T

as Pseudomonas spp.). Reducing equivalents (NADH) are produced by oxidation of substrates within the cytoplasm. In Fig. 9.5B, reverse electron transport is used to generate NADH, which is used to reduce CO2 in the Calvin cycle. Electron transport through the respiratory chain to Box generates the AP needed both to synthesize ATP and to drive reverse electron transport (e.g., chemolithotrophs such as Nitrobacter).

Photosynthesis is the ultimate source of energy to allow soil organisms to work. Photosynthesis requires no oxygen (although it may partially cycle O2) and may or may not generate O2. If photosynthesis uses HOH as an electron donor, as it does for photoaquatrophs, then it generates O2 and is called oxygenic photosynthesis (Table 9.3; Eq. [1]).1 Photosynthesis that uses a reduced mineral such as H2S as electron donor, as is the case with photolithotrophs, produces S0 (Eq. [2]) (or SO4~; Eq. [3]), which is called anoxygenic photosynthesis (Staley and Orians, 1992).

All anoxygenic photosynthetic organisms represented in Fig. 9.5C can use H2 as H donor; in addition the purple nonsulfur bacteria (Rhodospirillaceae) and the green gliding bacteria (Chloroflexaceae) can use organic substrates, the purple sulfur bacteria (Chromatiaceae) can use H2S and organic substrates, and the green sulfur bacteria (Chlorobiaceae) can use H2S, but not organic substrates. Photon activation of electrons in the photosynthesis reaction site (PS Rxn) generates cyclic transfer of electrons through a series of redox couples, resulting in formation of ATP, and eventually the electron returns to its ground state and to the PS Rxn site. NAD+ is reduced using reverse electron transfer driven by AP generated from photosynthesis. Subsequent transhydrogenation allows NADH to reduce

TABLE 9.3 Anoxygenic and Oxygenic Photosynthesis

Oxygenic photosynthesis, e.g., Panicum, Nostoc hu + 2HOH + 3(ADP + P¡) — + 4H+ + O2 + 3ATP

3ATP + CO2 + 4e~ + 4H+ — CH2O + HOH + 3(ADP + P¡) Overall: hu + CO2 + HOH — CH2O + O2 [1]

Anoxygenic photosynthesis, e.g., Chromatium (purple S bacteria)

2H2S hu + pigment 3(ADP + Pi) + [pigment+ + e~]a 3ATP + CO2 + 4e~ + 4H+ Overall: hu + CO2 + 2H2S

3ATP + pigment

"Complex including several components of the photosynthetic system and the respiratory chain. (Reproduced from McGill (1996), with permission from SBCS and SLCS.)

NADP+ to NADPH, which is used to reduce C in the Calvin cycle (Nicholls and Ferguson, 1992). In the Chlorobiaceae CO2 is reduced by reversal of the TCA cycle, all other anoxygenic photosynthetic organisms use the Calvin cycle (see Gottschalk, 1986, for additional reading). NADH is used to reduce CO2, so the electron and H transfer to CO2 in anoxygenic phototrophic bacteria can be summarized by Eq. [2] in Table 9.3.

In oxygenic photosynthesis (Table 9.3, Eq. [1]) ATP is synthesized during transfer from HOH to NADP+, and pseudocyclic transfer without formation of NADPH meets additional ATP needs (Nicholls and Ferguson, 1992). The reducing equivalents (NADPH + H+ ) are used in the Calvin cycle for reduction of C. In Fig. 9.5D the electrons released from splitting of water are passed linearly to NAD+ along a series of redox couples during which AP is generated for ATP synthesis. Two PS Rxn sites are involved. This linear transfer of electrons may not generate enough AP to produce enough ATP for CO2 reduction. Cyclic electron transfer can make up the shortfall.

examples of soil microbial transformations

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