Glucose promotes iron bi-directional redox transformation mediated by a single strain Comamonas sp. HJ-2

In anaerobic underground environments, the iron redox cycle is fundamentally driven by microbial activities, wherein the metabolic capacity of individual strains plays a pivotal role. This study investigated the regulatory mechanisms of three carbon sources — acetate, glucose, and lactate — on the iron redox cycle mediated by Comamonas sp. HJ-2 under neutral pH anaerobic conditions in Fe(III)–Fe(II) coexisting and Fe(III)-free systems. The results demonstrated that glucose enhanced Fe(II) bio-oxidation and Fe(III) bio-reduction by promoting energy metabolism. Glucose promoted sustained FMN accumulation, which is consistent with a potential contribution of the glycolysis-pentose phosphate pathway-riboflavin synthesis axis, which accumulated to a final concentration of approximately 0.40 μM. The accumulated FMN may function as an electron shuttle and thereby contribute to Fe(III) reduction. Mineralogical analyses further showed system-dependent effects of glucose. In ferrihydrite-containing systems, glucose inhibited the transformation of ferrihydrite toward more crystalline iron phases, whereas in ferrihydrite-free systems it shifted the product distribution toward a higher proportion of ferrihydrite and a lower proportion of lepidocrocite. Glucose also increased the content of surface-bound Fe(II) on minerals. In conclusion, glucose emerged as the most effective carbon source for optimizing the HJ-2-driven iron redox coupling process, likely due to its association with enhanced energy metabolism, FMN accumulation, and altered mineral phase transformation. This study elucidates the carbon source-dependent mechanisms governing microbial iron cycling in anaerobic environments and highlights the critical role of carbon source selection in enhancing iron biotransformation efficiency.