[B-CG03] Earth and Planetary Science Frontiers for Life and Global Environment

[BCG03-P05] Oxidation and reduction of iron by a single Rhodoferax isolate at circumneutral pH

*Shingo Kato¹, Takashi Itoh¹, Moriya Ohkuma¹ (1.RIKEN)

Keywords:Iron cycle, Bacteria, Cultivation, Genome, Chemolithoautotrophy

Iron cycling (oxidation/reduction and dissolution/precipitation) is involved in geochemical cycling of various elements such as carbon, sulfur, and heavy metals. In natural environments, the iron cycling is mainly driven by iron-oxidizing/reducing microorganisms. Although recent culture-independent metagenomics has suggested the presence of phylogenetically diverse iron-oxidizing/reducing microorganisms, only small number of cultivated species have been reported so far. Here we report cultivation of four novel freshwater iron-oxidizing bacteria (FeOB) capable of growing at circumneutral pH under the microaerobic conditions. Iron-rich microbial mats were collected in a wetland, Tsukuba, Japan, and used as inoculum for soft-agarose-gradient cultivation with FeCO₃ plug. Eventually, three isolates (strains MIZ01, MIZ03, and MIZ09) and one co-culture (strain MIZ02 with MIZ09) were obtained. Typical iron-bands for known microaerophilic FeOB were observed in these gradient cultures. Whole-genome sequences of these strains were determined by Illumina and Nanopore sequencing. Phylogenomic analysis indicated that MIZ01 and MIZ02 were affiliated in the genus Sideroxydans, MIZ03 was affiliated in Rhodoferax, and MIZ09 was affiliated in Thiomonas, respectively. Remarkably, our cultivation experiments demonstrated that Rhodoferaxsp. MIZ03 could grow not only by microaerobic iron oxidation, but also by anaerobic iron reduction, which was consistent with its genome content. Moreover, MIZ03 could grow by hydrogen and thiosulfate oxidation under the (micro)aerobic conditions. Thus, MIZ03 is the first Rhodoferax isolate growing chemolithoautotrophically, and, to our best knowledge, is the first organism capable of microaerobic iron oxidation and anaerobic iron reduction at circumneutral pH among the three domains of life. Considering the fact that DNA sequences related to Rhodoferax have been often detected in subsurface environments, Rhodoferax spp. may play a more significant role in biogeochemical cycling than previously recognized.