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Project Progress

Guaymas Project: The novel, deeply branching dsrAB genes found earlier during the Guaymas project were included in a phylogenetic analysis of dissimilatory and assimilatory sulfite reductases, based on a comprehensive dataset of both enzymes. The phylogeny was based on the highly conserved active centers only, to avoid problems with insufficient full-length homology of these diversified enzymes. Assimilatory and dissimilatory enzymes diverged into two major phylogenetic domains that corresponded to assimilatory and dissimilatory function. Within the assimilatory domain, the phylogeny supports the hypothesis that a small, monomeric reductase was the ancestral form which underwent gene duplication and modification of subunits. Within the dissimilatory domain, the ancestral gene duplication into dsrA and dsrB genes appears to have predated the bacteria/archaea divergence; as a corollary, the split into assimilatory and dissimilatory sulfite reductases must have predated this divergence (Dhillon, Goswami, Riley, Teske, and Sogin, 2004).

We have continued the Guaymas key gene survey with a key gene for methanogenesis, mcrA, which codes for the alpha subunit of methyl-Coenzyme M reductase, the last enzyme in diverse methanogenic pathways. So far, we have identified mcrA genes of major methanogenic families (Methanosarcinales, Methanomicrobiales, Methanococcales) in the Guaymas sediments. The results in detail indicate a diversified, partially thermophilic methanogen community, with several uncultured lineages among the Methanosarcinales and Methanomicrobiales. Analogous to the previous dissimilatory sulfite reductase survey, we are working towards a more comprehensive view of the phylogenetic depth and divergence pattern of methanogenic terminal anaerobic degraders (Dhillon, Lever, Lloyd, Sogin, and Teske, 2004).

Deep Subsurface Project: In collaboration with the University of Rhode Island (URI) "Subsurface Biosphere" NAI group, the phylogenetic analysis of deep marine subsurface sediments is continued with emphasis on organic-poor, low activity sites (Ocean Drilling Program (ODP) site 1225, 1231 in the Equatorial Pacific and Peru Basin). Here, microbial communities persist in low-activity sediments that show very little geochemical evidence of the predominant marine anaerobic remineralization pathways, sulfate reduction and methanogenesis. So far, the archaeal community in these subsurface sites overlaps with major archaeal lineages found in marine sediments (Benthic marine crenarchaeotal Groups A and B) and with archaea dominant in the deep-sea water column (Pelagic Group I). Several newly found euryarchaeotal and crenarchaeotal lineages at Site 1231 have their closest relatives in hydrothermal vent sites, indicating shared populations between the hydrothermal and the non-hydrothermal subsurface (Sørensen, Lauer, and Teske, 2004).

As a collaborative project with ODP Leg 201 participants, organic-rich sediments of the Peruvian continental shelf (site 1227) and the Peru Trench (site 1230) are surveyed; interestingly, one of the dominant archaeal groups is the Benthic marine crenarchaeotal group B (synonymous with DSAG, Deep Sea Archaeal Group) (Inagaki et al. 2004). This ongoing project attempts a comprehensive inventory of deep-surface microbial lineages, and their occurrence range in the marine subsurface and other habitats.