A Candidate Magnetic Sense Organ in the Yellowfin Tuna, Thunnus albacares

Extremely low frequency magnetic field distracts zebrafish from a visual cognitive task

Electromagnetic fields emitted from overhead power lines and subsea cables are widely regarded to be a disruptive factor for animals using the natural magnetic field as orientation cue for guiding their directed movements. However, it is not known if anthropogenic electromagnetic fields also have the potential to disturb animals attending to information from other sensory modalities. To find out, we trained adult zebrafish (Danio rerio) individually to perform avoidance behavior in response to a visual signal (green LED light spot), which in the exposure group was presented simultaneously with a sinusoidally changing magnetic field (0.3 Hz, group A: 0.015 mT, group B: 0.06 mT). Despite the salience of the visual signal, which was both sufficient and necessary to elicit conditioned avoidance responses, the 0.06 mT magnetic condition had a negative impact on learning performance and response behavior. This suggests that extremely low frequency technical magnetic fields of Earth strength amplitude can act as cross-modal distractor that diverts the attention of animals away from environmentally relevant cues based on nonmagnetic sensory modalities. Our research highlights the need to study the role of anthropogenic magnetic fields as sensory pollutant beyond the scope of magnetic orientation behavior.

Chain-Like Structures of Biogenic and Nonbiogenic Magnetic Nanoparticles in Vascular Tissues

In this paper, slices of organs from various organisms (animals, plants, fungi) were investigated by using atomic force microscopy and magnetic force microscopy to identify common features of localization of both biogenic and nonbiogenic magnetic nanoparticles. It was revealed that both biogenic and nonbiogenic magnetic nanoparticles are localized in the form of chains of separate nanoparticles or chains of conglomerates of nanoparticles in the walls of the capillaries of animals and the walls of the conducting tissue of plants and fungi. Both biogenic and nonbiogenic magnetic nanoparticles are embedded as a part of the transport system in multicellular organisms. In connection with this, a new idea of the function of biogenic magnetic nanoparticles is discussed, that the chains of biogenic magnetic nanoparticles and chains of conglomerates of biogenic magnetic nanoparticles represent ferrimagnetic organelles of a specific purpose. Besides, magnetic dipole-dipole interaction of biogenic magnetic nanoparticles with magnetically labeled drugs or contrast agents for magnetic resonance imaging should be considered when designing the drug delivery and other medical systems because biogenic magnetic nanoparticles in capillary walls will serve as the trapping centers for the artificial magnetic nanoparticles. The aggregates of both artificial and biogenic magnetic nanoparticles can be formed, contributing to the risk of vascular occlusion. Bioelectromagnetics. 43:119-143, 2022. © 2021 Bioelectromagnetics Society.

Magnetoreception and magnetic navigation in fishes: a half century of discovery

As the largest and most diverse vertebrate group on the planet, fishes have evolved an impressive array of sensory abilities to overcome the challenges associated with navigating the aquatic realm. Among these, the ability to detect Earth's magnetic field, or magnetoreception, is phylogenetically widespread and used by fish to guide movements over a wide range of spatial scales ranging from local movements to transoceanic migrations. A proliferation of recent studies, particularly in salmonids, has revealed that fish can exploit Earth's magnetic field not only as a source of directional information for maintaining consistent headings, but also as a kind of map for determining location at sea and for returning to natal areas. Despite significant advances, much about magnetoreception in fishes remains enigmatic. How fish detect magnetic fields remains unknown and our understanding of the evolutionary origins of vertebrate magnetoreception would benefit greatly from studies that include a wider array of fish taxa. The rich diversity of life-history characteristics that fishes exhibit, the wide variety of environments they inhabit, and their suitability for manipulative studies, make fishes promising subjects for magnetoreception studies.

Numerical tests of magnetoreception models assisted with behavioral experiments on American cockroaches

Many animals display sensitivity to external magnetic field, but it is only in the simplest organisms that the sensing mechanism is understood. Here we report on behavioural experiments where American cockroaches (Periplaneta americana) were subjected to periodically rotated external magnetic fields with a period of 10 min. The insects show increased activity when placed in a periodically rotated Earth-strength field, whereas this effect is diminished in a twelve times stronger periodically rotated field. We analyse established models of magnetoreception, the magnetite model and the radical pair model, in light of this adaptation result. A broad class of magnetite models, based on single-domain particles found in insects and assumption that better alignment of magnetic grains towards the external field yields better sensing and higher insect activity, is shown to be excluded by the measured data. The radical-pair model explains the data if we assume that contrast in the chemical yield on the order of one in a thousand is perceivable by the animal, and that there also exists a threshold value for detection, attained in an Earth-strength field but not in the stronger field.

