Nitrate concentration mediates iron transformation by an iron-oxidizing–reducing bacterium in the Fe(ii)–Fe(iii) co-existing system

The bacterium Comamonas terrigena strain HJ-2 has been reported as a nitrate-dependent iron-oxidizing bacterium, surprisingly with Fe(III) reduction ability. The control of iron redox cycling by the strain HJ-2 in the Fe(II)-Fe(III) co-existing system is mysterious and worthy of exploration. The present study demonstrates that HJ-2 utilizes nitrate as an electron acceptor to rapidly oxidize Fe(II), with negligible Fe(III) reduction, under neutral pH and anaerobic conditions in a co-existing Fe²⁺ and ferrihydrite system. Nitrate significantly influences the iron transformation mediated by HJ-2. The final conversion ratio of Fe(II) increased from 27.94% to 96.67%, and the bio-oxidation rate of Fe(II) escalated from 0.000143 to 0.013 h^-1 as nitrate concentrations rose from 0.1 to 10 mM. X-ray diffraction results indicated that in the absence of HJ-2, goethite was the sole product, regardless of the nitrate concentration. While in the presence of HJ-2, the minerals formed transitioned from crystalline goethite and lepidocrocite to amorphous or weakly crystalline minerals with increasing nitrate concentration. Scanning electron microscopy and energy-dispersive X-ray spectroscopy showed that the morphology of the final mineral formed under high nitrate conditions resembled ferrihydrite, with higher carbon content on the mineral surface at elevated nitrate concentrations, suggesting enhanced production of bio-iron oxides and increased adsorption capacity for aqueous Fe(II). X-ray photoelectron spectroscopy results confirmed higher Fe(II) content on the amorphous minerals surface than goethite. This work provides new insights into microbial-mediated Fe-N cycling in natural environments, highlighting the significance of nitrate in driving iron redox processes and the iron mineral transformation by strain HJ-2.