Intracellular uptake and reduction of hexavalent chromium by the cyanobacterium Synechocystis sp PUPCCC 62

A hexavalent chromium-tolerant cyanobacterial strain isolated from the Satluj River, Ludhiana, India was identified as Synechocystis sp. strain PUPCCC 62 based on the 16S rRNA gene and phycocyanin cpcBA-intergenic spacer sequence analysis. Cr IC50 for the organism was determined to be 100 mu mol L-1. The organism removed nearly four times more Cr(VI) from imidazole-HCl buffer (200 mmol L-1, pH 6.0) (250 nmol Cr(VI) mg(-1) protein) compared with basal medium (59 nmol Cr(VI) mg(-1) protein) in 8 h. Temperature of 28 A degrees C, pH of 6.0, and biomass load of 200 mu g protein mL(-1) were found to be the optimum parameters for Cr(VI) bioremoval from the buffer solution. Cr(VI) bioremoval by the cyanobacterium was light/photosynthesis dependent. Along with Cr(VI) bioremoval, its reduction to Cr(III) was also demonstrated. In a 6-h period, the amount of Cr(VI) in the solution decreased to 7.3 mu mol and the amount of Cr(III) increased to 2.7 mu mol from initial amount of 10 mu mol Cr(VI). Both Cr(VI) and Cr(III) were observed in the solution only and not on the surface or inside the cells. It has been demonstrated that the organism reduced Cr(VI) to Cr(III) intracellularly and excreted it outside the cells. This is the first report on intracellular reduction of Cr(VI) by a cyanobacterium.