It is essential to investigate microbial and chemical oxidation of ferrous phosphate mineral vivianite, toward understanding the importance of iron redox reactions and mineral transformations for the fate of phosphorus in the environment. This study explored vivianite oxidation by a variety of Fe(II)-oxidizer strains, including microaerophilic Fe(II)-oxidizers, anoxygenic phototrophic Fe(II)-oxidizers (pFeOx), and nitrate-reducing Fe(II)-oxidizers (NRFeOx), and compared it with abiotic vivianite oxidation by O₂ and nitrite. The results showed that only microaerophilic Fe(II)-oxidizing culture SG15, R. ferrooxidans SW2 and C. ferrooxidans KoFox could achieve significant oxidation of solid-Fe(II) vivianite at pH 7. In contrast, microaerophilic SG15 only promoted 20% more of vivianite oxidation when compared to 50% abiotic oxidation of vivianite by air. R. palustris TIE-1 and Acidovorax sp. BoFeN1 could not oxidize vivianite. Autotrophic NRFeOx culture KS only oxidized ~30% of vivianite. No matter how much vivianite was oxidized, we did not observe any release of dissolved phosphate. The different oxidation extents and phosphate immobilization may be attributed to different oxidation mechanisms by these Fe(II)-oxidizers, and the interaction between Fe(II)-oxidizer cells and vivianite minerals. Characterization via XRD, FTIR, and ⁵⁷Fe Mössbauer spectroscopy tracked the transformation of Fe minerals and Fe/P speciation. Quantitative analyses of Fe K-edge EXAFS and P K-edge XANES spectroscopy indicated that less than 10% of vivianite were oxidized by nitrite, R. palustris TIE-1, and Acidovorax sp. BoFeN1. Microbial structures of culture KS community may change after a few days, leading to only 35% oxidation since SEM images showed only one type of cell morphology was observed instead of the expected mixture community. Microaerophilic Fe(II)-oxidizing culture SG15, R. ferrooxidans SW2 and C. ferrooxidans KoFox oxidized most of the vivianite into iron(III) phosphate solids (FePO₄), further explaining the limited net release of dissolved phosphate into solution. These results imply that microbial activity could be an important factor contributing to vivianite transformation and provide a new understanding on vivianite presence in soils and sediments, considering its resistance to full oxidation by air at circumneutral pH. In sediments or soils, vivianite minerals showing a high oxidation ratio (>50%) and cell-mineral attachment could be considered to be a biosignature. This study gives a new insight of Fe(II) and P coupled biogeochemical cycling via chemical and microbial reactions in natural environments.
 

iron biogeochemical cycle,vivianite,iron(II)-oxidizers,iron,phosphorus