Magnetosome magnetite biomineralization in a flagellated protist: evidence for an early evolutionary origin for magnetoreception in eukaryotes

The most well-recognized magnetoreception behaviour is that of the magnetotactic bacteria (MTB), which synthesize membrane-bounded magnetic nanocrystals called magnetosomes via a biologically controlled process. The magnetic minerals identified in prokaryotic magnetosomes are magnetite (Fe₃O₄) and greigite (Fe₃S₄). Magnetosome crystals, regardless of composition, have consistent, species-specific morphologies and single-domain size range. Because of these features, magnetosome magnetite crystals possess specific properties in comparison to abiotic, chemically synthesized magnetite. Despite numerous discoveries regarding MTB phylogeny over the last decades, this diversity is still considered underestimated. Characterization of magnetotactic microorganisms is important as it might provide insights into the origin and establishment of magnetoreception in general, including eukaryotes. Here, we describe the magnetotactic behaviour and characterize the magnetosomes from a flagellated protist using culture-independent methods. Results strongly suggest that, unlike previously described magnetotactic protists, this flagellate is capable of biomineralizing its own anisotropic magnetite magnetosomes, which are aligned in complex aggregations of multiple chains within the cell. This organism has a similar response to magnetic field inversions as MTB. Therefore, this eukaryotic species might represent an early origin of magnetoreception based on magnetite biomineralization. It should add to the definition of parameters and criteria to classify biogenic magnetite in the fossil record.     

Newly isolated but uncultivated magnetotactic bacterium of the phylum Nitrospirae from Beijing, China

Magnetotactic bacteria (MTB) in the phylum Nitrospirae synthesize up to hundreds of intracellular bullet-shaped magnetite magnetosomes. In the present study, a watermelon-shaped magnetotactic bacterium (designated MWB-1) from Lake Beihai in Beijing, China, was characterized. This uncultivated microbe was identified as a member of the phylum Nitrospirae and represents a novel phylogenetic lineage with ≥6% 16S rRNA gene sequence divergence from all currently described MTB. MWB-1 contained 200 to 300 intracellular bullet-shaped magnetite magnetosomes and showed a helical swimming trajectory under homogeneous magnetic fields; its magnetotactic velocity decreased with increasing field strength, and vice versa. A robust phylogenetic framework for MWB-1 and all currently known MTB in the phylum Nitrospirae was constructed utilizing maximum-likelihood and Bayesian algorithms, which yielded strong evidence that the Nitrospirae MTB could be divided into four well-supported groups. Considering its population densities in sediment and its high numbers of magnetosomes, MWB-1 was estimated to account for more than 10% of the natural remanent magnetization of the surface sediment. Taken together, the results of this study suggest that MTB in the phylum Nitrospirae are more diverse than previously realized and can make important contributions to the sedimentary magnetization in particular environments.     

Diverse phylogeny and morphology of magnetite biomineralized by magnetotactic cocci

Magnetotactic bacteria (MTB) are diverse prokaryotes that produce magnetic nanocrystals within intracellular membranes (magnetosomes). Here, we present a large-scale analysis of diversity and magnetosome biomineralization in modern magnetotactic cocci, which are the most abundant MTB morphotypes in nature. Nineteen novel magnetotactic cocci species are identified phylogenetically and structurally at the single-cell level. Phylogenetic analysis demonstrates that the cocci cluster into an independent branch from other Alphaproteobacteria MTB, that is, within the Etaproteobacteria class in the Proteobacteria phylum. Statistical analysis reveals species-specific biomineralization of magnetosomal magnetite morphologies. This further confirms that magnetosome biomineralization is controlled strictly by the MTB cell and differs among species or strains. The post-mortem remains of MTB are often preserved as magnetofossils within sediments or sedimentary rocks, yet paleobiological and geological interpretation of their fossil record remains challenging. Our results indicate that magnetofossil morphology could be a promising proxy for retrieving paleobiological information about ancient MTB.     

