In vitro study of lipid biosynthesis in an anaerobically methane-oxidizing microbial mat

The anaerobic oxidation of methane (AOM) is a key process in the global methane cycle, and the majority of methane formed in marine sediments is oxidized in this way. Here we present results of an in vitro (CH4)-C-13 labeling study (delta(CH4)-C-13, similar to 5,400 parts per thousand) in which microorganisms that perform AOM in a microbial mat from the Black Sea were used. During 316 days of incubation, the C-13 uptake into the mat biomass increased steadily, and there were remarkable differences for individual bacterial and archaeal lipid compounds. The greatest shifts were observed for bacterial fatty acids (e.g., hexadec-11-enoic acid [161 Delta 11]; difference between the delta C-13 at the start and the end of the experiment [Delta delta C-13(start-end)], similar to 160 parts per thousand). In contrast, bacterial glycerol diethers exhibited only slight changes in delta C-13 (Delta delta C-13(start-end), similar to 10 parts per thousand). Differences were also found for individual archaeal lipids. Relatively high uptake of methane-derived carbon was observed for archaeol (Delta delta C-13(start-end), similar to 25 parts per thousand), a monounsaturated archaeol, and biphytanes, whereas for sn-2-hydroxyarchaeol there was considerably less change in the delta C-13 (Delta delta C-13(start-end), similar to 2 parts per thousand). Moreover, an increase in the uptake of C-13 for compounds with a higher number of double bonds within a suite of polyunsaturated 2,6,10,15,19-pentamethyleicosenes indicated that in methanotrophic archaea there is a biosynthetic pathway similar to that proposed for methanogenic archaea. The presence of group-specific biomarkers (for ANME-1 and ANME-2 associations) and the observation that there were differences in C-13 uptake into specific lipid compounds confirmed that multiple phylogenetically distinct microorganisms participate to various extents in biomass formation linked to AOM. However, the greater C-13 uptake into the lipids of the sulfate-reducing bacteria (SRB) than into the lipids of archaea supports the hypothesis that there is autotrophic growth of SRB on small methane-derived carbon compounds supplied by the methane oxidizers.