or anaerobically to CO2. (Reproduced from McGill (1996), with permission from SBCS and SLCS.)

CH2O

Anoxygenic

Oxygenic

FIGURE 9.7 Schematic outline of electron flow through oxygenic and anoxygenic cycles starting with anoxygenic (not oxygen-producing) on the left and oxygenic photosynthesis (oxygen-producing) on the right. Anoxygenic photosynthesis (photolithotrophs) generates oxidized S species from reduced S under anaerobic conditions, thereby maintaining anaerobic conditions. Oxygenic photosynthesis (photoaquatrophs) generates oxygen from water, thereby producing aerobic conditions. Both yield CH2O for biomass production. In contrast, however, photolithotrophs serve the mineral-oxidizing function of chemolithotrophs in oxygenic cycles, but do not produce oxygen. Chemo-organotrophs unite both sets of cycles by oxidizing reduced C aerobically or anaerobically to CO2. (Reproduced from McGill (1996), with permission from SBCS and SLCS.)

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