Influence of sulphate and phosphate ions on Cr(VI) removal by Streptomyces sp. MC1

PEREIRA, C. E; VILLEGAS L. B; ABATE C.M

Tucumán

Congreso; SAMIGE (Sociedad Argentina de Microbiologia General); 2011

SAMIGE

Chromium (Cr) is a natural element which is found in rocks, soil, plants, animals, volcanic dust and gases. It can exist in several oxidation states, but the most stable and common forms in the environment are trivalent [Cr(III)] and hexavalent [Cr(VI)] chromium. Cr(III) is an essential micronutrient required for the growth of many organisms but toxic at high concentrations. However Cr(VI) is toxic, mutagenic, carcinogenic, teratogenic at all concentrations and it is the most frequently used in industrial processes like leather tanning. While, Cr(VI) is highly soluble in water, mobile and biologically available in the ecosystems, Cr(III) shows poor solubility and is easily adsorbed on mineral surfaces. Due to the problems that high Cr(VI) concentrations produce in the environment, the treatment strategy could include the reduction of Cr(VI) to Cr(III). Reduction ability has been found in a large range of eukaryotic and prokaryotic microorganisms. Streptomyces sp. MC1, isolated from sugar cane, was selected because it showed significant growth and capacity to remove Cr(VI) in liquid minimal medium, abilities that might be useful for bioremediation It is believed that Cr(VI) uses sulfate and phosphate transport routes to penetrate the cellular membrane due to its structural similarity to these anions. The aim of this work was to study the influence of sulfate and phosphate ions on the resistance and Cr(VI) removal. Cells of Streptomyces sp. MC1 were grown in liquid minimal medium supplemented with glucose and with or without 20 mg/L of K2Cr2O7 as Cr(VI) source and 5 mM of Na2SO4 or K2HPO4 for 120 h at 30 ºC in an orbital shaker (170 rpm). Biomass was determined by dry weight, Cr(VI) concentration in the medium by the colorimetric method of 1,5-diphenylcarbazide, total chromium by flame atomic absorption spectroscopy and residual glucose by the method of dinitrosalicylic. The presence of sulfate or phosphate ions induced the growth of Streptomyces sp. MC1. However, only sulfate ion enhanced the Cr(VI) removal significantly. On the other hand, total chromium concentration in the supernatant was constant in all assayed conditions. It is known that Cr(III) is a stable and soluble component formed by Cr(VI) reduction. Therefore, it could be inferred that chromium remaining in the supernatant will be as Cr(III). This finding may indicate that the Cr(VI) removal by Streptomyces sp. MC1 was due to reduction to trivalent form but not to chromium bioaccumulation under these conditions. Cr(VI) reduction to Cr(III) by microorganisms is an interesting strategy of bioremediation. These results may have significant implications in the biological treatment of Cr(VI) in environmental polluted. Furthermore studies are required for future applications in bioremediations processes.