Methanogenesis may have been one of the oldest biological processes on Earth and probably played a major role in the evolution of Earth’s atmosphere. Earth evolved from an ancient anoxic atmosphere to an oxygenated one (the so-called ‘Great Oxidation Event’) about 2.4 billion years ago. Although methanogenesis does not directly produce free O2, it may have helped make Earth habitable for aerobic life. Contemporary methanogens are microorganisms that form methane as a major product of their energy metabolism. They share several features:
- They are all Archaea and members of the Euryarchaeota phylum
- They are obligate CH4-producers and obtain all or most of their energy for growth from CH4 production
- They are obligate anaerobes and are restricted to strictly anoxic environments
- They utilize a limited number of substrates for methanogenesis, mostly H2 + CO2 or formate (hydrogenotrophic methanogens), methyl-containing C–1 compounds (methylotrophic methanogens) and acetate (aceticlastic methanogens)
Sulfur is an essential element required for the growth of all known organisms and is present in a wide variety of molecules with distinctive physiological functions. Although genomic sequences of methanogens provide a general idea about their metabolism, many essential enzymes of established sulfur assimilation pathways are missing. Recent studies indicated the presence of a tRNA-dependent pathway for Cys biosynthesis in methanogens, but how sulfur is incorporated into Cys is still unknown. Moreover, the biosynthesis of other sulfur compounds, e.g. Fe–S clusters and Met, remains to be elucidated. Box 2 lists questions that address a few of the many possible avenues for future research. Given the antiquity of methanogenesis and the obligately anaerobic lifestyle of methanogens, studying the sulfur metabolism of methanogens may lead to a better understanding of ancient sulfur metabolism:
Methanogens: a window into ancient sulfur metabolism. Trends Microbiol. 09 Mar 2012
Methanogenesis is an ancient metabolism that originated on the early anoxic Earth. The buildup of O(2) about 2.4 billion years ago led to formation of a large oceanic sulfate pool, the onset of widespread sulfate reduction and the marginalization of methanogens to anoxic and sulfate-poor niches. Contemporary methanogens are restricted to anaerobic habitats and may have retained some metabolic relics that were common in early anaerobic life. Consistent with this hypothesis, methanogens do not utilize sulfate as a sulfur source, Cys is not utilized as a sulfur donor for Fe-S cluster and Met biosynthesis, and Cys biosynthesis uses an unusual tRNA-dependent pathway.