趋磁细菌的磁小体

趋磁细菌是一类对磁场有趋向性反应的细菌,其菌体能吸收外界环境中铁元素并在体内合成包裹有膜的纳米磁性颗粒Fe₃O₄或Fe₃O_{3S4晶体即磁小体。综述了趋磁细菌的磁小体生物矿化的条件,以及趋磁细菌的铁离子吸收、磁小体囊泡的形成、铁离子的转运到磁小体囊泡及囊泡中受控的Fe3O4生物矿化的分子生物学和生物化学等方面的研究进展,重点介绍了趋磁细菌磁小体合成机制的研究进展及未来研究磁小体的发展方向。}     

Genetics and cell biology of magnetosome formation in magnetotactic bacteria

The ability of magnetotactic bacteria (MTB) to orient in magnetic fields is based on the synthesis of magnetosomes, which are unique prokaryotic organelles comprising membrane-enveloped, nano-sized crystals of a magnetic iron mineral that are aligned in well-ordered intracellular chains. Magnetosome crystals have species-specific morphologies, sizes, and arrangements. The magnetosome membrane, which originates from the cytoplasmic membrane by invagination, represents a distinct subcellular compartment and has a unique biochemical composition. The roughly 20 magnetosome-specific proteins have functions in vesicle formation, magnetosomal iron transport, and the control of crystallization and intracellular arrangement of magnetite particles. The assembly of magnetosome chains is under genetic control and involves the action of an acidic protein that links magnetosomes to a novel cytoskeletal structure, presumably formed by a specific actin-like protein. A total of 28 conserved genes present in various magnetic bacteria were identified to be specifically associated with the magnetotactic phenotype, most of which are located in the genomic magnetosome island. The unique properties of magnetosomes attracted broad interdisciplinary interest, and MTB have recently emerged as a model to study prokaryotic organelle formation and evolution.     

Acidthiobacillus ferrooxidans中磁小体的提取

At.f和趋磁细菌在生理特性和生长环境有一定的相似性,而且镜检发现At.f具有趋磁性,所以本文采用了趋磁细菌中磁小体的提取方法尝试提取At.f中的磁小体,用超声波破碎At.f后,以磁铁吸取其体内的磁性颗粒,经过检测,发现其体内确实存在含铁元素的磁性颗粒。提取粗样品经过电镜分析,证实其体内存在着少量由脂质包裹的磁小体。磁小体悬浮液经过蔗糖密度梯度离心纯化后,对其作透射电镜,可以清晰的看到磁小体。实验结果表明,At.f体内存在少量的磁小体,正是由于磁小体的存在,才使得At.f在外加磁场作用下发生磁生物效应。这是首次发现从酸性矿坑水分离的At.f具有趋磁性,并从中提取到了磁小体,可以利用At.f的趋磁性将其按照不同磁性进行分离,从而获得活性高的、对不同磁性矿物有特异性的高效浸矿菌种。     

Manganese in biogenic magnetite crystals from magnetotactic bacteria

Magnetotactic bacteria produce either magnetite (Fe₃O₄) or greigite (Fe₃S₄) crystals in cytoplasmic organelles called magnetosomes. Whereas greigite magnetosomes can contain up to 10 atom% copper, magnetite produced by magnetotactic bacteria was considered chemically pure for a long time and this characteristic was used to distinguish between biogenic and abiogenic crystals. Recently, it was shown that magnetosomes containing cobalt could be produced by three strains of Magnetospirillum . Here we show that magnetite crystals produced by uncultured magnetotactic bacteria can incorporate manganese up to 2.8 atom% of the total metal content (Fe+Mn) when manganese chloride is added to microcosms. Thus, chemical purity can no longer be taken as a strict prerequisite to consider magnetite crystals to be of biogenic origin.     

