Magnetotactic bacteria (MtB) are prokaryotes that possess genes for the synthesis of membranebounded crystals of magnetite or greigite, called magnetosomes. Despite over half a century of studying MTB, only about 60 genomes have been sequenced. Most belong to Proteobacteria, with a minority affiliated with the Nitrospirae, Omnitrophica, Planctomycetes, and Latescibacteria. Due to the scanty information available regarding MTB phylogenetic diversity, little is known about their ecology, evolution and about the magnetosome biomineralization process. This study presents a large-scale search of magnetosome biomineralization genes and reveals 38 new MTB genomes. Several of these genomes were detected in the phyla Elusimicrobia, Candidatus Hydrogenedentes, and Nitrospinae, where magnetotactic representatives have not previously been reported. Analysis of the obtained putative magnetosome biomineralization genes revealed a monophyletic origin capable of putative greigite magnetosome synthesis. The ecological distributions of the reconstructed MTB genomes were also analyzed and several patterns were identified. These data suggest that open databases are an excellent source for obtaining new information of interest.

Magnetotactic bacteria are widely represented microorganisms that have the ability to synthesize magnetosomes. The magnetotactic cocci of the order Magnetococcales are the most frequently identified, but their classification remains unclear due to the low number of cultivated representatives. This paper reports the analysis of an uncultivated magnetotactic coccus UR-1 collected from the Uda River (in eastern Siberia). Genome analyses of this bacterium and comparison to the available Magnetococcales genomes identified a novel species called “Ca. Magnetaquicoccus inordinatus,” and a delineated candidate family “Ca. Magnetaquicoccaceae” within the order Magnetococcales is proposed. We used average amino acid identity values <55–56% and <64–65% as thresholds for the separation of families and genera, respectively, within the order Magnetococcales. Analyses of the genome sequence of UR-1 revealed a potential ability for a chemolithoautotrophic lifestyle, with the oxidation of a reduced sulfur compound and carbon assimilation by rTCA. A nearly complete magnetosome genome island, containing a set of mam and mms genes, was also identified. Further comparative analyses of the magnetosome genes showed vertical inheritance as well as horizontal gene transfer as the evolutionary drivers of magnetosome biomineralization genes in strains of the order Magnetococcales.

Magnetotactic bacteria (MTB) belong to several phyla. This class of microorganisms exhibits the ability of magneto-aerotaxis. MTB synthesize biominerals in organelle-like structures called magnetosomes, which contain single-domain crystals of magnetite (Fe3O4) or greigite (Fe3S4) characterized by a high degree of structural and compositional perfection. Magnetosomes from dead MTB could be preserved in sediments (called fossil magnetosomes or magnetofossils). Under certain conditions, magnetofossils are capable of retaining their remanence for millions of years. This accounts for the growing interest in MTB and magnetofossils in paleo- and rock magnetism and in a wider field of biogeoscience. At the same time, high biocompatibility of magnetosomes makes possible their potential use in biomedical applications, including magnetic resonance imaging, hyperthermia, magnetically guided drug delivery, and immunomagnetic analysis. In this review, we attempt to summarize the current state of the art in the field of MTB research and applications.

20Magnetotactic bacteria (MTB) belong to different taxonomic groups according to 16S 21 rRNA or whole-genome phylogeny. Magnetotactic representatives of the class 22 Alphaproteobacteria and the order Magnetococcales are the most frequently isolated 23 MTB in environmental samples. This bias is due in part to limitations of currently 24 2 available methods to isolate MTB. Here we describe a new approach for isolation of 25 MTB cells that does not depend on cell motility and will allow collecting bacteria 26 both south-and north-seeking movement. We also designed a specific primer system 27 for the gene encoding the MamK protein that effectively detects diverse MTB 28 phylogenetic groups in any sample type. The combination of these two approaches 29 allowed the identification of a novel MTB belonging to the family Syntrophaceae of 30 the class Deltaproteobacteria. Moreover, we found that Nitrospirae bacteria 31 predominated in the MTB fraction of a sample taken from Lake Beloe Bordukovskoe 32 near Moscow, Russia. We describe the novel dominant Nitrospirae bacterium 33 'Candidatus Magnetomonas plexicatena' and propose its taxonomic name. 34 IMPORTANCE 35Among magnetotactic bacteria (MTB), the members of phyla Proteobacteria, 36 Nitrospirae and 'Ca. Omnitrophica' have been studied extensively using the existing 37 approaches. However, in recent years, analyses of the metagenomic databases have 38 revealed the presence of MTB in phylogenetic groups, which had not been previously 39 detected using standard approaches. This finding indicates that the biodiversity of 40 MTB is much broader than is currently known. The difficulty of identifying MTB 41 based on comparative analysis of 16S rRNA genes lies in the existence of closely 42 related species of non-magnetotactic bacteria. Moreover, there is an absence of 16S 43 rRNA MTB sequences from such taxonomic groups as 'Latescibacteria' and 44 Planctomycetes. In addition, the standard methods of separating MTB can benefit 45 bacteria with high motility. Developing novel strategies for investigation offers great 46 3 promise towards identifying MTB groups. We have proposed new approach to 47 separate MTB cells from environmental samples and have also proposed a specific 48 primer system for the MTB identification. 49 INTRODUCTION 50 Prokaryotes having directed active movement that is guided by geomagnetism are 51 collectively called magnetotactic bacteria (MTB) (1). The term MTB has no 52 taxonomic meaning such that it representatives are physiologically, morphologically 53 and phylogenetically different and share only the ability to synthesize special 54 organelles called magnetosomes. Magnetosomes consist of nanosized magnetite 55 (Fe 3 O 4 ) (2) or greigite (Fe 3 S 4 ) (3-5) crystals surrounded by a lipid bilayer membrane 56having proteins specific to the organelle (6, 7). Magnetosomes frequently assemble 57 into chains inside the cell (8). MTB evolved the ability to conduct a special type of 58 movement called magnetotaxis, which is based on orientation relative...

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According to 16S rRNA gene-or genome-based phylogeny, magnetotactic bacteria (MTB) belong to diverse taxonomic groups. Here we analyzed the diversity of MTB in a sample taken from the freshwater lake Beloe Bordukovskoe near Moscow, Russia, by using molecular identification based on sequencing of the 16S rRNA gene and mamK, a specific marker gene for these bacteria. A protein encoded by the mamK gene is involved in magnetosome chain arrangement inside the cell. As a result, six operational taxonomic units (OTUs) of MTB were identified. Among them, OTUs affiliated with the phylum Nitrospirae were predominant. 'Ca.Etaproteobacteria' and Alphaproteobacteria represented the minor groups of MTB. We also identified a novel MTB belonging to the family Syntrophaceae of the Deltaproteobacteria class.Using a combination of fluorescence and transmission electron microscopy, the bacteria belonging to these new MTB groups were visualized. Electron microscopy revealed that Syntrophaceae MTB were rod-shaped and synthesized elongated magnetosomes, arranged as a disorganized cluster. Among the Nitrospirae group, two groups with vibrioid cell shape and one group of ovoid-shaped bacteria were identified, all of which had elongated magnetosome crystals consisting of magnetite.

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