The recently described bacterium "Candidatus Methylomirabilis oxyfera" couples the oxidation of the important greenhouse gas methane to the reduction of nitrite. The ecological significance of "Ca. Methylomirabilis oxyfera" is still underexplored, as our ability to identify the presence of this bacterium is thus far limited to DNA-based techniques. Here, we investigated the lipid composition of "Ca. Methylomirabilis oxyfera" to identify new, gene-independent biomarkers for the environmental detection of this bacterium. Multiple "Ca. Methylomirabilis oxyfera" enrichment cultures were investigated. In all cultures, the lipid profile was dominated up to 46% by the fatty acid (FA) 10-methylhexadecanoic acid (10MeC(16:0)). Furthermore, a unique FA was identified that has not been reported elsewhere: the monounsaturated 10-methylhexadecenoic acid with a double bond at the Delta 7 position (10MeC(16:1 Delta 7)), which comprised up to 10% of the total FA profile. We propose that the typical branched fatty acids 10MeC(16:0) and 10MeC(16:1 Delta 7) are key and characteristic components of the lipid profile of "Ca. Methylomirabilis oxyfera." The successful detection of these fatty acids in a peatland from which one of the enrichment cultures originated supports the potential of these unique lipids as biomarkers for the process of nitrite-dependent methane oxidation in the environment.
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