Comparative genomic analysis of magnetotactic bacteria from the Deltaproteobacteria provides new insights into magnetite and greigite magnetosome genes required for magnetotaxis

Magnetotactic bacteria (MTB) represent a group of diverse motile prokaryotes that biomineralize magnetosomes, the organelles responsible for magnetotaxis. Magnetosomes consist of intracellular, membrane‐bounded, tens‐of‐nanometre‐sized crystals of the magnetic minerals magnetite (Fe₃O₄) or greigite (Fe₃S₄) and are usually organized as a chain within the cell acting like a compass needle. Most information regarding the biomineralization processes involved in magnetosome formation comes from studies involving Alphaproteobacteria species which biomineralize cuboctahedral and elongated prismatic crystals of magnetite. Many magnetosome genes, the mam genes, identified in these organisms are conserved in all known MTB. Here we present a comparative genomic analysis of magnetotactic Deltaproteobacteria that synthesize bullet‐shaped crystals of magnetite and/or greigite. We show that in addition to mam genes, there is a conserved set of genes, designated mad genes, specific to the magnetotactic Deltaproteobacteria, some also being present in Candidatus Magnetobacterium bavaricum of the Nitrospirae phylum, but absent in the magnetotactic Alphaproteobacteria. Our results suggest that the number of genes associated with magnetotaxis in magnetotactic Deltaproteobacteria is larger than previously thought. We also demonstrate that the minimum set of mam genes necessary for magnetosome formation in Magnetospirillum is also conserved in magnetite‐producing, magnetotactic Deltaproteobacteria. Some putative novel functions of mad genes are discussed.     

趋磁细菌及磁小体在肿瘤治疗中的应用

提高抗肿瘤药物的靶向性是肿瘤治疗、降低药物副作用的重要手段。在肿瘤组织内部由于癌细胞的快速增殖致使其形成低氧区,低氧区会对多种肿瘤治疗方案产生耐受。趋磁细菌 (Magnetotactic bacteria, MTB) 是一类能在细胞内产生外包生物膜、纳米尺寸、单磁畴磁铁矿 (Fe3O4) 或硫铁矿 (Fe3S4) 晶体颗粒-磁小体的微生物的统称。在磁场的作用下,趋磁细菌可凭借鞭毛运动至厌氧区。趋磁细菌在动物体内毒性较低且生物相容性良好,其磁小体与人工合成的磁性纳米材料相比优势显著。文中在介绍趋磁细菌及其磁小体生物学特点、理化性能的基础上,综述了趋磁细菌作为载体偶联药物进入肿瘤内部,并通过感受低氧信号定位于肿瘤低氧区,以及趋磁细菌竞争肿瘤细胞铁源的研究进展,总结了磁小体运载化疗药物、抗体、DNA疫苗靶向结合肿瘤的研究进展,分析了趋磁细菌及磁小体肿瘤治疗中面临的问题,并对趋磁细菌和磁小体在肿瘤治疗中的应用进行了展望。     

The biomineralization of magnetosomes in <Emphasis Type="Italic">Magnetospirillum gryphiswaldense</Emphasis>

Magnetotactic bacteria (MTB) are major constituents of natural microbial communities in sediments and chemically stratified water columns. The ability of MTB to migrate along magnetic field lines is based on specific intracellular structures, the magnetosomes, which, in most MTB, are nanometer-sized, membrane-bound magnetic particles consisting of the iron mineral magnetite (Fe₃O₄). A broad diversity of morphological forms has been found in various MTB. The unique characteristics of bacterial magnetosomes have attracted a broad interdisciplinary research interest. The magnetosome membrane (MM) in Magnetospirillum gryphiswaldense contains a number of specific Mam proteins. Several mam genes were analyzed and assigned to different genomic regions. Many of the Mam proteins are highly conserved in other MTB but display low sequence similarity to any proteins from nonmagnetic organisms. Electronic Publication     

