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pH influences the occurrence and distribution of microorganisms. Microbes typically live over a range of 3-4 pH units and are described as acidophiles, neutrophiles, and alkaliphiles, depending on the optimal pH for growth. Their growth rates vary with pH along bell-or triangle-shaped curves, which reflect pH limits of cell structural integrity and the interference of pH with cell metabolism. We propose that pH can also affect the thermodynamics and kinetics of microbial respiration, which then help shape the composition and function of microbial communities. Here we use geochemical reaction modeling to examine how environmental pH controls the energy yields of common redox reactions in anoxic environments, including syntrophic oxidation, iron reduction, sulfate reduction, and methanogenesis. The results reveal that environmental pH changes energy yields both directly and indirectly. The direct change applies to reactions that consume or produce protons whereas the indirect effect, which applies to all redox reactions, comes from the regulation of chemical speciation by pH. The results also show that energy yields respond strongly to pH variation, which may modulate microbial interactions and help give rise to the pH limits of microbial metabolisms. These results underscore the importance of pH as a control on microbial metabolisms and provide insight into potential impacts of pH variation on the composition and activity of microbial communities. In a companion paper, we continue to explore how the kinetics of microbial metabolisms responds to pH variations, and how these responses control the outcome of microbial interactions, including the activity and membership of microbial consortia.

Natural arsenic contamination of groundwater, increasingly recognized as a threat to human health worldwide, is characterized by arsenic concentrations that vary sharply over short distances. Variation in arsenic levels in the Mahomet aquifer system, a regional glacial aquifer in central Illinois, appears to arise from variable rates of bacterial sulfate reduction in the subsurface, not differences in arsenic supply. Where sulfate-reducing bacteria are active, the sulfide produced reacts to precipitate arsenic, or coprecipitate it with iron, leaving little in solution. In the absence of sulfate reduction, methanogenesis is the dominant type of microbial metabolism, and arsenic accumulates to high levels.

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