Chromate reduction by the photosynthetic bacterium Rhodobacter sphaeroides

Chromate is a highly soluble and toxic non-essential oxyanion for most organisms. A number of chromate resistant bacteria have been investigated and diverse resistance mechanisms were found [1]. We are investigating the potentialities of the photosynthetic facultative bacterium Rhodobacter sphaeroides, known for its ability to tolerate high concentrations of several heavy metal ions [2] and bioaccumulate some of them, such as nickel and cobalt [3, 4], in the bioremediation of chromate polluted sites. Employing an interdisciplinary approach, the response to chromate stress was investigated by combining biochemical and spectroscopic measurements, proteomic characterization and cell imaging. An efficient resistance mechanism to chromate is suggested both by the high EC50 value and the lag-phase lengthening induced at concentrations above 0.05 mM. R. sphaeroides is also able to reduce chromate to the less toxic and soluble form Cr(III) with reductase activity preferentially associated with the protein soluble fraction. Chromate effect on soluble enzymes was investigated by a proteomic approach: soluble protein expression profiles of cells exposed to chromate were compared with those of untreated control cells through two-dimensional gel electrophoresis analysis. Upon exposure to chromate at least 30 soluble proteins were differentially expressed. The wide variety of differentially expressed proteins suggests that different metabolic pathways are involved as response to chromate exposure. The accompanying physiological response to Cr(VI) exposure included marked changes in cellular morphology as revealed by atomic force microscopy.