Molecular sequence analysis of prokaryotic diversity in the anoxic sediments underlying cyanobacterial mats of two hypersaline ponds in Mediterranean salterns

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Abstract
Small-subunit (16S) ribosomal DNA clone libraries were constructed using DNA isolated from the anoxic sediments underlying the cyanobacterial mats from two sampling stations of different salinity (Station A, 150–200‰ salinity; Station B, 250–320‰ salinity) located in the Mediterranean salterns of Salin-de-Giraud (France). Previous studies have shown that the mats at these two sites differ greatly in physicochemical and microbial composition. Sequence analysis of the clone libraries indicated that prokaryotic diversity was high in the sediments from both stations, in both the Bacteria and Archaea domains. Clones related to the α- and δ-Proteobacteria (phylum Proteobacteria), the strictly anaerobic fermentative bacteria (phylum Firmicutes), and the Cytophaga-Flavobacterium-Bacteroides (CFB) group (phylum Bacteroidetes) were found in the libraries from both sediments and accounted for the majority of Bacteria domain clones. The results indicated that the populations of δ-Proteobacteria (principally sulfate-reducing bacteria) were significantly different in the two sediments. In addition, several clones from Station A were related either to the γ-Proteobacteria (phylum Proteobacteria) or to the Spirochaeta, whereas the library from Station B contained several clones related to the uncultured, deep-branching ‘KTK group’ of Bacteria. Among the Archaea domain clones, all from Station B and the majority from Station A were related to the order Halobacteriales (phylum Euryarchaeota, class Halobacteria). In addition, 12% of the Archaea domain clones from Station A were related to the Methanococci group (phylum Euryarchaeota, class Methanobacteria) and 32% to the phylum Crenarchaeota. This study represents the first molecular analysis of the diversity of halophilic prokaryotes present in these sediments and the results are discussed in the light of our current knowledge of the microbial ecology of these hypersaline ecosystems.

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