Aerobic chromate reduction by chromium-resistant bacteria isolated from serpentine soil

A group of 34 chromium-resistant bacteria were isolated from naturally occurring chromium percolated serpentine soil of Andaman (India). These isolates displayed different degrees of chromate reduction under aerobic conditions. One of the 34 isolates identified as Bacillus sphaericus was tolerant to 800 mg l⁻¹ Cr(VI) and reduced >80% Cr(VI) during growth. In Vogel Bonner broth, B. sphaericus cells (10¹⁰ cells ml⁻¹) reduced 62% of 20 mg l⁻¹ of Cr(VI) in 48 h with concomitant discoloring of yellow medium to white one. Reduction of chromate was pronounced by the addition of glucose and yeast extract as electron donors. In the presence of 4.0 g l⁻¹ of glucose, 20 mg l⁻¹ of Cr(VI) was reduced to 2.45 mg l⁻¹ after 96 h of incubation. Optimum pH and temperature for reduction were 6.0 and 25 °C, respectively. Increase in cell density and initial Cr(VI) concentration increased chromate reduction but was inhibited by metal ions like, Ni²⁺, Co²⁺, Cd²⁺ and Pb²⁺. Experiments with cell-free extracts indicated that the soluble fraction of the cell was responsible for aerobic reduction of Cr(VI) by this organism.