Methane is a potent greenhouse gas, which can be converted by microorganism at the expense of oxygen, nitrate, nitrite, metal-oxides or sulfate. The bacterium 'Candidatus Methylomirabilis oxyfera,' a member of the NC10 phylum, is capable of nitrite-dependent anaerobic methane oxidation. Prolonged enrichment of 'Ca. M. oxyfera' with cerium added as trace element and without nitrate resulted in the shift of the dominant species. Here, we present a high quality draft genome of the new species 'Candidatus Methylomirabilis lanthanidiphila' and use comparative genomics to analyze its metabolic potential in both nitrogen and carbon cycling. To distinguish between gene content specific for the 'Ca. Methylomirabilis' genus and the NC10 phylum, the genome of a distantly related NC10 phylum member, CSP1-5, an aerobic methylotroph, is included in the analysis. All genes for the conversion of nitrite to N2 identified in 'Ca. M. oxyfera' are conserved in 'Ca. M. lanthanidiphila,' including the two putative genes for NO dismutase. In addition both species have several heme-copper oxidases potentially involved in NO and O₂ respiration. For the oxidation of methane 'Ca. Methylomirabilis' species encode a membrane bound methane monooxygenase. CSP1-5 can act as a methylotroph, but lacks the ability to activate methane. In contrast to 'Ca. M. oxyfera,' which harbors three methanol dehydrogenases (MDH), both CSP1-5 and 'Ca. M. lanthanidiphila' only encode a lanthanide-dependent XoxF-type MDH, once more underlining the importance of rare earth elements for methylotrophic bacteria. The pathways for the subsequent oxidation of formaldehyde to carbon dioxide and for the Calvin-Benson-Bassham cycle are conserved in all species. Furthermore, CSP1-5 can only interconvert nitrate and nitrite, but lacks subsequent nitrite or NO reductases. Thus, it appears that although the conversion of methanol to carbon dioxide is present in several NC10 phylum bacteria, the coupling of nitrite reduction to the oxidation of methane is a trait so far unique to the genus 'Ca. Methylomirabilis.'.

Thiosulphate dehydrogenase (EC 1.8.2.2; thiosulphate:acceptor oxidoreductase) was purified to apparent homogeneity from Thiobacillus acidophilus by a combination of ammomium sulphate precipitation, hydrophobic interaction chromatography, anion-exchange chromatography and gel filtration. The enzyme catalysed the oxidation of thiosulphate (S2O32-) to tetrathionate (S4O6-2) with potassium ferricyanide as an artificial electron acceptor. The molecular mass of the native enzyme, as determined by gel filtration, was 102 ± 4.2 kDa. The enzyme contained two different subunits with a molecular mass of 24 ± 0.9 and 20 ± 1.0 kDa (SDS-PAGE), respectively. Both subunits contained c553-type haem with absorption bands at 553, 524 and 416 nm. A 77 K spectrum of purified thiosulphate dehydrogenase revealed that the absorption at 553 nm is due to different haem groups. A cytochrome content of 5.3 mole c-type haem per mole of native enzyme was calculated. The pH optimum of the purified enzyme was 3. Apart from ferricyanide, Wurster's blue (the free radical of tetramethyl p-phenylenediamine) and horse heart cytochrome c could also serve as electron acceptors, though less effectively than ferricyanide. At pH 7.0, the K(m) for thiosulphate was 0.54 mM. The K(m) could not be determined at the pH optimum due to the chemical reactivity of thiosulphate at low pH values. Sulphite was a potent inhibitor of enzyme activity.