ECOLOGICAL ROLE OF THIOSULFATE AND SULFIDE UTILIZING PURPLE NONSULFURBACTERIA OF A RIVERINE ECOSYSTEM

The screening of purple nonsulfur bacteria (PNSB) from the polluted an d unpolluted zones of the river Ganga and from the discharged effluent s (pollutants), revealed that the population densities of PNSB were hi ghest in the discharged pollutants followed by polluted and unpolluted river waters in decreasing order. The allochthonous input of PNSB aff ected the ambient PNSB population. In all the monitored sites, the PNS B population utilizing malate as electron donor was higher than those utilizing thiosulfate or sulfide or both as electron donors. The physi co-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 mo rphophysiological variant of the species, Rhodobacter capsulatus and R hodocyclus gelatinosus (=Rubrivivax gelatinosus), were screened from r iver Ganga. The surviving ability of PNSB population under simulated c ondition differed amongst the monitored sites. The isolated strains of PNSB were able to convert thiosulfate to sulfate or to elemental sulf ur and also were able to oxidize sulfide. However, there was heterogen eity with respect to the rates at which they transformed these sulfur compounds. The overall thiosulfate transformation rate by the PNSB iso lates in the river water was 117.3 mu mol d(-1) mg(-1) and that of sul fide transformation was in the order of 0.15 mu mol d(-1) mg(-1). The ecological significance of sulfide utilization in the aquatic bodies b y the isolates of PNSB is discussed.