High overall diversity and dominance of microdiverse relationships in salt marsh sulphate-reducing bacteria

The biogeochemistry of North Atlantic salt marshes is characterized by the interplay between the marsh grass Spartina and sulphate-reducing bacteria (SRB), which mineralize the diverse carbon substrates provided by the plants. It was hypothesized that SRB populations display high diversity within the sediment as a result of the rich spatial and chemical structuring provided by Spartina roots. A 2000-member 16S rRNA gene library, prepared with delta-proteobacterial SRB-selective primers, was analysed for diversity patterns and phylogenetic relationships. Sequence clustering detected 348 16S rRNA sequence types (ribotypes) related to delta-proteobacterial SRB, and it was estimated that a total of 623 ribotypes were present in the library. Similarity clustering showed that approximately 46% of these sequences fell into groups with < 1% divergence; thus, microheterogeneity accounts for a large portion of the observable genetic diversity. Phylogenetic comparison revealed that sequences most frequently recovered were associated with the Desulfobacteriaceae and Desulfobulbaceae families. Sequences from the Desulfovibrionaceae family were also observed, but were infrequent. Over 80% of the delta-proteobacterial ribotypes clustered with cultured representatives of Desulfosarcina, Desulfococcus and Desulfobacterium genera, suggesting that complete oxidizers with high substrate versatility dominate. The large-scale approach demonstrates the co-existence of numerous SRB-like sequences and reveals an unexpected amount of microdiversity.