Environmental determinants of soil methane oxidation and methanotrophs

Methane (CH4) is one of the strongest greenhouse gases. Sources of CH4 are anthropogenic and natural, former playing ∼60% role. Major sink for CH4 are the atmospheric OH and Cl radicals (originating from CFCs), and biological system. Biological CH4 sink is mediated through the CH4 oxidation by the specialized group of bacteria called methanotrophs (MB). Methanotrophs have been reported from almost all the soil systems such as sediments, oceans, extremes of pH, salinity, and temperature. They oxidize methane aerobically in the presence of the enzyme methane monooxygenase (MMO). Anaerobic methane oxidation (AOM) also occurs in marine ecosystem where sulfate is final electron acceptor. Methanotrophs are of two types, first is cultured and low affinity group while the second is uncultured and high affinity group. Most of them can be grouped as Type I and Type II belonging to γ- and α-Proteobacteria, respectively. They may constitute up to 2% of total bacterial population in soil depending on physical factors such as water, temperature, soil depth, pH, texture, gaseous atmosphere (methane, oxygen, and CO2), soil organic content, and biological factors such as vegetation and microbial consortia. Besides, anthropogenic factors such as fertilizers, agro- and organochemicals, and land use pattern have strong influence over them. Global climate change including acid rain, high temperature, increasing rainfall, and drought have potential to affect the global methane sink activity. The authors attempt to review the recent advances made regarding CH4 oxidation and methanotrophic population size as well as community structure as affected by the various natural and anthropogenic factors.