Characterization Of Marine Magnetotactic Bacteria From Intertidal Sediments Of Jiaozhou Bay

Magnetotactic bacteria (MTB) consist of gram negative bacteria with diverse morphologies, which can orient and navigate along geomagnetic field lines. Magnetotactic bacteria contain intracellular membrane-bounded, nano-sized, and single-domain crystals, called magnetosomes, which mainly consist of iron oxide (magnetite, Fe₃O₄) or iron sulfide (greigite, Fe₃S₄). Magnetosome formation is a genetically controlled process, and the magnetic crystals display species-specific morphologies and exhibit specific arrangements within the cell. Magnetotactic bacteria are ubiquitous in water columns and sediments of freshwater or marine habitats, and are believed to play an important role in biogeochemical circulation of Fe and S。To assess the correlation between their evolutionary relatedness and geographic distribution, we analyzed 239 16S rDNA sequences available in the GenBank, which were affiliated with the Proteobacteria and Nitrospira phylum. It was found that magnetotactic bacteria of freshwater distinguish from that of maine environment; and magnetotactic bacteria are closer from ocean or freshwater, that is similar species have a large geographic distribution and tend to adapt the similar habitats. In addition, morphotypes of magnetotactic bacteria and their living environment conditions have a significant relevance. This observation suggests that magnetotactic bacteria may have multiple evolutionary origins.In the present study, we also studied the diversity of uncultured magnetotactic bacteria from intertidal sediments of Jiaozhou Bay in Qingdao, China, using traditional methods, such as the microscope technology. We found three kinds of magnetotactic bacteria which were reported for the first time, and all of them are dominant ones in some mirco environment, and show some interesting features in many respects, such as morphology, magnetosomes type, organization and motility. They represent novel models in the study of magnetotactic bacteria natural resources.We characterized a group of uncultivated marine magnetococci collected from intertidal sediments of Huiquan Bay. They displayed peculiar bean-like morphology with an average size of 2.8±0.3μm×2.0±0.2μm and synthesized iron-oxide type of magnetosomes arranged most as cluster, or less occurently in two chains. Two 16S rRNA gene sequences with 91.8% identity between them were obtained, and their authenticity to bean-like magnetococci was confirmed by Fluorescence in Situ Hybridization. They were affiliated with the Alphaproteobacteria, and most closely related to two uncultured magnetococci with sequence identity of 92.7% and 92.4%, respectively. Bean-like magnetococci had two bundles of flagella enveloped in a sheath. Individual cells displayed north-seeking helical motion. Interestingly, when inspected in droplets under aerobic conditions, we observed a circumfluent motion of the bacterial swarm. They first accumulated at the north edge of a droplet. Then they swam against the magnetic field lines toward interior of the droplet. The cell swarm diverged toward east and west and at the droplet edges returned back toward the north along the droplet border. The circumfluent swarm of magnetotactic bacteria has not been reported before and implies a complex magneto-aerotaxis mechanism.