[B-CG07] Earth and Planetary Science Frontiers for Life and Global Environment

[BCG07-P01] The discovery and comparative genome characterizations of magnetosome-bearing bacteria from a deep-sea metal sulfide chimney

*Shinsaku Nakano¹, Shingo Kato², Michinari Sunamura¹, Mariko Kouduka¹, Toshitsugu Yamazaki³, Yohey Suzuki¹ (1.Department of Earth and Planetary Science, The University of Tokyo, 2.Japan Agency for Marine-Earth Science and Technology, 3.Atmosphere and Ocean Research Institute, The University of Tokyo)

Magentotactic bacteria (MTB) are known to produce magnetosomes: single-domain magnetite (Fe₃O₄) and/or greigite (Fe₃S₄) encapsulated in cytoplasmic membrane. To orient cells along the geomagnetic field, magnetosomes are aligned into chains for strong dipole moment. Although intensive studies have been conducted for MTB inhabiting terrestrial and shallow marine sediments, MTB have not been found from deep-sea environments, despite the frequent detection of magentofossils in deep-sea sediments. In this study, a metal sulfide chimney formed on the seafloor of South Mariana Trough was investigated for the occurrence of MTB. During magnetic separation from non-magnetic material, magnetic particles mainly composed of vivianite [Fe₃(PO4)₂·8H₂O] attracted magnetic cells. After further separation of magnetic cells from vivianite particles, the visualization of microbial cells by catalyzed reporter deposition-fluorescence in situ hybridization and transmission electron microscopy unveiled the occurrence of metabolically active bacteria associated with magnetosomes from the metal sulfide chimney. Teardrop shaped magnetosomes and dominant 16S rRNA gene sequences related to Nitrospirae MTB in the metal sulfide chimney suggest the magnetosome-bearing cells might belong to Nitrospirae. However, the lack of magnetosome chains in the observed cells is distinct from those observed in Nitrospirae MTB. A near-complete genome that is representative of the dominant chimney Nitrospirae was reconstructed after metagenomics analysis of a whole microbial community. In contrast to the fact that previously known MTB have magnetosome-related genes bundled within magnetosome island, magnetosome-related genes were sparsely positioned in the reconstructed Nitrospirae genome. As homologues genes involved in the formation of magnetosome chains were deficient in the Nitrospirae genome, it is likely that the Nitrospirae genome is reconstructed from microbial cells associated with magnetosomes without chains. The ecological function of unaligned magnetosomes is speculated to attach magnetic vivianite particles for phosphate uptake, because phosphate is generally scarce in deep-sea environments. The further ecological and genomic investigations will shed light on the antiquity and evolutionary history of MTB.