Riboflavin secreted by Shewanella sp. FDL-2 facilitates its reduction of Se(iv) and Te(iv) by promoting electron transfer.

The biological reduction of selenite (Se(iv)) or tellurite (Te(iv)) to Se0 or Te0 has received increasing attention, as related studies have favored the development of Se/Te pollution control methods. In the presence of the electron donor, the microbes acquired energy and transferred electrons to Se(iv) or Te(iv) to achieve their detoxication. However, the microbial electron transfer pathways involved in this process are still not fully understood. In this study, we reported that marine Shewanella sp. FDL-2 (FDL-2) was capable of reducing Se(iv) and Te(iv) through a novel riboflavin-involved pathway. The results showed that FDL-2 can effectively reduce 10 mM Se(iv) and 5 mM Te(iv) to Se0 and Te0, which was further confirmed by XPS and XRD analyses. RT-qPCR results indicate the upregulation of genes coding flavin-related proteins, and the production of flavin-related substances by strain FDL-2 during Se(iv)/Te(iv) bioreduction was proven by fluorescence chromatography analysis. In addition, the presence of riboflavin enhanced the electron transfer efficiency, indicating its promoting effect on the bioreduction of Se(iv)/Te(iv). Overall, our results highlight a riboflavin-involved electron transfer pathway during Se(iv)/Te(iv) bioreduction and thus deepen our understanding of the corresponding mechanism.