Reduction of hexavalent chromium by Streptomyces griseus

Hexavalent chromium (Cr6+) is a common and toxic pollutant in soils and wastewaters. Conversion of the mobile Cr6+ to less mobile and less toxic trivalent Cr3+ can be achieved with conventional chemical reduction and its subsequent precipitation. Alternatively, Cr6+ can also be reduced to Cr3+ by microorganisms and has potential for application in decontamination of industrial effluents. During our studies on chromium toxicity in actinomycetes, a strain of Streptomyces griseus was found to grow and reduce highly toxic Cr6+ to less toxic Cr3+ from media containing 5–60 ppm of Cr6+. The concentration of Cr3+ increased with simultaneous decrease in Cr6+ during growth with complete conversion in 3–4 days. Reduction was faster when addition of chromium (especially high concentrations of chromium) was done after 24 h growth than at the time of inoculation. Conversion of Cr6+ to Cr3+ was complete within 24–48 h of chromium addition with significant uptake in the biomass. Reduction of chromium by resting cells and effect of various factors on chromium reduction are discussed.