Proteins encoded by genes within
Magnetotactic bacteria are able to align their swimming direction to the geomagnetic field lines because they possess a magnetic moment. These bacteria biomineralize magnetic nanoparticles, magnetite or greigite, inside a membrane. The membrane + nanoparticle set is known as magnetosome and intracellular magnetosomes are disposed in a linear chain. Cytoskeleton-like filaments are resposible for the mechanical stability of this chain. The genes responsible for the magnetosome membrane and for the cytoskeleton proteins have been largely studied: the mam genes. The magnetosome chain also confers to the bacterial body a magnetic moment that can be measured through different physical techniques. Because of their response to magnetic field inversions, magnetotactic bacteria are good models to study bacterial motion. Theoretical and experimental studies show that magnetotactic bacteria swim following a trajectory similar to cylindrical helix. Magnetotactic microorganisms have been observed avoiding regions with UV or violet-blue light of high intensity. If the intensity is lower, magnetotactic microorganims show photokinesis, increasing their velocity in the presence of red light and decreasing their velocity in the presence of green light, both relative to the velocity with blue light.
- magnetotactic bacteria
- magnetic moment
- mam genes
Bacteria are one of the simplest organisms found in nature. They are distinguished from eukaryotes and superior organisms because their genetic material is not contained in a nucleus but is free in the cytoplasm. However, despite their relative simplicity, bacteria inhabit Earth for longer than many other organisms and constitute the most abundant type of cell on our planet . Magnetotactic bacteria (MTB) are microorganisms that biomineralize magnetic nanoparticles inside their cytoplasm. These magnetic nanoparticles are involved by a lipidic membrane, and each “membrane + magnetic nanoparticle” set is known as magnetosome . Magnetosomes are arranged in linear chains in the cytoplasm, conferring a magnetic moment to the bacterial body, being able to interact with the geomagnetic field to orient its navigational direction to the geomagnetic field lines. This response is known as magnetotaxis, resulting from the magnetic torque among the geomagnetic field and the magnetic moment of the magnetosome chain, and for that reason, MTB are described as “living compasses.” MTB were discovered independently by Salvatore Bellini in 1963 and Richard Blakemore in 1975 . The first to observe MTB was Salvatore Bellini, an Italian physician from Pavia, Italy. In 1958, physicians from Pavia were asked to analyze the quality of water for human consume. Bellini was part of the team that studied the water quality, and he observed in water samples some bacteria that consistently accumulate in one side of water drops. After trying different stimuli, he discovered that they were affected by magnets. He called that bacteria as magnetosensitive. The first published observation of MTB was done in 1975 by Richard Blakemore in Massachusetts, USA . His discovery was accidental, because his goal was to isolate
2. Biology of magnetotactic bacteria
2.1 Cell biology
MTB comprise a diverse group of prokaryotes that share the ability to synthesize magnetosomes, which are composed by a magnetic nanocrystal, magnetite (Fe3O4), or greigite (Fe3S4), enclosed by a lipid bilayer (Figure 1; ). Thus, MTB has no taxonomic meaning regarding phylogeny, morphology, and physiology. The morphology of cultured and uncultured MTB described until now are cocci, spirilla, rods, ovoids, vibrios, and multicellular spherical/ellipsoidal forms . In all morphotypes, magnetosomes are arranged in one or multiple chains along the major axis of the cell, imparting the cell a magnetic moment, as mentioned previously . However, in some uncultured MTB, apparently disorganized chains have already been observed [6, 7]. Besides the magnetosomes, other common features are the Gram-negative cell wall structure, motility by flagella and lipid, polyphosphate and sulfur inclusions .
MTB present microaerophilic or anaerobic metabolism and all inhabit aquatic environments characterized by vertical chemical stratification . When observing a drop of environmental sample containing MTB on a light microscope with a magnet next to the slide, MTB tend to accumulate in one of the borders of the drop, which correspond to a magnetic pole, North or South. In the Southern Hemisphere, when MTB are being observed they usually accumulate at the border of the drop of sample corresponding to the magnet’s North magnetic pole, swimming to Geomagnetic South pole indicated by a compass. Therefore, these bacteria are called South-seeking. In the Northern Hemisphere, the opposite situation occurs and MTB presenting this behavior are called North-seeking.
