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Summary The number of thiobacilli and the thiosulfate oxidation potency were determined in the water and in the sediment of the river Saar in order to study the aerobic part of the sulfur cycle. Samples were taken along the entire course of the river between summer 1978 and spring 1981, i.e. before the lower part of the river was canalized. In the water samples, a close correlation was observed between the total number of thiobacilli and the thiosulfate oxidation potency. In the sediment of the middle part of the river, which is canalized, the observed concentration and metabolic activity of the thiobacilli, the presence of sulfide and the diminished concentration of sulfate indicated that both oxidation and reduction of sulfur occurred in this reach. Highest concentrations (numbers) and activities of thiobacilli, both in the water and in the sediment, were observed at low waters in August 1978; 10 3 cells/ml water and 10 5 cells/g sediment were detected. Lowest concentrations and activities were measured during high waters in February 1980. In a longitudinal study of the river the following species were enumerated in water and sediment by means of selective media: T. thioparus, T. neapolitanus, T. denitrificans, T. thiooxidans and T. ferrooxidans . Downstream from the source, the number of all these species increased, but in case of T. thioparus and T. neapolitanus maximum concentrations were already observed in the upper part of the river, which is not canalized and not polluted by industrial wastes. The remaining three species increased in number along the entire river. The concentrations of the individual species were considerably higher in the sediments than in the overlaying water. In general, the detected numbers were about 10 3 -times higher in the sediment samples, regardless whether selective or non-selective media were used.

Summary 8 mutants of Hydrogenomonas eutropha H 16 were isolated after treatment with the mutagen HNO 2 and enrichment in a medium containing urea and colistin. The mutants could no longer utilize urea and needed for optimal growth an increased concentration of NH 4 Cl which strongly inhibited the growth of the wild type. The more intensively investigated mutant U 5 showed an exponential growth with glutamate, but not with NH 4 Cl Like the wild type, the mutant also produced urease and NADP specific glutamate dehydrogenase; during nitrogen starvation an increase in specific urease activity was observed in both strains. The possibility of damage to the urea and ammonia transport system is discussed.