The chemical biology of methanogenesis

Two distinct pathways account for most of the CH(4) produced in the majority of the diverse and vast anaerobic environments of Earth's biosphere by microbes that are classified in the Archaea domain of life: conversion of the methyl group of acetate to CH(4) in the aceticlastic pathway and reduction of CO(2) with electrons derived from H(2), formate or CO in the CO(2) reduction pathway. Minor, albeit ecologically important, amounts of CH4 are produced by conversion of methylotrophic substrates methanol, methylamines and methyl sulfides. Although all pathways have terminal steps in common, they deviate in the initial steps leading to CH(4) and mechanisms for synthesizing ATP for growth. Hydrogen gas is the major reductant for CO(2)-reducing methanogens in the deep subsurface, although H(2) is also utilized by CO(2)-reducing microbes from the Bacteria domain that produce acetate for the aceticlastic methanogens. This review presents fundamentals of the two major CH(4)-producing pathways with a focus on understanding the potential for biologically-produced CH(4) on Mars. (C) 2010 Elsevier Ltd. All rights reserved.