Methane production and oxidation—A review on the pmoA and mcrA genes abundance for understanding the functional potentials of the agricultural soil

Global efforts to avert climate change cannot be successfully addressed without tackling the emission of methane from soil and other ecosystems. Methane is a greenhouse gas that retains heat in the atmosphere and causes global warming. Its production is the last step of organic matter decomposition and is produced by methanogenic archaea bearing the functional gene – mcrA (Methyl-coenzyme M reductase). Methane production involves the reduction of acetate or carbon dioxide in a microaerophilic or anaerobic environment under the catalytic actions of methyl coenzyme M to generate methane. On the other hand, methane oxidizing bacteria, also known as methanotrophs, through the catalytic actions of the pmoA gene (particulate methane monooxygenase), oxidize methane and reduce its emission to the atmosphere. In essence, production and consumption of methane both happen within the soil. Methanotrophs and methanogenic microbes inhabit the same soil environment. In fact, a shift in the balance between methanogens and methanotrophs activities and abundance could influence either increase in soil methane emission and control or the corresponding effects on global warming. In this review we highlight recent advances on the drivers of methane flux, pmoA and mcrA genes abundance, methane emission and control, linking microbial functional genes abundance to soil function, and examining methods for studying the abundance of pmoA and mcrA genes in the soil. We also highlight gaps that need to be filled and the impact of the mcrA to pmoA gene ratio in driving the methane emission rate in the soil. We also discussed the various abiotic factors that control pmoA and mcrA gene abundance