Cr(VI) bioreduction by fulvic acid-mineral-microbe interactions: The critical role of nitrogen-centered environmentally persistent free radicals

Environmentally persistent free radicals (EPFRs) can interact with various soil components and drive biogeochemical cycles. However, few studies have investigated the mechanisms by which EPFRs promote extracellular electron transfer (EET). The effect of fulvic acid and iron-bearing minerals on the Cr(VI) bioreduction by MR-1 was investigated. The electrochemical characterization, intracellular reactive oxygen species (ROS) detection and transcriptomic analysis of function genes were selected to explore the EPFRs-mediated EET by Shewanella oneidensis MR-1 (MR-1) and the expression of genes related to Cr(VI) bioreduction. Kinetic experiments showed that fulvic acid significantly enhanced Cr(VI) bioreduction by both MR-1 and the ΔomcA/mtrC mutant. Moreover, in the presence of iron-bearing minerals and fulvic acid, Cr(VI) bioreduction by MR-1 was greater than that by the ΔomcA/mtrC and ΔcymA mutants. Thus, fulvic acid acted as an electron shuttle in EET via metal-reducing pathways. Electron paramagnetic resonance spectroscopy and GO enrichment analysis revealed that nitrogen-centered EPFRs in fulvic acid decreased intracellular ROS by promoting glutathione synthesis, thereby accelerating Cr(VI) bioreduction via intracellular reductase. Combined with heat map results, these findings indicated that decreasing intracellular ROS could upregulate the expression of cymA , fccA , and omcA . In the presence of iron-bearing minerals, fulvic acid also significantly decreased intracellular ROS. Furthermore, on the surface of iron-bearing minerals, nitrogen-centered EPFRs were transformed into oxygen-centered EPFRs, thereby facilitating electron transfer between outer-membrane cytochromes and iron-bearing minerals. These findings provide new insights into the EPFR-mediated electron shuttling model and exhibit implications for the remediation of Cr(VI) pollution.