A New Rate Law Describing Microbial Respiration

Abstract

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...

Sulfur Isotope Enrichment during Maintenance Metabolism in the Thermophilic Sulfate-Reducing Bacterium Desulfotomaculum putei

“…The variation in ε 34 S values has been attributed to the SO 4 2Ϫ concentration, the type of electron donor and its concentration, the SO 4 2Ϫ reduction rate per cell (csSRR) (22), temperature, and species-specific isotope enrichment effects. In these laboratory experiments, doubling times are on the order of hours and csSRRs range from to 0.1 to 18 fmol cell Ϫ1 h Ϫ1 (7,12,17,22,30,32,39,40).…”

“…where r sample is equal to 34 S sample / 32 S sample i and r VCDT is equal to 0.0441638 (24). Each set of 12 samples was bracketed by three working standards ranging in isotopic composition from Ϫ32.5‰ to ϩ38.7‰ based on calibration to the International Atomic Energy Agency (IAEA) S-1 (Ϫ0.30 ‰), IAEA S-3 (Ϫ32.5‰), and NBS-127 (ϩ20.3 ‰) external standards.…”

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“…The stored energy is used for cell maintenance and growth, and biomass synthesis, etc. [20][21][22]. It follows that bacteria are only liable to conduct a TEAP reaction if a minimum Gibbs energy is overcome.…”

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“…The term in Eq. (4) approximately accounts for thermodynamic constraints on kinetics, because net reaction rate must approach zero as reaction free energy tends toward zero (Jin and Bethke, 2003;Boudart, 1976), which is equivalent to a growth efficiency of 1. The leading ε 2 i term in Eq.…”