Nitrate and iron salts enhanced Shewanella MR-1 bioreduction on Cr(VI) by stimulating cytochrome c electron transmission system

Bioreduction of Cr(VI) by Shewanella through extracellular electron transmission (EET) attracts increasing interest due to cytochrome c (cyts c ) system especially conductive flagella (“nanowire”) structure. The responses of bioreduction efficiency, “nanowire” growth, cyts c synthesis, and e (electron) transmission intensity through CymA→MtrA→MtrB→MtrC→OmcA (Route 1) and CymA→NapA→NapB (Route 2) to iron ion (Fe 3 + ) and nitrate (NO 3 - ) were evaluated. Results showed that Fe 3+ and NO 3 - of 1–1.5 mM significantly ( p  < 0.01) enhanced bioreduction of Cr(VI) from 67% to 82% and 79%, respectively with promoted cyts c -mediated c onductivity (C p ). Both Fe 3+ and NO 3 - stimulated the growth of “nanowire” and synthesis of cyts c participating in EET with Fe 3+ showing stronger stimulation on Routes 1 while NO 3 - on Routes 2. The genes expression of cyts c on inner membrane (CymA), periplasmic space (NapA/NapB), and “nanowire” (MtrA/MtrB/MtrC, OmcA) show synchronous responses to Fe 3+ and NO 3 - . However, contents of mobile cyts c in extracellular polymeric substances (EPS) and structural cyts c in cell membrane showed inconsistent responses to Fe 3+ and NO 3 - . Overall, this study illustrated the regulation mechanism of e acceptors on cyts c- electron transmission system of Shewanella , which help improving the electrochemical efficiency on Shewanella and provide novel idea in chromium remediation strategy .