Isolation, Identification And Evolution Of Multicellular Magnetotactic Prokaryotes In Intertidal Zone

Multicellular magnetotactic prokaryotes (MMPs) are a peculiar group ofmagnetotactic bacteria that are assembled from10to100cells with magnetosomes.As a model organisms, MMPs can be applied to study on life origin and evolution,cell differentiation and biomineralization. Currently, two MMP morphotypes havebeen phylogenetically identified: spherical mulberry-like (several species) andellipsoidal pineapple-like (previously one species). In this study，three species ofpineapple-like MMPs and one species of unpublished pinecone-like MMPs werediscovered in intertidal sediments of Lake Yuehu, Drummond Island and Marseilles,respectively. After magnetic collection of the samples from various intertidal sites,four species of MMPs were isolated and studied with respect to their microstructure,motility, division and phylogenetics via microscopic, micromanipulation, andmolecular biological technique approaches. Their evolutionary affiliation wasdetermined though phylogenetic analysis.The three species of pineapple-like MMPs were collected from Lake Yuehu,Drummond Island and Marseilles, respectively. Analysis of16S rRNA gene sequencesrevealed that they display7.8%sequence divergence, thus probably belonging todifferent species but affiliated to same genus. Although they are different in sizes, thethree kinds of ellipsoidal MMPs assembled of cells arranged in six to eight interlacedcircles. They showed autofluorescence when exposed to light with differerntwavelengths. The cells contain lipid granules and bullet-shaped magnetitemagnetosomes. All MMPs presented negative phototaxis and complicated ping-pongmotility. For the pineapple-like MMPs in Lake Yuehu, a morphological differentiation was discovered due to existence of the triangle-shaped and H-shaped cells in MMPs.Although most MMPs contained bullet-shaped magnetite magnetosomes, someMMPs could produce bullet-shaped magnetite plus equidimensional greigitemagnetosomes. During reproduction, they divided along either their long-or short-body axes.A morphotype of MMPs named pinecone-like MMPs have been previously observedin the samples collected in Qingdao by our laboratory. Here they were discovered atthe sampling site in Lake Yuehu. The MMP，looked like a pinecone via lightmicroscope, was spherical aggregates with a diameter of13.4±1.3μm and composedwith33±5cells arranged radiationally around its center. They also displayedautofluorescence when exposed to light with differernt wavelengths. The cellssynthesized lipid granules and bullet-shaped magnetite magnetosomes. With thecooperation of peritrichous flagella and cells, the MMPs carried out free motion,rotation, and walking, but no apparent ping-pong motion. Same to mulberry-like andpineapple-like MMPs, this type of MMPs also did negative phototaxis. Duringreproduction, they divided along their short-body axes as a heart-shaped torsion.Analysis of16S rRNA gene sequences determined for the first time that thepinecone-like MMPs were affiliated to Deltaproteobacteria but diverge from otherknown MMPs by12.6%, representing thus a novel genus.Taken together, phylogenetic analysis and the cellular characterization have classifiedmulberry-like, pineapple-like and pinecone-like MMPs into three novel cladesbelonging to family Desulfobacterales in order of Deltaproteobacteria. The novelDesulfobacterales family consist species of multicellular magnetotactic organisms.This work contribute to a comprehensive understanding of the evolution of MTB andMMPs.