Anaerobic co-reduction of chromate and nitrate by bacterial cultures of Staphylococcus epidermidis L-02

Industrial wastewater is often polluted by Cr(VI) compounds, presenting a serious environmental problem. This study addresses the removal of toxic, mutagenic Cr(VI) by means of microbial reduction to Cr(III), which can then be precipitated as oxides or hydroxides and extracted from the aquatic system. A strain of Staphylococcus epidermidis L-02 was isolated from a bacterial consortium used for the remediation of a chromate-contaminated constructed wetland system. This strain reduced Cr(VI) by using pyruvate as an electron donor under anaerobic conditions. The aims of the present study were to investigate the specific rate of Cr(VI) reduction by the strain L-02, the effects of chromate and nitrate (available as electron acceptors) on the strain, and the interference of chromate and nitrate reduction processes. The presence of Cr(VI) decreased the growth rate of the bacterium. Chromate and nitrate reduction did not occur under sterile conditions but was observed during tests with the strain L-02. The presence of nitrate increased both the specific Cr(VI) reduction rate and the cell number. Under denitrifying conditions, Cr(VI) reduction was not inhibited by nitrite, which was produced during nitrate reduction. The average specific rate of chromate reduction reached 4.4 micromol Cr 10(10 )cells(-1 )h(-1), but was only 2.0 micromol Cr 10(10 )cells(-1 )h(-1) at 20 degrees C. The maximum specific rate was as high as 8.8-9.8 micromol Cr 10(10 )cells(-1 )h(-1). The role of nitrate in chromate reduction is discussed.