Magnetotactic bacteria: nanodrivers of the future

Magnetotactic bacteria (MTB) represent a heterogeneous group of Gram-negative aquatic prokaryotes with a broad range of morphological types, including vibrioid, coccoid, rod and spirillum. MTBs possess the virtuosity to passively align and actively swim along the magnetic field. Magnetosomes are the trademark nano-ranged intracellular structures of MTB, which comprise magnetic iron-bearing inorganic crystals enveloped by an organic membrane, and are dedicated organelles for their magnetotactic lifestyle. Magnetosomes endue high and even dispersion in aqueous solutions compared with artificial magnetites, claiming them as paragon nanomaterials. MTB and magnetosomes offer high technological potential in modern science, technology and medicines. This review focuses on the applicability of MTB and magnetosomes in various areas of modern benefits.     

Uncultivated Magnetotactic Cocci from Yuandadu Park in Beijing,China

In the present study, we investigated a group of uncultivated magnetotactic cocci, which was magnetically isolated from a freshwater pond in Beijing, China. Light and transmission electron microscopy showed that these cocci ranged from 1.5 to 2.5 μm and contained two to four chains of magnetite magnetosomes, which sometimes were partially disorganized. Overall, the size of the disorganized magnetosomes was significantly smaller than that arranged in chains. All characterized magnetosome crystals were elongated (shape factor = 0.64) and fall into the single-domain size range (30 to 115 nm). Comparative 16S rRNA gene sequence analysis and fluorescence in situ hybridization showed that the enriched bacteria were a virtually homogeneous population and represented a novel lineage in the Alphaproteobacteria. The closest cultivated relative was magnetotactic coccoid strain MC-1 (88% sequence identity). First-order reversal curve diagrams revealed that these cocci had relatively strong magnetic interactions compared to the single-chain magnetotactic bacteria. Low-temperature magnetic measurements showed that the Verwey transition of them was ∼108 K, confirming magnetite magnetosomes, and the delta ratio δ_FC/δ_ZFC was >2. Based on the structure, phylogenetic position and magnetic properties, the enriched magnetotactic cocci of Alphaproteobacteria are provisionally named as “Candidatus Magnetococcus yuandaducum.”Magnetotactic bacteria (MTB) can mineralize intracellular nanosized iron oxides or sulfides called magnetosomes, which in most MTB are normally single-domain (SD) magnetite with a narrow range of grain sizes from 30 to 120 nm (3). The chain configuration of magnetosomes renders MTB able to navigate the oxic-anoxic interface in chemically stratified environments by swimming along the Earth''s magnetic field (13). Diverse MTB, including coccoid, spirillar, rod-shaped bacteria and multicellular magnetotactic prokaryotes with one, two, or more chains of magnetosomes, thrive in a broad range of aquatic environments, which sometimes even are dominant strains of the microbial biomass in sediment (10, 47). Based on their phylogeny, all currently known MTB can be divided into two taxonomic groups: Proteobacteria and Nitrospira phyla (2).When MTB die, the magnetosomes can be preserved in sediment as fossil magnetosomes (or magnetofossils) (6). Fossil magnetosomes have been found in lacustrine and deep-sea sediments (6, 35, 43, 51), which are stable carriers of natural remanence and may play substantial contributions to the bulk magnetization of sediments due to their SD sizes (6, 19, 26, 32, 33). Moreover, since most known MTB are microaerophilic or anaerobic and are concentrated in the oxic-anoxic transition zone, the presence and characteristics of MTB species in vertically stratified sediments can be used as a potential paleoenvironmental proxy (19, 44, 45). However, how to identify bacterial magnetite or greigite (Fe₃S₄) in sediments is still challenging. Recently, characterizing the magnetic properties of MTB has attracted increasing interests because magnetic techniques are fast and effective in distinguishing bacterial crystals from abiogenic magnetic minerals in sediments (11, 26, 27, 33, 38).In spite of their wide distribution and abundance in aquatic environments, most MTB are intractable, and so far only a few of them, e.g., Magnetospirillum gryphiswaldense strain MSR-1 (41), M. magnetotacticum strain AMB-1 (17), M. magnetotacticum strain MS-1 (4), and magnetotactic coccoid strain MC-1 (24), can be grown in pure culture. Until recently, most insights into the molecular characterizations and magnetic properties of MTB have been based on pure cultures, which have a single magnetosome chain per cell (10, 11, 19, 20, 25, 27, 28, 37, 38, 42, 52). However, knowledge of uncultivated MTB, especially strains with multiple chains of magnetosomes that are commonly encountered in natural environments, remains limited. In the present study, we investigated a population of uncultivated magnetotactic cocci with multiple magnetosome chains, which were abundant in the pond in Yuan Dynasty Capital City Wall Relics Park (Yuandadu Park) in Beijing, China, in order to characterize their morphological features, phylogenetic positions, and magnetic properties and finally to classify them in a provisional Candidatus taxon.     

