The thermodynamics and kinetics of microbial metabolism

Abstract

The various kinetic rate laws commonly used to describe microbial metabolism are derived considering only forward reaction progress and hence are inconsistent with the requirements of thermodynamics. These laws may be applied without significant error where abundant energy is available to drive the metabolic reaction, so the forward reaction overwhelms the reverse. The laws are, however, unsuitable where little energy may be available. In previous papers we derived a new rate law for microbial respiration considering that reaction progresses simultaneously in both the forward and reverse directions. In this paper, we demonstrate in a new and rigorous way how the rate law can account quantitatively for the thermodynamic driving force for reaction. We refine our previous work on microbial respiration to better account for details of the electron transfer process. We furthermore extend the theory to account for enzymatic reaction and microbial fermentation. We show that commonly used rate laws of simple form can be modified to honor thermodynamic consistency by including a thermodynamic potential factor. Finally, we consider how the rate of biomass synthesis can be determined from the rate of respiration or fermentation. We apply these results to describe (1) the enzymatic reaction by which benzoyl-CoA forms, (2) crotonate fermentation, and (3) glucose fermentation; for each process we demonstrate how the reaction rate is affected by the thermodynamic driving force. Results of the study improve our ability to predict microbial metabolic rates accurately over a spectrum of geochemical environments, including under eutrophic and oligotrophic conditions.