In Vitro Reduction of Hexavalent Chromium by Cytoplasmic Fractions of Pannonibacter phragmitetus LSSE-09 under Aerobic and Anaerobic Conditions

Abstract

Hexavalent chromate reductase was characterized and was found to be localized in the cytoplasmic fraction of a chromium-resistant bacterium Pannonibacter phragmitetus LSSE-09. The Cr(VI) reductase activity of cell-free extract (S₁₂) was significantly improved by external electron donors, such as NADH, glucose, acetate, formate, citrate, pyruvate, and lactate. The reductase activity was optimal at pH 7.0 with NADH as the electron donor. The aerobic and anaerobic Cr(VI)-reduction enhanced by 0.1 mM NADH were respectively 3.5 and 3.4 times as high as that without adding NADH. The Cr(VI) reductase activity was inhibited by Mn²⁺, Cd²⁺, Fe³⁺, and Hg²⁺, whereas Cu²⁺ enhanced the chromate reductase activity by 29% aerobically and 33% anaerobically. The aerobic and anaerobic specific Michaelis-Menten constant K(m) of S₁₂ fraction was estimated to be 64.95 and 47.65 μmol L⁻¹, respectively. The soluble S₁₅₀ fractions showed similar activity to S₁₂ and could reduce 39.7% and 53.4% of Cr(VI) after 1 h of incubation aerobically and anaerobically while the periplasmic contents showed no obvious reduction activity, suggesting an effective enzymatic mechanism of Cr(VI) reduction in the cytoplasmic fractions of the bacterium. Results suggest that the enzymatic reduction of Cr(VI) could be useful for Cr(VI) detoxification in wastewater.