Michael A. Tanner, Christine L. Everett, William J. Coleman, Mary M. Yang and Douglas C. Youvan

We have used 16S rRNA phylogenetic analysis to investigate the microbial diversity of sulfur-rich black mud marine sediments. Two distinct environments were examined: mud from a hypersaline marsh and mud from a brackish marsh. Only two out of one hundred cloned 16S rDNA sequences were identical, indicating a very complex community of microbial species. Both environments exhibited significant sulfate-reducing d -Proteobacteria, indicating that these organisms are likely the primary source of reduced forms of sulfate required by other species. Since seawater has abundant quantities of sulfate, microorganisms that reduce this molecule are important for the cycling of sulfur in the global ecosystem. PCR of the 16S rDNAs extracted directly from the mud without culturing revealed that most of the population was less than 95% identical to known 16S rDNAs, and eight of the one hundred sequences analyzed were only 80-85% identical to known sequences. Four sequences from this more divergent group were identified as new candidate divisions. Although sulfate-reducing bacteria dominated both ecosystems, the hypersaline black mud also harbored organisms related to known halophiles. A search for Archaea uncovered a diverse collection of halophilic species that showed taxonomic affiliation with the Halobacteriales. From our sampling of 100 rDNA clones, we estimate that the diversity of organisms in these black mud samples is approximately 10,000 species – greater than the approximately 6000 Archaea and Bacteria currently recognized by culture studies. Additionally, we note a lack of predominant species in our sample. This suggests a modestly ambitious DNA sequencing effort of rDNAs from black mud would yield thousands of new species and many new candidate divisions.