The study of MTB movement in an oxygen gradient showed that MTB change flagellar rotation and, consequently, the direction of movement depending on the oxygen concentration, migrating to the optimal conditions . These observations were used to infer MTB dislocation along the vertical chemical gradient in the environment, which is based on the vertical component of the magnetic field and depends on the cellular state regarding oxygen. In the upper layers of the chemically stratified environment, where oxygen is abundant and higher than the optimal for MTB growth, the bacterium is on the oxidized state and rotate flagella to migrate downward, where the environment is more reduced. While in reducing conditions, in which oxygen is not abundant and concentrations are lower than that required for MTB growth, bacteria rotate the flagella in the opposite direction to reach upper layers of the gradient with optimal oxygen concentrations. Therefore, the presence of specific cell structures in MTB, as the magnetosome chain, flagella, and storage inclusions, represent adaptive advantages for dislocation across the chemical gradient to explore resources in the environment. For example, during the day, the oxygen gradient changes among the stratified layers of the environment and microorganisms dislocate across these layers to reach regions with suitable conditions for survival and growth. Because MTB are microaerophilic and/or capable of anaerobic respiration, which means that they are sensitive to high oxygen concentrations, an efficient mechanism to orient and migrate in the environment guarantees species survival.
Only a few species of MTB have been isolated in axenic cultures , and fewer type strains are available in cell line repositories. Many uncultured MTB species have been characterized from environmental samples, because it is possible to purify these cells based on their response to an applied magnetic field using a magnet . Figure 2 shows examples of cultured and uncultured species of MTB and their characteristics according to their phylogenetic affiliation. Note that MTB are spread among different phyla in Bacteria domain and that greigite magnetosomes are only synthesized by MTB belonging to Deltaproteobacteria class.
According to studies performed in
In the environment, magnetotactic cocci are the most abundant morphotype of MTB and present high phylogenetic diversity and variety of size, shape, and organization of magnetosomes [10, 19]. For example, magnetotactic cocci have been found in marine sediments from Antarctica, suggesting the existence of psychrophilic MTB . Interestingly, these samples presented at least three types of magnetotactic cocci based on the magnetosomes crystal size, shape, and organization . Ultrastructure characterization of cultured magnetotactic cocci showed that these cells present two bundles of flagella and can achieve speed of approximately 300 μm/s, which is extremely high if we consider that the bacterium has nearly 1 μm. Each flagellar apparatus of
The origin of magnetotaxis and its distribution among the different phyla of the Bacteria domain are not well understood. Despite the great phylogenetic diversity, MTB have unique genes related to biomineralization, which are located in a generally unstable region in the genome . The genomic and ultrastructure characterizations of nonmagnetotactic spontaneous mutants of
The genes for biomineralization are grouped into four operons in
|Protein||Function||Effect on cell upon deletion||Reference/strain used on the study|
|MamA||Activation of biomineralization||Decrease in the number of magnetosomes|| Strain AMB-1|
|MamB||Transport of iron into the magnetosome vesicle; interacts with PDZ domains (MamE)||Magnetosomes are not produced; no magnetic response|| Strain AMB-1;  strain MSR-1|
|MamE||Crystal maturation||Synthesis of magnetosomes smaller than <20 nm; cells without magnetic response||[29, 33] Strain AMB-1|
|MamH||Balance in the redox state of iron on the magnetosome membrane||Decrease in magnetosome number and size; less efficient magnetic response|| Strain AMB-1;  strain MSR-1|
|MamI||Synthesis of the magnetosome membrane||Magnetosomes are not produced; no magnetic response|| Strain AMB-1|
|MamJ||Alignment of the magnetosome chain: it interacts with the surface of the magnetosome and with a structure similar to the cytoskeleton (MamK filament)||Magnetosomes are not aligned in chains; reduced magnetic orientation|| Strain MSR-1|
