Sulphate reduction, methanogenesis and phylogenetics of the sulphate reducing bacterial communities along an estuarine gradient

Four sites were investigated along the salinity gradient in the estuary of the River Colne, UK. Sulphate reduction rates at all sites decreased with depth in the sediment, but the annual rate of sulphate reduction over the 0 to 20 cm profile did not decrease significantly, even at a low salinity site up the estuary where sulphate concentrations were much lower than at the estuary mouth. Methane was formed at all of the sites, at rates 2 orders of magnitude lower than sulphate reduction, and was greatest at a site on the marsh top site where organic matter was high. Methane emissions to the atmosphere were substantially lower than sedimentary methanogenesis, suggesting methane oxida- tion occurred in the sediments before emission, with the proportion oxidized before emission increas- ing down the estuary. Nucleic acids were extracted from the sediments at each site, slot-blotted onto nylon membranes, and 16S rRNA quantified with radiolabelled oligonucleotide probes against differ- ent groups of sulphate reducing bacteria (SRB). Autoradiographs were quantified by laser densito- metry compared to known standards of RNA, and expressed as a percentage of the signal from a total bacterial probe. The 4 SRB genera analysed (Desulfovibrio, Desulfobacter, Desulfobulbus and Desul- fobacterium) were all detected at all 4 sites. Desulfovibrio was the largest group of SRB at the 3 lower estuary sites, while Desulfobacter, which appeared to be the major acetate utiliser present along the estuary, and Desulfobulbus were detected at lower levels. Signal from the physiologically diverse Desulfobacterium occurred at significantly higher levels in the upper estuary at the Hythe, and it was possible that this was associated with its use of electron acceptors other than sulphate, such as nitrate.