The chemical biology of methanogenesis

Two distinct pathways account for most of the CH₄ 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 CH₄ in the aceticlastic pathway and reduction of CO₂ with electrons derived from H₂, formate or CO in the CO₂ 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 CH₄ and mechanisms for synthesizing ATP for growth. Hydrogen gas is the major reductant for CO₂-reducing methanogens in the deep subsurface, although H₂ 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 CH₄-producing pathways with a focus on understanding the potential for biologically-produced CH₄ on Mars.