Diversity And Characterization Of Marine Magnetotactic Bacteria In Intertidal Sediments

Magnetotactic bacteria (MTB) are Gram-negative, producing magnetosomes,motile prokaryotes that can orient and migrate along the geomagnetic lines. They areubiquitous in sediments and stratified water column and distribute predominantly inthe oxic-anoxic transition zone (OATZ). MTB have various morphotypes, of whichmulticellular magnetotactic prokaryotes (MMPs) are the special group. MMPs sharewith other MTB the unique capacity to swim along geomagnetic field lines, but aredistinguished by their multicellularity, swimming behavior, life cycle, andphylogenetic affiliation. Currently, two morphotypes of MMPs have been described:spherical mulberry-like and ellipsoidal pineapple-like.In this study, a combination of light and electron microscopic observation,ecological investigation, micromanipulation and molecular analysis have been used tostudy the vertical distribution of marine MTB and their relationships withenvironmental factors in the Huiquan Bay, to disclose the diversity of marine singleMTB and to characterize spherical MMPs in Lake Yuehu. Two sampling sites wereexploited; a typical intertidal zone Huiquan Bay is, and a relative stable lagoon-tidalinlet system-Lake Yuehu.Results showed that the most abundant MTB are coccoid-shaped ones which swimtoward the north pole in intertidal sediments of Huiquan Bay. MTB were concentratedin the reduced environments with a redox potential ranging from179.7mV to-107.0mV. The maximum abundance of MTB was observed at the yellow-black layerstransition with a depth of5-9cm in redox cline, where is an oxic-anoxic transitionzoon. The MTB exhibited a certain correlation with the vertical distribution of thegrain size, water content and sulfate concentration in the sediment.A large number of marine single MTB were collected from the intertidal sediments of Lake Yuehu, and the abundance could reach up to103-104cells/cm3. Diversemorphotypes were observed, including cocci, oval, vibrio, spirilla, rod-shaped, elliptic,handled-shaped and bar-shaped. And the magnetococci was most abundant. Thesemagnetococci have flagella arranged in parallel and dispersed in the tail within abundle. The length of flagella was1-2time longer than the size of cells bodies and thediameter was70.8±14.6nm. The majority of magnetosomes arranged in one, two ormultiple chains. Irregular arrangement was also evident. Morphology of themagnetosome crystals was species-specific. For example, prismatic-shapedmagnetosomes distributed in magnetococci, bullet-shaped magnetosomes existed inrod-shaped or magnetovibrio, also had cubo-octahedral and tooth or flake shaped.Composition of all these crystals were Fe3O4. In addition, These MTB containedseveral (1-4individuals) phosphorus granules.Morphological diversity of marine single MTB was parallel with restrictionfragment length polymorphism (RFLP) and16S rDNA sequence analysis. At least14different taxonomic groups were identified. Among them, thirteen belonged toAlphaproteobacteria and one exception was Gammaproteobacteria. Ten new speciesand four new genera were identified. The dominant species were uncultured marinemagnetococci. Compared to known axenic and uncultured marine MTB, the16SrDNA sequences of most MTB collected from Lake Yuehu exhibited sequenceidentities ranging from90.1%to96.2%(＜97%). These results indicate a specificgeo-distribution of MTB from Lake Yuehu.Spherical MMPs were mainly found in the subsurface layer of gray-black intertidalsediments (5-6cm in depth) of Lake Yuehu. These MMPs were5.57±0.91μm indiameter, composed of approximately16-32ovoid cells with a helical arrangement,had peritrichous flagella and lipid granules. MMPs contained both bullet-shapedmagnetite magnetosomes and irregular greigite magnetosomes that were arranged inchains or clusters and distributed near the cell periphery.While swimming freely, spherical MMPs moved along the magnetic field lineswith a helical trajectory and the velocity varied from17to82μm/s. MMPs displayedtypical Ping-Pong motion and negative phototaxis. Velocities of excursion and return were about124±53μm/s and93±39μm/s, respectively. Spherical MMPs swamquickly away from the illumination source (in the direction opposite to the appliedmagnetic field) when exposed to blue light (wave length:450-480nm), violet light(400-410nm) and UV light (330-385nm). In contrast, they did not react togreen-light illumination (510-550nm).The16S rRNA genes of micromanipulation-purified spherical MMPs were clonedand sequenced. Phylogenetic analysis revealed that the MMP species was affiliatedwith Deltaproteobacteria and displayed>2.8%sequence divergence with respect toall previously reported MMPs. We propose to name them as “CandidatusMagnetomorum rongchengroseum”. This is the first phylogenetic identification of aspherical MMP that produces both magnetite and greigite magnetosomes.This report increases and updates the biological characteristics of MMPs andenriches the unexpected diversity of marine MTB. It will help us to understand theadaptation mechanism of marine MTB to intertidal zone habitats and their role inbio-geochemical cycles.