|MamK||Controls the assembly, segregation, and positioning of the magnetosome chain inside the cell||Alignment of the magnetosomes in short chains and decrease in the number of magnetosomes|| Strain MSR-1|
|MamM||Transport of iron into the magnetosome: magnetite nucleation, crystal growth, and stabilization of MamB||Increase in the formation of polycrystalline particles of magnetite; formation of crystals of hematite; no magnetic response|| Strain AMB-1;  strain MSR-1|
|MamO||Nucleation of the magnetite crystal||Magnetosomes are not produced; cells without magnetic response|| Strain AMB-1;  strain MSR-1|
|MamP||Magnetite crystal size control||Decrease in the size of magnetosomes; less efficient magnetic response||[29, 38] Strain AMB-1|
|Magnetite crystal size control||Decrease in the size of magnetosomes; less efficient magnetic response||[29, 38] Strain AMB-1|
|MamU||Possibly lipid metabolism||No changes were observed|| Strain AMB-1|
3. Physical characteristics of magnetosomes
Two different processes of mineral formation by living beings have been recognized. One process of mineral formation is known as biologically induced mineralization (BIM), and is characterized by bulk extracellular and/or intercellular mineral formation, without the elaboration of organic matrices. It produces minerals having crystal habits similar to those produced by precipitation from inorganic solutions. BIM processes are less controlled than organic matrix-mediated mineralization, and looks like a primitive stage in the evolution of biogenic mineral formation. The other process is known as biologically controlled mineralization (BCM). In general, the organism constructs an organic mold into which the appropriate ions are actively introduced to crystallize a mineral. The mineral type, orientation of crystallographic axes, and microarchitectures are under genetic control .
Magnetotactic bacteria distinguish from other bacteria because they biomineralize, through BCM, magnetic nanoparticles of magnetite, or greigite. Magnetite is a very interesting iron oxide because it is magnetic and a good conductor. Its free charge density is similar to that of some metals . It is also a hard mineral, being used by chitons for tooth hardening . Several studies show that greigite has similar electrical  and hardening use  as magnetite. The magnetic properties of nanoparticles have a strong dependence on the size: very small particles have a magnetic moment nonstable in the body, changing randomly its orientation and producing a null average magnetic moment. Those particles are known as superparamagnetic. If the size increases, the anisotropic energy also increases and creates an energy barrier that maintains the magnetic moment in a fixed direction. In that case, the nanoparticles behave as stable magnets and are known as single domains . Magnetosomes are in the size range of magnetic single domains. The magnetic flux lines created by the magnetosome in the chain can be observed using the magnetic electron holography technique , showing the flux lines entirely aligned to the chain as corresponds with a dipolar field created by a single magnetic moment. So, it is appropriate to say that the magnetosome chain behaves as a compass needle. The linear arrange of magnetosomes is not energetically stable, because after some number of magnetosomes the best configuration is a ring. To maintain the linear configuration, magnetosomes are attached to the cytoskeletal filaments .
The first analysis done in magnetosomes was energy-dispersive X-ray microanalysis (EDS or EDX), showing that they are composed mainly by iron and oxygen . To show that they are the iron oxide magnetite, the Mossbauer technique was used , showing that the Mossbauer spectra behave as a mixture of magnetite and maghemite. Also, electron diffraction shows the diffraction patterns corresponding to magnetite . Several studies with EDS show that this magnetite is highly pure. However, in some cases, some metallic ions can be absorbed in the magnetosome structure, depending on the ambient pollution . Studies done with high-resolution electron microscopy show that magnetosomes are produced in specific geometric morphologies . Those morphologies are truncated cuboctahedron, elongated cuboctahedron, and hexagonal prisms. In the case of greigite, the crystalline morphologies are truncated cuboctahedrons and elongated rectangular prisms . This iron sulfide was discovered in magnetosomes of multicellular magnetotactic prokaryotes, and identified through EDX spectroscopy and electron diffraction .
