methanogens

Methanogens are a group of strictly anaerobic archaea that produce methane as a metabolic byproduct. They inhabit anaerobic environments where oxygen is absent or scarce and play a key role in global carbon cycling.

Methanogens perform methanogenesis, a specialized form of metabolism. They can use carbon dioxide and hydrogen (CO2 + 4 H2 → CH4 + 2 H2O) (hydrogenotrophic), acetate (acetoclastic: CH3COO− + H+ → CH4 + CO2), or methylated compounds such as methanol or methylamines (methylotrophic). These reactions require unique coenzymes, notably coenzyme M (CoM) and coenzyme B (CoB). The final step is catalyzed by methyl-coenzyme M reductase, producing methane.

They are found in a wide range of habitats, including wetlands (rice paddies, marshes), the guts of ruminants and termites, sediments (marine and freshwater), hot springs, hydrothermal vents, and anaerobic digesters used in wastewater treatment. They are obligate anaerobes, and oxygen can be toxic to them.

Ecology and significance: Methanogens are a central source of methane in natural ecosystems and contribute to atmospheric methane, a potent greenhouse gas. In the gut, they help maintain hydrogen balance and enable fermentation by other microbes; in industry, methanogens drive biogas production in anaerobic digesters.

Taxonomy and evolution: Methanogens are Archaea, primarily within the phylum Euryarchaeota, including orders Methanobacteriales, Methanococcales, Methanomicrobiales, and Methanosarcinales. They are considered among the most ancient organisms, with methanogenesis thought to be one of the oldest metabolic pathways.