Facultative methanotrophs and environmental regulation of methane oxidation

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The concentration of methane in the atmosphere is currently 1.8 ppm and about 20% of global warming has been attributed to this potent greenhouse gas. It is estimated that 400-640 Tg of methane is produced annually by methanogenic bacteria in anaerobic soils and sediments much of which is oxidised at the aerobic-anaerobic interface in wetlands or overlying aerobic soils before it is released to the atmosphere, thereby mitigating the global emissions of methane into the atmosphere. Wetlands and soils are therefore important sinks for methane, consuming an estimated 27 and 40 Tg CH4 per year respectively. This is carried out by aerobic methane oxidising bacteria. Bacteria which grow on methane as sole carbon source have been obtained from many environments. Cultivation-independent techniques indicate that there are more novel methanotrophs in the environment that await isolation. Generally, methanotrophs have been regarded as obligate, ie only growing on methane (or methanol). Recent exciting discoveries of facultative methanotrophs which grow on both methane and multi-carbon compounds such as acetate now make it imperative that we develop a thorough understanding of how these newly discovered methanotrophs, which appear to be widespread in wetlands and soils, regulate their methane oxidation systems and how they contribute to the global methane cycle. We propose to study the environmental regulation of methane oxidation in a model facultative methanotroph Methylocella silvestris. Another exciting breakthrough is that we will soon have the genome sequence of Methylocella silvestris available to us which provides a blueprint for the metabolic pathways in this bacterium and enables us to elucidate the pathways of facultative metabolism. We have developed a number of physiological and molecular biological techniques to enable us to study this bacterium in cultures in the laboratory and to investigate pathways of acetate utilisation and to determine what environmental factors regulate metabolism in this bacterium. We have also developed a number of cutting edge molecular ecology tools which enable us to parallel the laboratory culture experiments with microcosm experiments which will address the role of these facultative methanotrophs in peat and landfill soils in order to address the hypothesis that in certain growth environments facultative methanotrophs have a competitive edge over obligate methanotrophs and that they make a significant contribution to the cycling of methane in the environment.