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Project Progress

We concluded our analysis of dissimilatory sulfite reductase genes, a key gene of sulfate reduction, in the Guaymas Basin hydrothermal vents. We have investigated the Guaymas Basin hydrothermal vents as a model system to study anaerobic microbial communities (methane oxidizers, and sulfate reducers) that catalyze a sulfate-and methane-driven anaerobic carbon cycle that is compatible with stable carbon and sulfur isotope evidence for early Earth ecosystems (Teske et al. 2003). A new phylotype of dissimilatory sulfite reductase from the Guaymas vents formed one of the deepest branches in the DSRtree, implying the existence of ancestral sulfate-reducing prokaryotes in the Guaymas Basin. These sulfate reducers have not been cultured, and would have remained undetected by 16S ribosomal ribonucleic acid (rRNA) sequencing (Dhillon et al. 2003). A comprehensive phylogenetic analysis of the dissimilatory sulfite reductase gene family (with assimilatory as well as dissimilatory enzymes, and the new Guaymas phylotypes) and a study of reaction center conservation of these enzymes is in progress, to gain insight into the evolution of sulfate- and sulfite assimilation and respiration (Dhillon, Riley et al., in preparation).

In collaboration with Kai-Uwe Hinrichs and Helen Sturt (WHOI), a lipid biomarker catalogue for sulfate-reducing bacteria and archaea is in development. We intend to complement ribosomal and functional genes with a similarly detailed database of biomarker molecules, for detection and identification of the full phylogenetic range of sulfate-reducing microorganisms in different environments (Sturt et al. 2003).

We also completed an analysis of 16S rDNA genes in the Forearc Basin of the Nankai Trough. The prokaryotic community of the deep subsurface sediments in the Forearc Basin of the Nankai Trough southeast of Japan (ODP site 1176A) was analyzed by 16S rDNA sequencing. Sediment samples from 1.15, 51.05, 98.50 and 193.96 meters below sea floor (mbsf) harbored highly diverse bacterial communities. The most frequently retrieved clones included members of the Green-non sulfur bacteria whose closest relatives come from deep subsurface environments, a new epsilon-Proteobacterial phylotype, and representatives of a cluster of closely related bacterial sequences from hydrocarbon- and methane-rich sediments around the world. Archaeal clones were limited to members of the genus Thermococcus, and were only obtained from the two deepest samples.