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

Analyses of C-13 contents of individual organic molecules in a marine sedim ent show that crocetane, 2,6,11,15-tetramethylhexadecane, an isomer of phyt ane, 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 st rait between Denmark and Sweden. Crocetane appears first 185 cm below the s ediment-water interface, in the zone marking the transition from sulfate re duction to methanogenesis. Its delta C-13 value is -90 +/- 10 parts per tho usand 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 int ersection of the diffusion gradients for sulfate and methane in sedimentary pore waters, the source organism apparently function as a methane-consumin g member of the microbial consortium responsible for the anaerobic oxidatio n of methane [Hoehler et al., 1994], in which, as first demonstrated quanti tatively in these sediments [Iversen and Jorgensen, 1985], electrons are tr ansferred from methane to sulfate. The presence of archaeal biomass through out 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 delta value of -47 parts per thousand well below the transition zone. Its isotopic depletion could reflect either methanoge nic or methanotrophic sources. The ether-bound lipids are isotopically unif orm throughout the section and are presumed to derive from archaea that uti lize a carbon source unaffected by the oxidation of methane.