Microbial reduction of Cr(VI) to Cr(III) can mitigate environmental chromium toxicity. A chromium, cadmium and nickel tolerating strain TCL with 97% 16S rRNA gene sequence homology to Bacillus cereus was isolated from a derelict open-cast, Tasra Coalmine Lake of Jharia, India. It could tolerate up to Cr2000 [2,000 mg L-1 Cr(VI)] and completely reduce Cr200 within 16 h under heterotrophic condition. TCL grown in ≥ Cr500 exhibited multifarious stress responses particularly in its prolonged lag-phase, like cell aggregation, up to two-fold elongation, increased exopolysaccharide production, and stress enzyme activities. These were relieved by increasing inoculum size or nutrient content. Chromium reduction was constitutive, with maximum activities detected in loosely-bound exopolysaccharides and membrane fractions, followed by cytoplasm and spent media. Cr(VI) was efficiently reduced to Cr(III) and >90% was released in spent media. Cells also expressed Cr-induced active efflux pumps. Growing cells or its crude enzyme extracts could efficiently reduce Cr(VI) in diverse temperatures (15-45 °C), pH (5-9); and in presence of other metals (Cd, Cu, Mo, Ni, Pb), oxyanions (SO4-2, NO2-), and metabolic inhibitors (phenol, NaN3, EDTA). Growth and reduction were also detected in nutrient-limited minimal salt media, and contaminated leather industry effluent thereby making TCL a potential candidate for bioremediation.
Keywords: Bioremediation; Chromium; Coalmine; Reduction mechanism; Stress response.
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