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The rate of microbial respiration can be described by a rate law that gives the respiration rate as the product of a rate constant, biomass concentration, and three terms: one describing the kinetics of the electron-donating reaction, one for the kinetics of the electron-accepting reaction, and a thermodynamic term accounting for the energy available in the microbe's environment. The rate law, derived on the basis of chemiosmotic theory and nonlinear thermodynamics, is unique in that it accounts for both forward and reverse fluxes through the electron transport chain. Our analysis demonstrates how a microbe's respiration rate depends on the thermodynamic driving force, i.e., the net difference between the energy available from the environment and energy conserved as ATP. The rate laws commonly applied in microbiology, such as the Monod equation, are specific simplifications of the general law presented. The new rate law is significant because it affords the possibility of extrapolating in a rigorous manner from laboratory experiment to a broad range of natural conditions, including microbial growth where only limited energy is available. The rate law also provides a new explanation of threshold phenomena, which may reflect a thermodynamic equilibrium where the energy released by electron transfer balances that conserved by ADP phosphorylation.Understanding the rate at which microbes respire in biological and geochemical systems is central to developing quantitative descriptions of a broad range of problems in microbiology, from the propagation of disease to the attenuation of contaminants in drinking-water supplies. Microbiologists have in recent years expended considerable effort in investigating microbial respiration rates under specific conditions (22,33,36,41).In virtually all cases, the results of such studies have been cast in terms of a semi-empirical rate law such as the Monod equation (15). Application of such rate laws is limited in two senses. First, because of their semi-empirical nature, they are best suited to interpolating the results of a set of experiments within the range of chemical conditions tested (7). Second, none of the laws accounts for the thermodynamic effects of the energy available in the cell's environment. As such, there is no basis for applying a rate law derived for a laboratory experiment, where the available energy is typically maintained at a high level to provide for microbial growth at acceptable rates, to natural conditions, where much less energy may be available. As a result, the rate laws invariably predict positive activities even where there is no energy available to drive the cell's metabolism forward.In a recent paper (13), we performed, on the basis of the chemiosmotic model of cellular respiration (19) and nonlinear nonequilibrium thermodynamics (6), a rigorous analysis of the problem of respiration in eukaryotic cells by mitochondria. In our analysis, we accounted for a simultaneous forward and backward flux of electrons through the respiratory chain. The resultin...

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.

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.

For 30 years it has been assumed that a single species of cyanobacteria, Phormidium corallyticum, is the volumetrically dominant component of all cases of black band disease (BBD) in coral. Cyanobacterium-specific 16S rRNA gene primers and terminal restriction fragment length polymorphism analyses were used to determine the phylogenetic diversity of these BBD cyanobacteria on coral reefs in the Caribbean and IndoPacific Seas. These analyses indicate that the cyanobacteria that inhabit BBD bacterial mats collected from the Caribbean and Indo-Pacific Seas belong to at least three different taxa, despite the fact that the corals in each case exhibit similar signs and patterns of BBD mat development.