Microaerobic iron oxidation and carbon assimilation and associated microbial community in paddy soil

Iron oxidation is a prevalent and important biogeochemical process in paddy soil, but little is known about whether and how microbially mediated iron oxidation is coupled with carbon assimilation, particularly under microaerobic conditions. Here, we investigated kinetics of CO2 assimilation and Fe(II) oxidation in an incubation experiment with paddy soil under suboxic conditions, and profiled the associated microbial community using DNA-stable isotope probing and 16S rRNA gene-based sequencing. The results showed that CO2 assimilation and Fe(II) oxidation in the gradient tubes were predominantly mediated by the microbes enriched in the paddy soil, primarily Azospirillum and Magnetospirillum, as their relative abundances were higher in the C-13 heavy fractions compared to C-12 heavy fractions. This study provided direct evidence of chemoautotrophic microaerophiles linking iron oxidation and carbon assimilation at the oxic-anoxic interface in the paddy soil ecosystem.