Arsenite biotransformation by Rhodococcus sp.: Characterization, optimization using response surface methodology and mechanistic studies.

A large population of the world is under increased health risk due to consumption of arsenic contaminated groundwater. The present study investigates the arsenic resistance and arsenic biotransforming ability in three bacterial species, namely Bacillus arsenicus, Rhodococcus sp. and Alcaligenes faecalis for employing them in potential groundwater bioremediation programmes. The tolerance to pH levels for the 3 organisms are 6-9 for A. faecalis, 5-10 for Rhodococcus and 5-9 for B. arsenicus. The arsenic bio-oxidation capacity was qualitatively confirmed by using the silver nitrate method and all three bacteria were able to convert arsenite to arsenate. The arsenite tolerance capacity (MIC values) were found to be 3 mM, 7 mM and 12 mM for B. arsenicus, A. faecalis and Rhodococcus sp. respectively. The changes in cellular morphology of these strains under various arsenic stress conditions were studied using advanced cell imaging techniques such as scanning electron microscopy and Atomic Force Microscopy. Rhodococcus sp. emerged as a potential candidate for bioremediation application. A response surface methodology was employed to optimize key parameters affecting arsenic removal (pH, Iron (II) soluble, concentration of humic acid and initial arsenic concentration) and at optimized conditions, experimental runs demonstrated 48.34% removal of As (III) (initial concentration = 500 μg/L) in a duration of 6 h, with complete removal after 48 h. Evidences from this work indicate that arsenic removal occurs through bioaccumulation, biotransformation and biosorption. The present study makes the first attempt to investigate the arsenic removal capability of Rhodococcus sp. in synthetic groundwater by employing bacterial whole cell assays. This study also sheds light on the arsenic tolerance and detoxification mechanisms employed by these bacteria, knowledge of which could be crucial in the successful implementation of in-situ bioremediation programmes.