Magnetotactic bacteria (MTB) are a group of microorganisms capable of orientating and swimming along magnetic fields because they contain intracellular biomineralized magnetosomes composed of magnetite (Fe3O4) or/and greigite (Fe3S4).They are ubiquitous in freshwater,brackish,and marine habitats,and are cosmopolitan in distribution.However,knowledge of their occurrence and distribution in seamount ecosystems is limited.We investigated the diversity and distribution of MTB in the Caroline Seamount (CM4).The abundance of living MTB in 12 stations in depth varying from 90 to 1545 m was 1.1×103-43.7×103 inds./dm3.Despite diverse shapes of MTB observed,magnetotactic cocci were the dominant morphotype and could be categorized into two types:1) typical cocci that appeared to have peritrichous flagella;and 2) those characterized by having a drop-shaped form and one bundle of flagella located at the thin/narrow end of the cell.Transmission electron microscopy (TEM) analysis revealed that the magnetosomes formed by those magnetotactic cocci are magnetite (Fe3O4) with octahedral crystal habit.A total of 41 operational taxonomic units (OTUs) of putative MTB (2702 reads) were acquired from nine stations,based on high-throughput sequencing.Of these,40 OTUs belonged to the Proteobacteria phylum and one belonged to the Nitrospirae phylum.We found apparent connectivity between the MTB populations on the Caroline and Kexue (Science in Chinese) seamounts,although the diversity of MTB on Caroline was much richer than on the Kexue Seamount.Our results imply that the unique topography of seamonnts and other as-yet unclear environmental factors could lead to evolution of different flagella arrangements in magnetotactic cocci,and the occurrence of octahedral magnetite magnetosomes.