Single‐cell genomics of uncultivated deep‐branching magnetotactic bacteria reveals a conserved set of magnetosome genes

While magnetosome biosynthesis within the magnetotactic Proteobacteria is increasingly well understood, much less is known about the genetic control within deep‐branching phyla, which have a unique ultrastructure and biosynthesize up to several hundreds of bullet‐shaped magnetite magnetosomes arranged in multiple bundles of chains, but have no cultured representatives. Recent metagenomic analysis identified magnetosome genes in the genus ‘Candidatus Magnetobacterium’ homologous to those in Proteobacteria. However, metagenomic analysis has been limited to highly abundant members of the community, and therefore only little is known about the magnetosome biosynthesis, ecophysiology and metabolic capacity in deep‐branching MTB. Here we report the analysis of single‐cell derived draft genomes of three deep‐branching uncultivated MTB. Single‐cell sorting followed by whole genome amplification generated draft genomes of Candidatus Magnetobacterium bavaricum and Candidatus Magnetoovum chiemensis CS‐04 of the Nitrospirae phylum. Furthermore, we present the first, nearly complete draft genome of a magnetotactic representative from the candidate phylum Omnitrophica, tentatively named Candidatus Omnitrophus magneticus SKK‐01. Besides key metabolic features consistent with a common chemolithoautotrophic lifestyle, we identified numerous, partly novel genes most likely involved in magnetosome biosynthesis of bullet‐shaped magnetosomes and their arrangement in multiple bundles of chains.     

Identification and characterization of magnetotactic Gammaproteobacteria from a salt evaporation pool,Bohai Bay,China

Magnetotactic bacteria (MTB) are phylogenetically diverse prokaryotes that can produce intracellular chain-assembled nanocrystals of magnetite (Fe₃O₄) or greigite (Fe₃S₄). Compared with their wide distribution in the Alpha-, Eta- and Delta-proteobacteria classes, few MTB strains have been identified in the Gammaproteobacteria class, resulting in limited knowledge of bacterial diversity and magnetosome biomineralization within this phylogenetic branch. Here, we identify two magnetotactic Gammaproteobacteria strains (tentatively named FZSR-1 and FZSR-2 respectively) from a salt evaporation pool in Bohai Bay, at the Fuzhou saltern, Dalian City, eastern China. Phylogenetic analysis indicates that strain FZSR-2 is the same species as strains SHHR-1 and SS-5, which were discovered previously from brackish and hypersaline environments respectively. Strain FZSR-1 represents a novel species. Compared with strains FZSR-2, SHHR-1 and SS-5 in which magnetite particles are assembled into a single chain, FZSR-1 cells form relatively narrower magnetite nanoparticles that are often organized into double chains. We find a good relationship between magnetite morphology within strains FZSR-2, SHHR-1 and SS-5 and the salinity of the environment in which they live. This study expands the bacterial diversity of magnetotactic Gammaproteobacteria and provides new insights into magnetosome biomineralization within magnetotactic Gammaproteobacteria.     

