Hoh

4SO4- -

+ 6CH2O + 8H+

[11]

In such autotrophic (obtain C from CO2) chemolithotrophs (energy from a chemical source and electrons from a mineral source), CO2 must be reduced to form biomass, represented here as CH2O. Electrons for reduction of CO2 are provided by the oxidation of S (Table 9.6; using S0 as an example). Consequently additional S must be oxidized to provide electrons to reduce CO2 [11]. Reverse e-transport reduces NAD+ to NADH for subsequent reduction of NADP+, probably by transhydrogenation, to NADPH (Nicholls and Ferguson, 1992), which is used in the Calvin cycle to reduce C. These reactions generate acidity in soil environments. For example, the combined set of reactions (Eq. [11]) releases 2 mol of H+ per mole of S0 oxidized during the processes of energy generation and of biomass formation. See Chap. 15 for further treatment of this. Such acidity influences soil pH and mineral weathering. By promoting mineral weathering, acidification during chemolithotrophic oxidation of N or S may also have influenced the course of planetary development through the formation of pedogenic clay (Kennedy et al., 2006).

Large quantities of S may be oxidized to generate ATP for growth and reducing equivalents for the Calvin cycle. A way to relate S oxidation to ATP formation would add precision to understanding about soil microbial ecosystems. ATP yield can be estimated roughly from free energy change. Because of the nonstandard condition of the cytoplasm, free energy change in organisms is less than the "standard" free energy change. ATP synthesis under "cytoplasmic" conditions captures only about 40% of the "standard" free energy change (Nicholls and Ferguson, 1992). Consequently, although ATP hydrolysis yields a standard free energy change of 30 kJ mol-1, a free energy change of about 30/0.4 = 75 kJ mol-1 will be expected to yield only 1 mol of ATP. Accepting this estimate, calculating the expected ATP yield using Eq. [10] yields 584.9/75 = 7.8 mol. The Calvin cycle requires 3 mol of ATP per mole of C fixed. Rounding to 8 mol of ATP per mole S0 oxidized (ATP/ 2e- = 2.7) and 3 mol ATP used per mole of C fixed, one can write an overall summary of the amount of S oxidized both to generate ATP and to provide the reducing equivalents needed to reduce CO2 to CH2O (Table 9.7, Eq. [12]).

Oxidation of N with Reduction of CO2

Nitrogen is an important agronomic, ecological, and environmental element that is oxidized by autotrophic chemolithotrophs during nitrification. The N atoms in NH+ are oxidized to release protons and electrons; and water contributes O for

TABLE 9.7 Estimate of Overall Stoichiometry of S Oxidation for Energy Production and C Reduction by Species such as Thiobacillus spp.

72(ADP + Pi) + 9S0 + 9HOH

+ 13.5O2

—>

9SO4- +

18H+ + 72ATP

C reduction: 16S0

+ 64HOH

—>

16SO-f~ -

f 96e~ + 128H+

72ATP + 24CO2 + 96e

^ + 96H+

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