Algal and archaeal polyisoprenoids in a recent marine sediment: Molecular isotopic evidence for anaerobic oxidation of methane

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Analyses of 13C contents of individual organic molecules in a marine sediment show that crocetane, 2,6,11,15‐tetramethylhexadecane, an isomer of phytane, is produced by microorganisms that use methane as their main source of carbon. The sediments lie at a water depth of 68 m in the Kattegat, the strait between Denmark and Sweden. Crocetane appears first 185 cm below the sediment‐water interface, in the zone marking the transition from sulfate reduction to methanogenesis. Its δ13C value is −90 ± 10‰ versus Vienna Pee Dee Belemnite (VPDB). Its structure, which includes four isoprene units arranged symmetrically around a tail‐to‐tail linkage, suggests that it is produced by a member of the archaea. Growing at the intersection of the diffusion gradients for sulfate and methane in sedimentary pore waters, the source organism apparently functions as a methane‐consuming member of the microbial consortium responsible for the anaerobic oxidation of methane [Hoehler et al., 1994], in which, as first demonstrated quantitatively in these sediments [Iversen and Jørgensen, 1985], electrons are transferred from methane to sulfate. The presence of archaeal biomass throughout the sediment section is indicated by significant concentrations of 2,6,10,15,19‐pentamethylicosane (PMI) and of ether‐bound phytane and biphytane. The PMI reaches a minimum δ value of −47‰ well below the transition zone. Its isotopic depletion could reflect either methanogenic or methanotrophic sources. The ether‐bound lipids are isotopically uniform throughout the section and are presumed to derive from archaea that utilize a carbon source unaffected by the oxidation of methane.