Common ancestry of iron oxide- and iron-sulfide-based biomineralization in magnetotactic bacteria

Magnetosomes are prokaryotic organelles produced by magnetotactic bacteria that consist of nanometer-sized magnetite (Fe₃O₄) or/and greigite (Fe₃S₄) magnetic crystals enveloped by a lipid bilayer membrane. In magnetite-producing magnetotactic bacteria, proteins present in the magnetosome membrane modulate biomineralization of the magnetite crystal. In these microorganisms, genes that encode for magnetosome membrane proteins as well as genes involved in the construction of the magnetite magnetosome chain, the mam and mms genes, are organized within a genomic island. However, partially because there are presently no greigite-producing magnetotactic bacteria in pure culture, little is known regarding the greigite biomineralization process in these organisms including whether similar genes are involved in the process. Here using culture-independent techniques, we now show that mam genes involved in the production of magnetite magnetosomes are also present in greigite-producing magnetotactic bacteria. This finding suggest that the biomineralization of magnetite and greigite did not have evolve independently (that is, magnetotaxis is polyphyletic) as once suggested. Instead, results presented here are consistent with a model in which the ability to biomineralize magnetosomes and the possession of the mam genes was acquired by bacteria from a common ancestor, that is, the magnetotactic trait is monophyletic.     

Mass collection of magnetotactic bacteria from the permanently stratified ferruginous Lake Pavin,France

Obtaining high biomass yields of specific microorganisms for culture-independent approaches is a challenge faced by scientists studying organism's recalcitrant to laboratory conditions and culture. This difficulty is highly decreased when studying magnetotactic bacteria (MTB) since their unique behaviour allows their enrichment and purification from other microorganisms present in aquatic environments. Here, we use Lake Pavin, a permanently stratified lake in the French Massif Central, as a natural laboratory to optimize collection and concentration of MTB that thrive in the water column and sediments. A method is presented to separate MTB from highly abundant abiotic magnetic particles in the sediment of this crater lake. For the water column, different sampling approaches are compared such as in situ collection using a Niskin bottle and online pumping. By monitoring several physicochemical parameters of the water column, we identify the ecological niche where MTB live. Then, by focusing our sampling at the peak of MTB abundance, we show that the online pumping system is the most efficient for fast recovering of large volumes of water at a high spatial resolution, which is necessary considering the sharp physicochemical gradients observed in the water column. Taking advantage of aerotactic and magnetic MTB properties, we present an efficient method for MTB concentration from large volumes of water. Our methodology represents a first step for further multidisciplinary investigations of the diversity, metagenomic and ecology of MTB populations in Lake Pavin and elsewhere, as well as chemical and isotopic analyses of their magnetosomes.     

Monophyletic origin of magnetotaxis and the first magnetosomes

Horizontal gene transfer (HGT), the transfer of genetic material other than by descent, is thought to have played significant roles in the evolution and distribution of genes in prokaryotes. These include those responsible for the ability of motile, aquatic magnetotactic bacteria (MTB) to align and swim along magnetic field lines and the biomineralization of magnetosomes that are responsible for this behaviour. There is some genomic evidence that HGT might be responsible for the distribution of magnetosome genes in different phylogenetic groups of bacteria. For example, in the genomes of a number of MTB, magnetosome genes are present as clusters within a larger structure known as the magnetosome genomic island surrounded by mobile elements such as insertion sequences and transposases as well as tRNA genes. Despite this, there is no strong direct proof of HGT between these organisms. Here we show that a phylogenetic tree based on magnetosome protein amino acid sequences from a number of MTB was congruent with the tree based on the organisms' 16S rRNA gene sequences. This shows that evolution and divergence of these proteins and the 16S rRNA gene occurred similarly. This suggests that magnetotaxis originated monophyletically in the Proteobacteria phylum and implies that the common ancestor of all Proteobacteria was magnetotactic.     

