Ecological role of thiosulfate and sulfide utilizing purple nonsulfur bacteria of a riverine ecosystem

The screening of purple nonsulfur bacteria (PNSB) from the polluted and unpolluted zones of the river Ganga and from the discharged effluents (pollutants), revealed that the population densities of PNSB were highest in the discharged pollutants followed by polluted and unpolluted river waters in decreasing order. The allochthonous input of PNSB affected the ambient PNSB population. In all the monitored sites, the PNSB population utilizing malate as electron donor was higher than those utilizing thiosulfate or sulfide or both as electron donors. The physico-chemical parameters of the river water related to the distribution of PNSB were recorded. Twenty-four isolates of PNSB utilizing sulfide and thiosulfate, assigned to Rhodopseudomonas palustris including a morphophysiological variant of the species, Rhodobacter capsulatus and Rhodocyclus gelatinosus (=Rubrivivax gelatinosus), were screened from river Ganga. The surviving ability of PNSB population under simulated condition differed amongst the monitored sites. The isolated strains of PNSB were able to convert thiosulfate to sulfate or to elemental sulfur and also were able to oxidize sulfide. However, there was heterogeneity with respect to the rates at which they transformed these sulfur compounds. The overall thiosulfate transformation rate by the PNSB isolates in the river water was 117.3 μmol d−1 mg−1 and that of sulfide transformation was in the order of 0.15 μmol d−1 mg−1. The ecological significance of sulfide utilization in the aquatic bodies by the isolates of PNSB is discussed.