Mechanisms of Cr (VI) reduced by nitrogen-fixing bacteria

Nitrogen-fixing bacteria can be a new microbial reduction process to remove Cr (VI) in nitrogen-deficient smelting wastewater. However, the reductive mechanism remains unknown. In this work, under the optimal conditions (pH = 7 and 35 °C), nitrogen-fixing bacteria reduced Cr (VI) at a reduction rate of 0.0834 ± 0.0046 h^-1. Clostridiaceae and Clostridium levels increased by 12.60 ± 0.43 % and 18.05 ± 0.20 %, respectively. Which were responsible for Cr (VI) reduction. Riboflavin was secreted as the mediator for the Cr (VI) reduction, and external addition of riboflavin increased the reduction rate maximum by 124.61 ± 4.98 %. Transcriptomic analysis revealed that genes involved in riboflavin biosynthesis for Cr (VI) reduction with fold increases in numbers ranging from 1.38 ± 0.22-127.15 ± 3.24. Moreover, signaling molecules enhanced Cr (VI) reduction through strengthening the synthesis of riboflavin and exogenous addition of signaling molecules increased the reduction rate maximum by 123.61 ± 10.13 %. This study broadens the application of nitrogen-fixing bacteria and provides insight into the remediation of Cr (VI) pollution.