Distribution of sulphur-cycle bacteria in the Ems-Dollard estuary

Summary A quantitative study of the sulphur cycle in the tidal flat-sediments of the Eems-Dollard estuary was started by determining the distribution of two physiologically different groups of bacteria: sulphate-reducing and sulphide-oxidizing bacteria. Viable counts of these bacterial groups were determined by most probable number techniques.The highest numbers of aerobic sulphide-oxidizing bacteria were found in the upper 2 cm of the sediment. A rapid decrease was observed with increasing depth.The anaerobic sulphate-reducing bacteria showed different distribution-patterns with depth. Frequently high numbers of these bacteria were found above the redox-discontinuity-layer. This may be attributed to the presence of anaerobic micro-pockets in this largely aerobic top-layer of the sediment.The horizontal distribution of the sulphide-oxidizing bacteria appeared to be highly correlated with sediment parameters such as organic carbon and clay content of the sediment. The sulphate-reducing bacteria showed only a small linear correlation with these parameters.By means of polyfactor-analysis mathematical models were made with bacterial numbers as the dependent variables and with some environmental parameters as independent variables. The parameters used in this models could explain the variance of the viable counts for approximately 70%. The clay content of the sediment and the number of sulphate reducing bacteria appeared to determine to a large extent the variance in numbers of sulphide-oxidizing bacteria. There are indications that a great deal of the sulphide-oxidizing bacteria might be mixotrophic.For the explanation of the variance in numbers of sulphate-reducing bacteria the most important parameters were the clay content of the sediment, the number of aerobic heterotrophic bacteria and temperature (or season). Therefore the numbers of these organisms were varying throughout the year. It is assumed that the heterotrophic bacteria supply the sulphate-reducing bacteria with organic substrates.