Molecular and isotopic analysis of anaerobic methane-oxidizing communitiesin marine sediments

Convergent lines of molecular, carbon-isotopic, and phylogenetic evidence h ave previously indicated (Hinrichs, K.U., Hayes, J.M., Sylva, S.P., Brewer. P.G.. DeLong, E.F., 1999. Methane-consuming archaebacteria in marine sedim ents. Nature 398, 802-805.) that archaea are involved in the anaerobic oxid ation of methane in sediments from the Eel River Basin. offshore northern C alifornia. Now, further studies of those same sediments and of sediments fr om a methane seep in the Santa Barbara Basin have confirmed and extended th ose results. Mass spectrometric and chromatographic analyses of an authenti c standard of sn-2-hydroxyarchaeol (hydroxylated at C-3 in the sn-2 phytany l moiety) have confirmed our previous, tentative identification of this com pound but shown that the previously examined product was the mono-TMS, rath er than di-TMS, derivative. Further analyses of C-13-depleted lipids, appre ciably more abundant in samples from the Santa Barbara Basin, have shown th at the archaeal lipids are accompanied by two sets of products that are onl y slightly less depleted in C-13. These are additional glycerol ethers and fatty acids. The alkyl substituents in the ethers (mostly monoethers, with some diethers) are non-isoprenoidal. The carbon-number distributions and is otopic compositions of the alkyl substituents and of the fatty acids are si milar, suggesting strongly that they are produced by the same organisms. Th eir structures, n-alkyl and methyl-branched n-alkyl, require a bacterial ra ther than archaeal source. The non-isoprenoidal glycerol ethers are novel c onstituents in marine sediments but have been previously reported in thermo philic, sulfate- and nitrate-reducing organisms which lie near the base of the rRNA-based phylogenetic tree. Based on previous observations that the a naerobic oxidation of methane involves a net transfer of electrons from met hane to sulfate, it appears likely that the non-archaeal, C-13-depleted lip ids are products of one or more previously unknown sulfate-reducing bacteri a which grow syntrophically with the methane-utilizing archaea. Their produ cts account for 50% of the fatty acids in the sample from the Santa Barbara Basin. At all methane-seep sites examined, the preservation of aquatic pro ducts is apparently enhanced because the methane-oxidizing consortium utili zes much of the sulfate that would otherwise be available for remineralizat ion of materials from the water column. Crown Copyright (C) 2000 Published by Elsevier Science Ltd. All rights reserved.