In Vitro Study of Lipid Biosynthesis in an Anaerobically Methane-Oxidizing Microbial Mat

Abstract

ABSTRACT 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 13 CH 4 labeling study (δ 13 CH 4 , ∼5,400‰) in which microorganisms that perform AOM in a microbial mat from the Black Sea were used. During 316 days of incubation, the 13 C 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 [16:1Δ11]; difference between the δ 13 C at the start and the end of the experiment [Δδ 13 C start-end ], ∼160‰). In contrast, bacterial glycerol diethers exhibited only slight changes in δ 13 C (Δδ 13 C start-end , ∼10‰). Differences were also found for individual archaeal lipids. Relatively high uptake of methane-derived carbon was observed for archaeol (Δδ 13 C start-end , ∼25‰), a monounsaturated archaeol, and biphytanes, whereas for sn -2-hydroxyarchaeol there was considerably less change in the δ 13 C (Δδ 13 C start-end , ∼2‰). Moreover, an increase in the uptake of 13 C 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 13 C uptake into specific lipid compounds confirmed that multiple phylogenetically distinct microorganisms participate to various extents in biomass formation linked to AOM. However, the greater 13 C 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.