Microaerobic Fe(II) oxidation coupled to carbon assimilation processes driven by microbes from paddy soil

Microaerobic Fe(II) oxidation process at neutral pH, driven by microbes can couple to carbon assimilation process in iron-rich freshwater and marine environments; however, few studies report such coupled processes in paddy soil of the critical zone in South China. In this study, rhizosphere soil from flooded paddy field was used as the inoculum to enrich the microaerophilic Fe(II)-oxidizing bacteria (FeOB) in gradient tubes with different Fe(II) substrates (FeS and FeCO3) and C-13-biocarbonate as inorganic carbon source to track the carbon assimilation. Kinetics of Fe(II) oxidation and biomineralization were analyzed, and the composition and abundance of the microbial community were profiled using 16S rRNA gene-based high-throughput sequencing. Results showed that microbial cell bands were formed 0.5-1.0 cm below the medium surface in the inoculated tubes with Fe(II) substances, while no cell band was found in the non-inocula controls. The protein concentrations in the cell bands reached the highest values at 18.7-22.9 mg mL(-1) on 6 d in the inocula tubes with Fe(II) substrates. A plateau of the yields of C-13-biocarbonate incorporation was observed during 6-15 d at 0.44-0.54% and 1.61-1.98% in the inocula tubes with FeS and FeCO3, respectively. The inocula tube with FeS showed a higher Fe(II) oxidation rate of 0.156 mmol L-1 d(-1) than that with FeCO3 (0.106 mmol L-1 d(-1)). Analyses of X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy revealed that amorphous iron oxide was formed on the surface of rod-shaped bacteria after Fe(II) oxidation. Relative to the agar only control, the abundances of Clostridium and Pseudogulbenkiania increased in the inocula tube with FeS, while those of Vogesella, Magnetospirillum, Solitalea, and Oxalicibacterium increased in the inocula tube with FeCO3, all of which might be the potential microaerophilic FeOB in paddy soil. The findings in this study suggest that microbes that couple microaerobic Fe(II) oxidation to carbon assimilation existed in the paddy soil, which provides an insight into the iron-carbon coupling transformation under microaerobic conditions in the critical zone of the iron-rich red soil.