Controlled Biomineralization of Magnetite (Fe(inf3)O(inf4)) and Greigite (Fe(inf3)S(inf4)) in a Magnetotactic Bacterium

A slowly moving, rod-shaped magnetotactic bacterium was found in relatively large numbers at and below the oxic-anoxic transition zone of a semianaerobic estuarine basin. Unlike all magnetotactic bacteria described to date, cells of this organism produce single-magnetic-domain particles of an iron oxide, magnetite (Fe(inf3)O(inf4)), and an iron sulfide, greigite (Fe(inf3)S(inf4)), within their magnetosomes. The crystals had different morphologies, being arrowhead or tooth shaped for the magnetite particles and roughly rectangular for the greigite particles, and were coorganized within the same chain(s) in the same cell with their long axes along the chain direction. Because the two crystal types have different crystallochemical characteristics, the findings presented here suggest that the formation of the crystal types is controlled by separate biomineralization processes and that the assembly of the magnetosome chain is controlled by a third ultrastructural process. In addition, our results show that in some magnetotactic bacteria, external environmental conditions such as redox and/or oxygen or hydrogen sulfide concentrations may affect the composition of the nonmetal part of the magnetosome mineral phase.     

Characterization of a homogeneous taxonomic group of marine magnetotactic cocci within a low tide zone in the China Sea

Magnetotactic bacteria are a heterologous group of motile prokaryotes, ubiquitous in aquatic habitats and cosmopolitan in distribution. Here, we studied the diversity of magnetotactic bacteria in a seawater pond within an intertidal zone at Huiquan Bay in the China Sea. The pond is composed of a permanently submerged part and a low tide subregion. The magnetotactic bacteria collected from the permanently submerged part display diversity in morphology and taxonomy. In contrast, we found a virtually homogenous population of ovoid-coccoid magnetotactic bacteria in the low tide subregion of the pond. They were bilophotrichously flagellated and exhibited polar magnetotactic behaviour. Almost all cells contained two chains of magnetosomes composed of magnetite crystals. Intriguingly, the combination of restriction fragment length polymorphism analysis (RFLP) and sequencing of cloned 16S rDNA genes from the low tide subregion samples as well as fluorescence in situ hybridization (FISH) revealed the presence of a homogenous population. Moreover, phylogenetic analysis indicated that the Qingdao Huiquan low tide magnetotactic bacteria belong to a new genus affiliated with the α-subclass of Proteobacteria . This finding suggests the adaptation of the magnetotactic bacterial population to the marine tide.     

Anomalous Magnetic Orientations of Magnetosome Chains in a Magnetotactic Bacterium: Magnetovibrio blakemorei Strain MV-1

There is a good deal of published evidence that indicates that all magnetosomes within a single cell of a magnetotactic bacterium are magnetically oriented in the same direction so that they form a single magnetic dipole believed to assist navigation of the cell to optimal environments for their growth and survival. Some cells of the cultured magnetotactic bacterium Magnetovibrio blakemorei strain MV-1 are known to have relatively wide gaps between groups of magnetosomes that do not seem to interfere with the larger, overall linear arrangement of the magnetosomes along the long axis of the cell. We determined the magnetic orientation of the magnetosomes in individual cells of this bacterium using Fe 2p X-ray magnetic circular dichroism (XMCD) spectra measured with scanning transmission X-ray microscopy (STXM). We observed a significant number of cases in which there are sub-chains in a single cell, with spatial gaps between them, in which one or more sub-chains are magnetically polarized opposite to other sub-chains in the same cell. These occur with an estimated frequency of 4.0±0.2%, based on a sample size of 150 cells. We propose possible explanations for these anomalous cases which shed insight into the mechanisms of chain formation and magnetic alignment.