4. Determination of bacterial magnetic moment
As the magnetosome chain determines a magnetic moment to MTB, let us talk about the different techniques used to estimate that magnetic moment. The first theoretical estimate for the magnetic moment was done counting the contribution of several nanoparticles arranged in a chain . For a magnetosome chain composed of 22 particles of magnetite with every nanoparticle having 1.25 × 10−16 cm3 of volume, it is possible to calculate the total magnetic moment as M = nVindMV, where n is the number of particles in the chain, Vind is the volume of each particle (assuming that all are equal), and MV is the magnetic moment per unit volume of the magnetic material. For magnetite, MV = 480 × 10−3 Am2/cm3. In this way, a magnetic moment of 1.3 × 10−15 Am2 is obtained. This magnetic moment value means a magnetic to thermal energy rate of about 16 (assuming a temperature of 300 K). This method can be used whenever is possible to observe and count the number of magnetosomes in the chain. This method is not applied in the case of live bacteria and for bigger microorganisms with lots of magnetosomes, as is the case for “
A statistical analysis of the swimming orientation of magnetotactic bacteria, assuming that they behave as paramagnetic particles, produces the orientation to be equivalent to the average of cosθ (<cosθ>), being θ the angle among the bacterial velocity and the magnetic field. Kalmijn showed that <cosθ> is function of the magnetic to thermal energy ratio : <cosθ> = L (MH/kT) = coth(MH/kT)−kT/MH, where M is the bacterium magnetic moment, H is the magnetic field intensity, k is the Boltzmann constant, T is the absolute temperature, and L(x) is the Langevin function: coth(x)—1/x. For MH/kT ≈ 10, the Langevin function is about 0.9, which means that the bacterial trajectory is well oriented to the magnetic field direction. The analysis of the velocity as function of the magnetic field  or of the orientation as function of the magnetic field  permits the estimative of the bacterial magnetic moment. Kalmijn stressed the fact that this kind of study is valid only for the orientation of a single bacterium and not for the average orientation of several bacteria . Using this method, it has been shown that “
Other techniques have been used for measuring the magnetic moment of magnetotactic bacteria. Using a SQUID magnetometer, an average magnetic moment of 1.8 ± 0.4 × 10−12 emu for bacteria from natural sediments had been determined . This method is interesting because it is a direct measurement and does not need to assume unknown values for parameters from the studied cell. There are two interesting physical techniques involving light for measuring the magnetic moment of magnetic bacteria. One is the analysis of the birefringence arising in a pull of magnetotactic bacteria when in presence of an external magnetic field . The birefringence transforms an input linear polarized light beam in an output elliptically polarized light beam, with a phase shift between the fast and slow components. This phase shift is measured and it depends on the intensity of the external magnetic field and on the magnetic moment. Experiments were done with live and dead bacteria, killed with drops of formalin. The measured values, at normal concentration conditions, for live bacteria were about 1.21 × 10−13 emu and for dead bacteria about 1.33 × 10−13 emu. Apparently, for dead bacteria, the measured values are higher than for live bacteria. It was assumed that this difference could be an effect of motile behavior in live bacteria and the concept of “effective temperature” Teff was introduced, meaning that live bacteria feels a disorienting thermal energy kTeff higher than the ambient thermal energy in 10–20%. The other technique is the analysis of the light scattered by a pull of magnetic bacteria , based on the fact that the presence of an external magnetic field determines an angular distribution in the orientation of bacteria. This angular distribution affects the structure factor in the scattered light intensity. With this method, the average length and average magnetic moment can be determined. For two different cultures of
|Technique||Organism||Magnetic moment (Am2)||References|
|Electron microscopy||Magnetotactic spirillum MS1||1.3 × 10−15|||
|U-turn analysis||Several microorganisms from fresh to marine water||0.3 × 10−15 to 54 × 10−15|||
|U-turn modified||6.1 × 10−16|||
|SQUID||Fresh water uncultured bacteria||1.8 × 10−15|||
|Light scattering||(live) 2.2 × 10−16|
(dead) 4.3 × 10−16
|Light scattering||25.3 ± 1.6 × 10−16|||
|Birefringence||1.21 × 10−16|||
|Rotating magnetic field||Natural samples||4 × 10−15|||
|Rotating magnetic field||Uncultivated coccus MYC-1||1.8 × 10−15|||
|Magnetic tweezers||(low H) 2.4 ± 1.1 × 10−16|
(high H) 7.7 ± 3.4 × 10−16
5. The movement of magnetotactic bacteria
Several experimental observations show that magnetotaxis functions together with aerotaxis, determining the so-called magneto-aerotaxis [8, 67]. Basically, two different behaviors have been identified in magneto-aerotaxis: polar magnetotaxis, that consists in the North-seeking or South-seeking behaviors in the search for the better oxygen concentrations; and axial magnetotaxis, in that case, MTB move in the magnetic field direction but without preferential sense. MTB from natural samples always present polar magnetotaxis. Axial magnetotaxis has been observed only in cultured MTB.
