Terrestrial and oceanic geothermal areas emit substantial amounts of hydrocarbons in the form of methane and the short-chain alkanes ethane and propane. Under hydrothermal conditions, these alkanes can also be oxidised to their respective alcohols and ketones, with a preference for the 2-position. The thermoacidophilic verrucomicrobial methanotroph Methylacidiphilum fumariolicum SolV, isolated from the Solfatara volcano, was previously shown to oxidise methane as well as the short-chain hydrocarbons propane and ethane. Here, we show the growth of strain SolV on the C3 compounds 2-propanol and acetone with growth rates of 0.054 h⁻¹ and 0.042 h⁻¹, respectively. In contrast to methanotrophic growth (rate 0.07 h⁻¹), growth was not dependent on CO₂ or lanthanides. Respiration experiments on steady-state continuous cultures showed an apparent affinity of 0.4 μM acetone and 5.4 μM 2-propanol. Transcriptomic analysis of these cultures showed that a gene cluster including a novel acetone monooxygenase (PMO3), previously identified in the closely related species Methylacidiphilum caldifontis, was highly upregulated under growth on C3 substrates. These results support the versatile metabolism of verrucomicrobial methanotrophs. The conversion of other compounds besides methane can be important in view of the ecological relevance of methanotrophs.

The trace amounts (0.53 ppmv) of atmospheric hydrogen gas (H2) can be utilized by microorganisms to persist during dormancy. This process is catalyzed by certain Actinobacteria, Acidobacteria, and Chloroflexi, and is estimated to convert 75 × 1012 g H2 annually, which is half of the total atmospheric H2. This rapid atmospheric H2 turnover is hypothesized to be catalyzed by high-affinity [NiFe] hydrogenases. However, apparent high-affinity H2 oxidation has only been shown in whole cells, rather than for the purified enzyme. Here, we show that the membrane-associated hydrogenase from the thermoacidophilic methanotroph Methylacidiphilum fumariolicum SolV possesses a high apparent affinity (Km(app) = 140 nM) for H2 and that methanotrophs can oxidize subatmospheric H2. Our findings add to the evidence that the group 1h [NiFe] hydrogenase is accountable for atmospheric H2 oxidation and that it therefore could be a strong controlling factor in the global H2 cycle. We show that the isolated enzyme possesses a lower affinity (Km = 300 nM) for H2 than the membrane-associated enzyme. Hence, the membrane association seems essential for a high affinity for H2. The enzyme is extremely thermostable and remains folded up to 95 °C. Strain SolV is the only known organism in which the group 1h [NiFe] hydrogenase is responsible for rapid growth on H2 as sole energy source as well as oxidation of subatmospheric H2. The ability to conserve energy from H2 could increase fitness of verrucomicrobial methanotrophs in geothermal ecosystems with varying CH4 fluxes. We propose that H2 oxidation can enhance growth of methanotrophs in aerated methane-driven ecosystems. Group 1h [NiFe] hydrogenases could therefore contribute to mitigation of global warming, since CH4 is an important and extremely potent greenhouse gas.