Methanotrophy

Methanotrophy is the biological oxidation of methane as an energy source and carbon substrate. Methanotrophs are aerobic bacteria that metabolize methane in oxic environments, including soils, wetlands, sediments, and aquatic systems, thereby acting as a major sink for methane, a potent greenhouse gas.

Metabolism and pathways: The initial step is the oxidation of methane to methanol by methane monooxygenase enzymes. Most methanotrophs use either particulate methane monooxygenase (pMMO) or, under copper limitation, soluble methane monooxygenase (sMMO). Methanol is further oxidized to formaldehyde and then to formate and CO2. Formaldehyde can be assimilated into biomass via two main carbon assimilation routes: the ribulose monophosphate (RuMP) cycle used by Type I gammaproteobacteria, and the serine cycle used by Type II alphaproteobacteria. These pathways provide both energy and carbon skeletons for growth.

Ecology and diversity: Methanotrophs are common in methane-rich habitats such as rice paddies, wetlands, peatlands, lakes, and landfill cover soils. They are influenced by methane availability, oxygen, copper levels (the copper switch between sMMO and pMMO), pH, and nitrogen compounds. Through methane consumption, they play a crucial role in mitigating methane emissions to the atmosphere.

Anaerobic methane oxidation: In marine and some freshwater sediments, anaerobic oxidation of methane (AOM) occurs in consortia of methane-oxidizing archaea (ANME) with sulfate-reducing bacteria, using sulfate or other electron acceptors such as nitrate or metal oxides. This represents an oxygen-independent form of methanotrophy.

Applications and significance: Understanding methanotrophy informs greenhouse gas mitigation and supports biotechnological approaches to convert methane into biomass or other useful products in bioreactors and treatment systems.

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