Bacterial phototrophic biomass as a bio-catalyst for the reduction of Chromium(VI) in waste-waters

Hexavalent chromium represents an outstanding risk for the environment and the health of human beings, as it is considerably involved in the genesis of cancer and other fatal diseases. Biological reduction of Cr(VI) to Cr(III) is a potentially useful mechanism to remediate chromium (VI) pollution and to detoxify contaminated wastes. The photosynthetic purple bacterium Rhodobacter sphaeroides is known for its ability to tolerate high concentrations of several heavy metal ions, to bioaccumulate nickel and cobalt, and to reduce oxyanions as tellurite, selenite and chromate. The response of the carotenoidless mutant R26 to chromate stress under phototrophic conditions has been recently investigated by biochemical and spectroscopic measurements, proteomic analysis and cell imaging, revealing good Cr(VI) reduction ability associated with morphological and compositional changes of the cell envelope, while no specific stress-induced chromate-reductase activity was found in the soluble proteome. Phototrophic biomass of Rhodobacter sphaeroides strain R26, harvested, washed, and stored at -20°C, was just thawed and used as Cr(VI) reduction catalyst. Chromate solutions, buffered at neutral pH and supplemented with a mixture of succinate, malonate and glucose as electron donors, have been employed for simulating the waste-water environment. The decrease of Cr(VI) concentration triggered by cells addition was evaluated by the diphenylcarbazide (DPC) assay. The analysis of reaction kinetics revealed that Rhodobacter sphaeroides resting biomass acts as an excellent bio-catalyst promoting chromate reduction by oxidizable carbon compounds. The role of abiotic variables such as pH, light, temperature and oxygen concentration was also assessed. Our data extend the information available about this phototrophic microorganism and elucidate its potential in Cr(VI) bioremediation applications.