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Diversity and Physiology of Prokaryotes in Selected Thermophilic and Mesophilic Environments that Might Resemble Early Earth’s Biosphere (dm)

Microbial communities in hydrothermal sediments of the Guaymas Basin (Gulf of California, Mexico) were analyzed by 16S rRNA sequencing and by 13C isotopic analysis of archaeal and bacterial lipids. In this way, microbial community composition can be correlated to carbon assimilation pathways that are reflected in the isotopic composition of diagnostic lipids. The Guaymas sediments contained a major lineage of uncultured euryarchaeota (ANME-1 including ANME-1b), and uncultured members (ANME-2) of the Methanosarcinales, the acetoclastic and methyl-disproportionating methanogens. The lipid profiles were congruent with the sequence profiles. Predominant archaeal lipids in the Guaymas Basin sediments included archaeol, diagnostic for non-thermophilic euryarchaota, and sn-2-hydroxyarchaeol, diagnostic for members of the Methanosarcinales. These lipids were extremely 13C depleted (â??13C = – 80 to – 60 â?°), indicating that they originated from anaerobic methanotrophic archaea that oxidized and assimilated 13C depleted methane that is abundant in Guaymas Basin vent fluids. The archaea occur associated with diverse bacterial populations, predominantly gamma, delta and epsilon-Proteobacteria, green non-sulfur bacteria, and the uncultured candidate subdivision OP11. Bacteria that consume hydrogen and acetate, the reaction products of archaeal methane oxidation, make this process thermodynamically feasible; delta-Proteobacterial sulfate reducers are good candidates.

The discovery of a highly structured, anaerobc methane-oxidizing archaeal ecosystem in the Guaymas sediments was a surprise. Originally, we were searching for novel lineages of deeply branching archaea and bacteria, without expecting detailed results on carbon cycling. We complemented the highly suggestive sequence data with 13C-isotopic signatures from specific archaeal lipids that proved the case for anaerobic methane oxidation. Archaeal and bacterial population structure of hydrothermal sediments at the Guaymas Basin vent sites derived from 16S rRNA sequence and 13C analysis of archaeal and bacterial lipids: indications for anaerobic methanotrophy.