Biogenic Metal Oxides

Biogenic metal oxides (MxOy) feature structures as highly functional and unique as the organisms generating them. They have caught the attention of scientists for the development of novel materials by biomimicry. In order to understand how biogenic MxOy could inspire novel technologies, we have reviewed examples of all biogenic MxOy, as well as the current state of understanding of the interactions between the inorganic MxOy and the biological matter they originate from and are connected to. In this review, we first summarize the origins of the precursors that living nature converts into MxOy. From the point-of-view of our materials chemists, we present an overview of the biogenesis of silica, iron and manganese oxides, as the only reported biogenic MxOy to date. These MxOy are found across all five kingdoms (bacteria, protoctista, fungi, plants and animals). We discuss the key molecules involved in the biosynthesis of MxOy, the functionality of the MxOy structures, and the techniques by which the biogenic MxOy can be studied. We close by outlining the biomimetic approaches inspired by biogenic MxOy materials and their challenges, and we point at promising directions for future organic-inorganic materials and their synthesis.

Magnetoreception in fishes: the effect of magnetic pulses on orientation of juvenile Pacific salmon

A variety of animals sense Earth's magnetic field and use it to guide movements over a wide range of spatial scales. Little is known, however, about the mechanisms that underlie magnetic field detection. Among teleost fish, growing evidence suggests that crystals of the mineral magnetite provide the physical basis of the magnetic sense. In this study, juvenile Chinook salmon (Oncorhynchus tshawytscha) were exposed to a brief but strong magnetic pulse capable of altering the magnetic dipole moment of biogenic magnetite. Orientation behaviour of pulsed fish and untreated control fish was then compared in a magnetic coil system under two conditions: (1) the local magnetic field and (2) a magnetic field that exists near the southern boundary of the natural oceanic range of Chinook salmon. In the local field, no significant difference existed between the orientation of the control and pulsed groups. By contrast, orientation of the two groups was significantly different in the magnetic field from the distant site. These results demonstrate that a magnetic pulse can alter the magnetic orientation behaviour of a fish and are consistent with the hypothesis that salmon have magnetite-based magnetoreception.

A magnetic compass guides the direction of foraging in a bat

Previously, two studies have provided evidence that bats can use magnetic field cues for homing or roosting. For insectivorous bats, it is well established that foraging represents one of the most fundamental behaviors in animals relies on their ability to echolocate. Whether echolocating bats can also use magnetic cues during foraging remains unknown, however. Here, we tested the orientation behavior of Chinese noctules (Nyctalus plancyi) during foraging in a plus-shaped, 4-channel apparatus under different magnetic field conditions. To minimize the effects of spatial memory on orientation from repeated experiments, naïve bats were tested only once in each experimental condition. As expected, under geomagnetic field and a food resource offered conditions, the bats significantly preferred to enter the channel containing food, indicating that they primarily relied on direct sensory signals unrelated to magnetic cues. In contrast, when we offered food simultaneously in all four channels and minimized any differences in all other sensory signals available, the bats exhibited a clear directional preference to forage along the magnetic field direction under either geomagnetic field or a magnetic field in which the horizontal component was rotated by 90°. Our study offers a novel evidence for the importance of a geomagnetic field during foraging.

On the origin of microbial magnetoreception

A broad range of organisms, from prokaryotes to higher animals, have the ability to sense and utilize Earth's geomagnetic field—a behavior known as magnetoreception. Although our knowledge of the physiological mechanisms of magnetoreception has increased substantially over recent decades, the origin of this behavior remains a fundamental question in evolutionary biology. Despite this, there is growing evidence that magnetic iron mineral biosynthesis by prokaryotes may represent the earliest form of biogenic magnetic sensors on Earth. Here, we integrate new data from microbiology, geology and nanotechnology, and propose that initial biomineralization of intracellular iron nanoparticles in early life evolved as a mechanism for mitigating the toxicity of reactive oxygen species (ROS), as ultraviolet radiation and free-iron-generated ROS would have been a major environmental challenge for life on early Earth. This iron-based system could have later been co-opted as a magnetic sensor for magnetoreception in microorganisms, suggesting an origin of microbial magnetoreception as the result of the evolutionary process of exaptation.

The symbiotic magnetic-sensing hypothesis: do Magnetotactic Bacteria underlie the magnetic sensing capability of animals?

The ability to sense Earth's magnetic field has evolved in various taxa. However, despite great efforts to find the 'magnetic-sensor' in vertebrates, the results of these scientific efforts remain inconclusive. A few decades ago, it was found that bacteria, known as magnetotactic bacteria (MTB), can move along a magnetic field using nanometric chain-like structures. Still, it is not fully clear why these bacteria evolved to have this capacity. Thus, while for MTB the 'magnetic-sensor' is known but the adaptive value is still under debate, for metazoa it is the other way around. In the absence of convincing evidence for any 'magnetic-sensor' in metazoan species sensitive to Earth's magnetic field, we hypothesize that a mutualism between these species and MTB provides one. In this relationship the host benefits from a magnetotactic capacity, while the bacteria benefit a hosting environment and dispersal. We provide support for this hypothesis using existing literature, demonstrating that by placing the MTB as the 'magnetic-sensor', previously contradictory results are now in agreement. We also propose plausible mechanisms and ways to test the hypothesis. If proven correct, this hypothesis would shed light on the forces driving both animal and bacteria magnetotactic abilities.

