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Hopanoids are lipids that are widespread in the bacterial domain and well established molecular biomarkers in modern and paleo environments. In particular, the occurrence of 13C-depleted 3-methylated hopanoids are characteristic of aerobic bacteria involved in methane oxidation. Previously the intra-aerobic methanotroph ‘Candidatus Methylomirabilis oxyfera’ (‘Ca. M. oxyfera’), which performs nitrite-dependent methane oxidation in anoxic environments, has been shown to synthesize bacteriohopanepolyols (BHPs) and their 3-methylated counterparts. However, since ‘Ca. M. oxyfera’ does not utilize methane as a carbon source, its biomass and lipids do not show the characteristic 13C-depletion. Therefore, the detection of ‘Ca. M. oxyfera’ in various environments is challenging, and still underexplored. Here, we re-investigated the hopanoid content of ‘Ca. M. oxyfera’ bacteria using enrichment cultures. We found the GC-amenable hopanoids of ‘Ca. M. oxyfera’ to be dominated by four demethylated hopanoids of which only one, 22,29,30-trisnorhopan-21-one, had been identified previously. The three novel hopanoids were tentatively identified as 22,29,30-trisnorhopan-21-ol, 3-methyl-22,29,30-trisnorhopan-21-one and 3-methyl-22,29,30-trisnorhopan-21-ol. These unique demethylated hopanoids are most likely biosynthesized directly by ‘Ca. M. oxyfera’ bacteria. Bioinformatical analysis of the ‘Ca. M. oxyfera’ genome revealed potential candidate genes responsible for the demethylation of hopanoids. For the sensitive detection of the four trisnorhopanoid biomarkers in environmental samples, a multiple reaction monitoring (MRM) method was developed and used to successfully detect the trisnorhopanoids in a peatland where the presence of ‘Ca. M. oxyfera’ had been confirmed previously by DNA-based analyses. These new biomarkers may be a novel tool to trace nitrite-dependent methane oxidation in various (past) environments.

The anaerobic oxidation of methane (AOM) is a microbial process present in marine and freshwater environments. AOM is important for reducing the emission of the second most important greenhouse gas methane. In marine environments anaerobic methanotrophic archaea (ANME) are involved in sulfate-reducing AOM. In contrast, Ca. Methanoperedens of the ANME-2d cluster carries out nitrate AOM in freshwater ecosystems. Despite the importance of those organisms for AOM in non-marine environments little is known about their lipid composition or carbon sources. To close this gap, we analysed the lipid composition of ANME-2d archaea and found that they mainly synthesise archaeol and hydroxyarchaeol as well as different (hydroxy-) glycerol dialkyl glycerol tetraethers, albeit in much lower amounts. Abundant lipid headgroups were dihexose, monomethyl-phosphatidyl ethanolamine and phosphatidyl hexose. Moreover, a monopentose was detected as a lipid headgroup that is rare among microorganisms. Batch incubations with 13C labelled bicarbonate and methane showed that methane is the main carbon source of ANME-2d archaea varying from ANME-1 archaea that primarily assimilate dissolved inorganic carbon (DIC). ANME-2d archaea also assimilate DIC, but to a lower extent than methane. The lipid characterisation and analysis of the carbon source of Ca. Methanoperedens facilitates distinction between ANME-2d and other ANMEs.