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Project Progress

The 16S rRNA sequencing survey of the Guaymas Basin and the C13-isotopic analysis of taxonomically informative prokaryotic lipids has been completed as described in the year 3 report, and the results were published in the April 2002 issue of Applied and Environmental Microbiology (see publications).

Briefly, lipid analysis and 16S rRNA sequencing showed that the archaeal community in the Guaymas basin hydrothermal vent sediments was dominated by anaerobic methanotrophic archaea (ANME-1 and ANME-2 lineages). These anaerobic methanotrophs, in combination with sulfate-reducing syntrophs and methanogenic archaea, constitute an anaerobic microbial ecosystem that produces and oxidizes methane, and thus drives an oxygen-independent carbon cycle.

This microbial ecosystem is studied further by extending the 16S rRNA survey using the SAR technique of sequencing concatenated 16S rRNA gene fragments for greater throughput (D. Kysela, C. Palacios, M.L. Sogin). The 16S rRNA data are complemented by a sequencing survey of the taxonomically informative key gene of sulfate reduction, dissimilatory sulfite reductase (A. Dhillon). Sulfate is the terminal electron acceptor of anaerobic methane oxidation, and sulfate-reducing prokaryotes are essential community members of the Guaymas ecosystem. So far, we have found at least three major phylogenetic groups of DSR genes in Guaymas, one group belonging to the Desulfobacteriaceae, a family of cultured sulfate-reducing bacteria that oxidize acetate and other substrates completely to CO₂, and at least two major phylogenetic branches of uncultured sulfate-reducing bacteria. These novel, near-complete DSR sequences will be checked for phylogenetic conservation of functional and structural motives, and searched for ancestral enzyme versions using paralogous gene trees based on alpha and beta subunits of the dsrAB gene. Detecting ancestral sulfate-reducing bacteria as part of methane-oxidizing microbial ecosystems would confirm the antiquity and early evolutionary emergence of anaerobic methane cycling.