Abstract
Methane-oxidizing bacteria (methanotrophs) play a vital role in reducing atmospheric methane emissions, and hence mitigating their potent global warming effects. A significant proportion of the methane released is thermogenic natural gas, containing associated short-chain alkanes as well as methane. It was one hundred years following the description of methanotrophs that facultative strains were discovered and validly described. These can use some multi-carbon compounds in addition to methane, often small organic acids, such as acetate, or ethanol, although Methylocella strains can also use short-chain alkanes, presumably deriving a competitive advantage from this metabolic versatility. Here, we review the diversity and molecular ecology of facultative methanotrophs. We discuss the genetic potential of the known strains and outline the consequent benefits they may obtain. Finally, we review the biotechnological promise of these fascinating microbes.
Highlights
Methane-o xidizing bacteria play a vital role in reducing atmospheric methane emissions, and mitigating their potent global warming effects
The initial oxidation of short-chain alkanes is usually catalysed by a monooxygenase, frequently an soluble diiron centre monooxygenases (SDIMOs) related to the group 3 methane monooxygenases, but which is instead from group 5 or group 6 of the SDIMO family [21], the butane monooxygenase of Thauera butanivorans is more closely related to the sMMO [22]
It is interesting to note that whereas all the facultative strains contain a number of relatively uncharacterized membrane transporters, which possibly allow organic acids to enter the cell, only the Methylocella strains possess close homologues of actP, an acetate-specific permease (67–71% amino acid identity with ActP characterized in Escherichia coli [94] and 68–70% identity with META1p2533 from Methylorubrum extorquens AM1 [95, 96])
Summary
The most abundant hydrocarbon in the atmosphere and a potent greenhouse gas, is one of the most significant contributors to climate change. The natural sources include macro- and micro-seeps, mud volcanoes, geothermal areas, volcanoes and submarine seeps (33–75 Tg y−1) [1] This thermogenic ‘natural gas’ contains substantial amounts of other climate-active gases, mainly ethane (a photochemical pollutant) and propane (an ozone precursor), 2–4 and 1–2.4 Tg y−1 from natural sources, respectively [5, 6]. The initial oxidation of short-chain alkanes is usually catalysed by a monooxygenase, frequently an SDIMO related to the group 3 methane monooxygenases, but which is instead from group 5 or group 6 of the SDIMO family [21], the butane monooxygenase of Thauera butanivorans is more closely related to the sMMO [22] These microbes are metabolically versatile compared to methanotrophs, and generally grow on a range of multicarbon compounds, but not methane [20]. With the discovery of more strains including methanotrophs that assimilate carbon autotrophically via the Calvin–Benson–Bassham (CBB) cycle [24, 25], these categories had to be adjusted and additional subdivisions were added, making this distinction less clearcut [26]
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