Involvement of microaerophilic iron-oxidizing bacteria in the iron-oxidizing process at the surface layer of flooded paddy field soil

To reveal whether microaerophilic Fe(II)-oxidizing bacteria (FeOB) participate in the Fe(II) oxidation at the oxic-anoxic interface in flooded paddy field soil, distribution of microaerophilic FeOB belonging to Gallionellaceae (Gallionella-related FeOB) in the surface layer of a flooded paddy soil microcosm and O2 conditions for the Fe(II) oxidation by a microaerophilic Fe(II) oxidizer, Ferrigenium kumadai An22, were investigated. Flooded paddy soil microcosms were incubated for 30 days. Five soil layers were sampled at 2-mm intervals from the soil surface after the incubation. The community structure of Gallionella-related FeOB was analyzed with qPCR and DGGE methods. In the culture experiment, O2 and Fe(II) profiles in F. kumadai An22-inoculated and non-inoculated gel-stabilized gradient tubes were analyzed, in which an opposing gradient of O2 and Fe(II) was formed. A thin oxic layer was formed at the soil surface after the incubation. The copy number of 16S rRNA genes of Gallionella-related FeOB was highest at the top 0–2 mm layer of the soil. DGGE analysis showed that several bands derived from Gallionella-related FeOB newly appeared at the top soil layer with high intensity. In the culture experiment, F. kumadai A22 grew at the oxic side of the oxic-anoxic interface with the formation of Fe oxides. The present study indicated that Gallionella-related microaerophilic FeOB proliferated in the surface layer of flooded paddy field soil, presumably by oxidizing Fe(II) in the oxic-anoxic interface.