Iron is the fourth most abundant element in the earth's crust, and its high redox activity allows it to participate extensively in the material cycles of the earth's various layers. In this study, a novel Fe-oxidizing-reducing bacterium, Comamonas terrigena strain HJ-2, isolated from river water-groundwater interaction zone sediments, was investigated for its unique Fe-oxidizing-reducing properties under neutral pH anoxic conditions, which can both utilize nitrate as an electron acceptor to achieve the biological oxidation of Fe(II) and mediate the reduction of Fe(III). It is worth noting that although HJ-2 possesses both Fe(II) oxidation and Fe(III) reduction functions, there is still a lack of systematic knowledge on the mechanism of microbial-mediated Fe redox transformation, especially the key functional enzymes and environmental regulators (nitrate, carbon source) that need to be analyzed. By constructing the Fe(II)/ ferrihydrite coexistence system, it was found that the nitrate concentration gradient (0-10 mM) significantly affected the iron transformation in the carbon source-free system. Low nitrate concentration (0-0.5 mM) promoted Fe(II) oxidation and drove the conversion of ferrihydrite to high crystallinity lepidocrocite/goethite with a negative correlation between the crystallinity and nitrate concentration, while the rapid oxidation of Fe(II) under high nitrate concentration (5-10 mM) led to the generation of highly adsorbed amorphous bio-oxides. After the introduction of organic carbon sources (acetate, glucose, and lactate), the system was characterized by oxidation followed by reduction: after the initial 120 h rapid oxidation phase of Fe(II), the carbon sources later mediated the reduction of iron oxides by stimulating HJ-2 to secrete flavin mononucleotide (FMN), with the most significant reduction of iron in the glucose group. D-ribose-5-phosphate from glucose metabolism promotes FMN biosynthesis through the riboflavin metabolism pathway, and FMN acts as an electron shuttle that can lead to a significant increase in the efficiency of iron reduction. Mineral characterization showed that the system eventually formed weakly crystalline iron oxides after undergoing the oxidation-reduction cycle, and XPS confirmed that the percentage of Fe(II) on the surface of minerals in the reduction stage reached 42.2%, which was significantly higher than that in the oxidation stage (32.4%). This study reveals for the first time the mechanism of Fe redox cycling mediated by a novel Fe-oxidizing-reducing bacterium, HJ-2, which is synergistically regulated by nitrate/carbon sources. It provides a new perspective for analyzing the microbial-driven mechanism of Fe-N-C coupled cycling in the subsurface environment.

Fe-oxidizing-reducing bacterium,nitrate,ferrihydrite,FMN,transformation