Energy conservation of anaerobic respiration

Abstract

Microbes save energy from the environment by synthesizing ATP. Understanding microbial energy conservation is important for both theoretical and practical applications. This paper focuses on the common metabolism of anoxic environments—ferric iron respiration, sulfate respiration, and methanogenesis—and analyzes microbial energy conservation on the basis of the thermodynamics as well as physiological models of respiratory reactions. The results of the analysis show that iron respiration synthesizes 1 to 4 ATPs by transferring eight electrons from H₂, acetate, lactate, and ethanol to ferric minerals; sulfate respiration makes 0.25 to more than 3 ATPs by transferring eight electrons from the same suite of electron donors to sulfate; methanogenesis yields 0 to 1 ATP by oxidizing four H₂ and by disproportionating one acetate. The ATP yields are compared to growth yields and energy thresholds of anaerobic metabolism to explore the impact of energy conservation. Specifically, energy conservation controls microbial growth: in geochemical systems, respiring microbes synthesize up to 5 g biomass per mole of ATP. Energy conservation also requires the environment to supply chemical energy at quantities greater than the energy saved by microbes. These results unify our view of microbial metabolism, and can be applied to evaluating the occurrence and significance of microbial life in natural environments.