Spatial dynamics of sulphate-reducing bacterial compositions in sediment along a salinity gradient in a UK estuary

The diversity of sulphate-reducing bacteria (SRB) was investigated in sediments along environmental gradients in the River Colne estuary, Essex, UK. DNA samples were collected from four sites; marine-dominated (Alresford Creek), brackish (the Hythe), predominantly freshwater (East Hill Bridge) and freshwater (the Weir) between September 2001 and May 2002. SRB community composition was assessed by PCR amplification, cloning and sequencing of part of the alpha subunit of dissimilatory sulphite reductase (dsrA) using directly extracted sediment DNA. The majority of the dsrA sequences were associated with members of the Desulfobacteraceae family, the Desulfobulbaceae family and a deeply branched group in the dsrA tree with no cultured representatives. There was some evidence of a salinity-related distribution within both the Desulfobacteraceae and Desulfobulbaceae groups. Clones related to Desulfotomaculum of the xenologues Firmicutes and a phylogenetically distinct Colne group 3 were detected only at the freshwater East Hill Bridge and Weir sites. Conversely, clones related to an uncultured group (Colne group 1) were found only at the marine and brackish sites. A statistical analysis of composition revealed that dsrA sequences from the marine-dominated Alresford Creek and the brackish site at the Hythe were not significantly different from each other (P>0.05), but were significantly different from those of the freshwater-dominated East Hill Bridge and the Weir (P<0.05). The sequences from East Hill Bridge and the Weir were not significantly different from each other (P>0.05). The data presented show a complex distribution of SRB along the estuary with some evidence to support the idea that salinity and sulphate concentrations are an important factor in determining SRB community structure.