MTB are easily identified because of their response to the inversion of the local magnetic field direction: after the inversion bacteria swim following the new magnetic field direction. It can be stated that magnetic field inversions stimulate MTB to swim, making them a model for the study of microorganism swimming. Bacteria swim in the low Reynolds number regime, where viscous forces and torques act to null the resultant force and torque . In that regime, microorganisms swim following an helical trajectory  whose parameterization in Cartesian coordinates (x, y, z), considering the helix axis as the z axis, can be written as (Rcos(ωt), Rsin(ωt), Vt), where R is the helix radius, V is the axial velocity, and ω = 2π
For the theoretical study of microorganisms, motion in the low Reynolds number regime is necessary to know all the forces and torques acting on the microorganism. Nogueira and Lins de Barros  developed a model in that regime, considering a spherical MTB with a single flagellum and a magnetosome chain aligned to the flagellum line. The equations to be considered are
The movement of magnetotactic microorganisms also depends on the presence of light, and the response depends on the wavelength and the intensity. This behavior has been studied mainly in multicellular magnetotactic prokaryotes. Negative photo response has been observed when they are illuminated with high-intensity UV light (365 nm), violet-blue light (395–440 nm filter) of about 80 W m−2 of intensity, and blue light (450–490 nm filter) of about 200 W m−2 of intensity. For longer wavelengths, no photo response was observed, and apparently long exposure to green light is lethal [70, 75]. That negative photo response is not observed when very low intensities are used. Photokinesis has been observed in multicellular magnetotactic prokaryotes, decreasing the velocity when illuminated with green light (517 nm, 0.46 W m−2) and increasing the velocity when illuminated with red light (628 nm, 0.16 W m−2), both respectively to the velocity observed when illuminated with blue light (469 nm, 0.8 W m−2) [76, 77]. Recently, De Melo and Acosta-Avalos  showed that the photokinesis in multicellular magnetotactic prokaryotes is related to the combined presence of monochromatic light and a constant magnetic field, and that it can be canceled in the presence of radio-frequency electromagnetic fields oscillating at the Zeeman resonance frequency associated to the constant magnetic field, showing the involvement of a radical pair mechanism, a very well-known magnetoreception mechanism used by migratory birds. Interestingly, magnetotactic microorganisms have the proper physical tools to sense the geomagnetic field with light. Perhaps, magnetotaxis and the radical pair mechanism are involved in a more elaborate magnetic sensing in MTB.
Since the discovery of MTB, they attracted the attention of the scientific community for several reasons. Firstly, they show clearly that living beings can interact with the geomagnetic field through magnetic nanoparticles and became a model that has been extensively used in magnetoreception research. The study of the magnetosome synthesis process within MTB is being used to develop new strategies to produce magnetic nanoparticles with potential use in Biotechnology. For example, genes responsible for magnetosome synthesis could be transferred and expressed in a host cell to increase the yield of magnetosomes production in bioreactors. If high amounts of magnetosomes were achieved at low cost, magnetosome use as biotechnological tools would be possible. For physicists, MTB are interesting to apply models of magnetism, used in solid-state theory, in living beings behavior. The different techniques developed to measure the MTB magnetic moment have shown that considering MTB as paramagnetic particles is as insufficient model, defying previous models about MTB magnetotaxis. The study of motion is also giving support to new understandings about magnetotaxis, because new characteristics of the interaction among MTB and the geomagnetic field are arising through the study of the movement as function of the applied magnetic field. There are some evidences that MTB use more than one mechanism to detect the magnetic field direction, and not only through the magnetic torque. So, a new magnetoreception mechanism shall be discovered in MTB in the near future.
We acknowledge the microscopy facility Unidade de Microscopia Multiusuário (UniMicro, UFRJ) and financial support from the Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), and Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ).
Conflict of interest
The authors declare no conflict of interest.