Biotransformation of chromium by root nodule bacteria Sinorhizobium sp. SAR1

Abstract

The present study aims to address the problem of chromium (Cr) toxicity by providing important insights into the mechanisms involved in its bioremediation. Among the 22 Rhizobium and Sinorhizobium isolates obtained from Sesbania sesban root nodules, Sinorhizobium sp. SAR1 (JX174035.1) tolerated the maximum Cr concentration (1mM) and hence was used for further studies. The excess secretion of extra polymeric substances, as seen from scanning electron micrographs, could be a probable mechanism of adaptation to the Cr stress. The Energy dispersive X-ray spectroscopy data did not show any peaks of Cr. The biosorption studies done on the isolate gave maximum adsorption capacity as 285.71mg/g. The isotherm studies showed a better fit to Langmuir isotherm. The Weber and Morris plot established that the phenomenon of adsorption was governed by film diffusion mechanism. The FTIR analysis suggested the role of cell wall components and extracellular polymeric substances in Cr adsorption to the biomass of Sinorhizobium. On the basis of these results a compiled mechanism of Cr (VI) adsorption and its biotransformation into Cr (III) by Sinorhizobium sp. SAR1 is explained. This work outlines a comprehensive detail for the exact phenomenon of Cr biotransformation by Sinorhizobium sp. SAR1. These results may further help in developing and enhancing effective bioremediation approaches.

Fig 1. Scanning electron micrographs of isolate SAR1 before and after treatment with Cr.

a: Control; b: After treatment with Cr.

Fig 2. Effect of pH on bioremoval of Cr SAR1.

Fig 3. Effect of biomass dosage on bioremoval of Cr.

Fig 4. Effect of Cr concentration on bioreduction and uptake by SAR1.

Fig 5. Adsorption isotherm for isolate SAR1 after Cr biosorption A: Langmuir isotherm; B: Freundlich isotherm.

Fig 6

a. The plot of Q vs √t; Weber and Morris for the rate-limiting step; b. The plot of log Q vs 0.5 log t; Weber and Morris for the mechanism of adsorption.

Fig 7

FTIR Spectra of isolate SAR1 A: before Control (–) and B: after Cr biosorption (–).

Fig 8. Steps in chromium biotransformation mechanism.

a. Adsorption of Cr (VI) to exopolysaccharide, lipopolysaccharide, lipoproteins, peptidoglycans; b. Biotransformation of Cr (VI) to Cr (III); c. Release of Cr III into the solution.