dMagnetotactic bacteria have the unique capacity of being able to swim along geomagnetic field lines. They are Gram-negative bacteria with diverse morphologies and variable phylogenetic relatedness. Here, we describe a group of uncultivated marine magnetococci collected from intertidal sediments of Huiquan Bay in the Yellow Sea. They were coccoid-ovoid in morphology, with an average size of 2.8 ؎ 0.3 m by 2.0 ؎ 0.2 m. Differential interference contrast microscopy, fluorescence microscopy, and transmission electron microscopy revealed that each cell was apparently composed of two hemispheres. The cells synthesized iron oxide-type magnetosomes that clustered on one side of the cell at the interface between the two hemispheres. In some cells two chains of magnetosomes were observed across the interface. Each cell had two bundles of flagella enveloped in a sheath and displayed north-seeking helical motion. Two 16S rRNA gene sequences having 91.8% identity were obtained, and their authenticity was confirmed by fluorescence in situ hybridization. Phylogenetic analysis revealed that the magnetococci are affiliated with the Alphaproteobacteria and are most closely related to two uncultured magnetococci with sequence identities of 92.7% and 92.4%, respectively. Because they display a >7% sequence divergence to all bacteria reported, the bean-like magnetococci may represent two novel genera.
Magnetotactic bacteria (MTB) were first discovered independently by Bellini in 1964 and Blakemore in 1975 (3-5, 9). They are a morphologically, metabolically, and phylogenetically diverse assemblage of motile prokaryotes that can orient and navigate along geomagnetic field lines (2, 28). MTB contain intracellular membrane-bound, nano-sized, single-domain crystals termed magnetosomes, which usually consist of iron oxide (magnetite, Fe 3 O 4 ) or iron sulfide (greigite, Fe 3 S 4 ) (2). Magnetosome formation is genetically controlled, and the magnetic crystals have species-specific morphologies and specific arrangements within the cell (2). Magnetosomes usually organize in chains and form a magnetic dipole moment in the cell. This makes it possible for the cell to align to the Earth's magnetic field, which enables the bacterium to find and maintain an optimum position in the oxygen and chemical gradient (10,11). Magnetotactic bacteria are ubiquitous in the water column and sediments of freshwater and marine habitats and are believed to play an important role in iron cycling (2, 7).MTB comprise a variety of morphological types (including coccoid, spiral, vibroid, rod-like, or aggregated) (8, 27, 29) and have a great phylogenetic diversity. MTB have been identified in Proteobacteria and Nitrospirae, and a recent study indicates the existence of large ovoid-shaped MTB belonging to candidate division OP3 (1,14,(18)(19)(20). The coccoid is the dominant morphology among magnetotactic bacteria in most habitats (21,22,24,25,31,32), and the interactions of bacterial species in a given habitat with the prevailing environmental conditions are likel...