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Project Progress

Environmental conditions in anaerobic marine sediments may closely resemble those under which eukaryotes diverged from a prokaryote precursor 2-3 billion years ago. Yet little is known about eukaryotic microbial populations in these environments, leaving a substantial gap in our understanding of vent biota. We have initiated a study of eukaryotic microbial community composition based upon SSU rDNA sequence analyses of sediments from Guaymas Basin (Gulf of California). Temperature profiles were measured concurrently at 5-cm depth intervals, and the cores were sectioned anaerobically into 1-cm segments. We used eukaryotic-specific primers to PCR amplify rDNA genes. Sequence of the amplified products were merged with a large data base of eukaryotic ribosomal RNAs and subjected to phylogenetic analyses.
PCR amplifications were achieved for samples from 0-3 cm (60°C) depth in core A and 0-3 cm (40°C) plus a seawater interface sample from core C. We have sequenced ~45 clones from each of the 1-cm horizons from both cores. Phylogenetic analysis of horizons 0-1 from core C and 0-1 and 1-2 from core A reveal a broad array of eukaryote taxa. Many sequences cluster among known taxa: metazoa, green plants, dinoflagellates, apicomplexa, ciliates, stramenopiles (e.g., diatoms), and fungi. Several sequences could not be assigned to any known eukaryotic phyla. Several closely related clones from the 0-1 and 2-3 cm horizons from core C branch deep within the eukaryote lineage between the parabasalids and the eukaryotic crown taxa. Of particular interest among the cloned rDNAs is the deep-branching clone C1_E027. Its position between described anaerobic, early eukaryotes is evidence of a primitive, previously non-described organism. Although this sequence was obtained from a relatively cool (3-25° C) sediment sample, the spatially and thermally dynamic nature of hydrothermal vents imply that the organism might reside at even higher temperatures. We have already demonstrated a remarkable diversity of eukaryote rDNAs in hydrothermal vent sediments, but we have only scratched the surface of the potential diversity in this environment.