Bacterial sulphate reduction in relation to sulphur geochemistry in two contrasting areas of saltmarsh sediment

Abstract

Sulphate reduction rates and the sulphur geochemistry of the sediments were investigated at two sites in a saltmarsh. Rates of dissimilatory sulphate reduction decreased rapidly with depth at both sites over the top 20 cm of sediment. There was no correlation between the measured rates and the numbers of sulphate reducing bacteria. Sulphate reduction rate also varied seasonally, and total annual sulphate reduction over the top 20 cm of sediment depth was equivalent to 0.44 mg-at. sulphate-S cm −2 of sediment surface (main creek site) and 0.31 mg-at. sulphate-S cm −2 (saltmarsh pan site). These rates are similar to those reported in the literature for similar sediments. Free sulphide was present in the sediment of the saltmarsh pan, but never at the creek site although sulphate reduction rates were similar. Pyrite and acid-volatile sulphide distributions with depth were also different between the two sites. The contrast in geochemistry was attributed to different stages of sulphur diagenesis. A large proportion of the sulphide was lost from the sediment by diffusion, tidal forces not contributing to transport of sulphide. The diagenetic difference between the two sites was attributed to the presence of a benthic algal mat in the saltmarsh pan which retarded diffusive loss of sulphide, maintained higher sedimentary sulphide concentrations, and thus kinetically favoured more rapid diagenesis at this site.