Bacterial reduction of hexavalent chromium.

Cr(VI)-reducing bacteria are widespread and Cr(VI) reduction occurs under both aerobic and anaerobic conditions. Under aerobic conditions, both NADH and endogenous cell reserves may serve as the electron donor for Cr(VI) reduction. Under anaerobic conditions, electron transport systems containing cytochromes appear to be involved in Cr(VI) reduction. High cell densities are necessary to obtain a significant rate of Cr(VI) reduction. Cr(VI) reduction by bacteria may be inhibited by Cr(VI), oxygen, heavy metals, and phenolic compounds. The optimum pH and temperature observed for Cr(VI) reduction generally coincide with the optimal growth conditions of cells. The optimum redox potential for Cr(VI) reduction has not yet been established.