Physiological origin of biogenic magnetic nanoparticles in health and disease: from bacteria to humans

The discovery of biogenic magnetic nanoparticles (BMNPs) in the human brain gives a strong impulse to study and understand their origin. Although knowledge of the subject is increasing continuously, much remains to be done for further development to help our society fight a number of pathologies related to BMNPs. This review provides an insight into the puzzle of the physiological origin of BMNPs in organisms of all three domains of life: prokaryotes, archaea, and eukaryotes, including humans. Predictions based on comparative genomic studies are presented along with experimental data obtained by physical methods. State-of-the-art understanding of the genetic control of biomineralization of BMNPs and their properties are discussed in detail. We present data on the differences in BMNP levels in health and disease (cancer, neurodegenerative disorders, and atherosclerosis), and discuss the existing hypotheses on the biological functions of BMNPs, with special attention paid to the role of the ferritin core and apoferritin.

A first test of the hypothesis of biogenic magnetite-based heterogeneous ice-crystal nucleation in cryopreservation

An outstanding biophysical puzzle is focused on the apparent ability of weak, extremely low-frequency oscillating magnetic fields to enhance cryopreservation of many biological tissues. A recent theory holds that these weak magnetic fields could be inhibiting ice-crystal nucleation on the nanocrystals of biological magnetite (Fe3O4, an inverse cubic spinel) that are present in many plant and animal tissues by causing them to oscillate. In this theory, magnetically-induced mechanical oscillations disrupt the ability of water molecules to nucleate on the surface of the magnetite nanocrystals. However, the ability of the magnetite crystal lattice to serve as a template for heterogeneous ice crystal nucleation is as yet unknown, particularly for particles in the 10-100 nm size range. Here we report that the addition of trace-amounts of finely-dispersed magnetite into ultrapure water samples reduces strongly the incidence of supercooling, as measured in experiments conducted using a controlled freezing apparatus with multiple thermocouples. SQUID magnetometry was used to quantify nanogram levels of magnetite in the water samples. We also report a relationship between the volume change of ice, and the degree of supercooling, that may indicate lower degassing during the crystallization of supercooled water. In addition to supporting the role of ice-crystal nucleation by biogenic magnetite in many tissues, magnetite nanocrystals could provide inexpensive, non-toxic, and non-pathogenic ice nucleating agents needed in a variety of industrial processes, as well as influencing the dynamics of ice crystal nucleation in many natural environments.

Magnetic particle-mediated magnetoreception

Behavioural studies underpin the weight of experimental evidence for the existence of a magnetic sense in animals. In contrast, studies aimed at understanding the mechanistic basis of magnetoreception by determining the anatomical location, structure and function of sensory cells have been inconclusive. In this review, studies attempting to demonstrate the existence of a magnetoreceptor based on the principles of the magnetite hypothesis are examined. Specific attention is given to the range of techniques, and main animal model systems that have been used in the search for magnetite particulates. Anatomical location/cell rarity and composition are identified as two key obstacles that must be addressed in order to make progress in locating and characterizing a magnetite-based magnetoreceptor cell. Avenues for further study are suggested, including the need for novel experimental, correlative, multimodal and multidisciplinary approaches. The aim of this review is to inspire new efforts towards understanding the cellular basis of magnetoreception in animals, which will in turn inform a new era of behavioural research based on first principles.

Magnetic-assembly mechanism of superparamagneto-plasmonic nanoparticles on a charged surface

One-dimensional magnetoplasmonic nanochains (MPNCs) were self-assembled using Au-coated Fe3O4 core-shell superparamagnetic nanoparticles (Fe3O4@Au NPs) by applying an external static magnetic field. The assembly mechanism of the Fe3O4@Au NPs was investigated thoroughly, revealing that substrate-particle interactions, van der Waals forces, and magnetic forces play important roles in the formation and control of the MPNCs. Magnetic force microscopy (MFM) and vibrating sample magnetometry (VSM) were used to study the magnetic properties of the MPNCs, which were compared with those of Fe3O4 nanochains.

Sub-micrometer-scale mapping of magnetite crystals and sulfur globules in magnetotactic bacteria using confocal Raman micro-spectrometry

The ferrimagnetic mineral magnetite Fe3O4 is biomineralized by magnetotactic microorganisms and a diverse range of animals. Here we demonstrate that confocal Raman microscopy can be used to visualize chains of magnetite crystals in magnetotactic bacteria, even though magnetite is a poor Raman scatterer and in bacteria occurs in typical grain sizes of only 35-120 nm, well below the diffraction-limited optical resolution. When using long integration times together with low laser power (<0.25 mW) to prevent laser induced damage of magnetite, we can identify and map magnetite by its characteristic Raman spectrum (303, 535, 665 cm(-1)) against a large autofluorescence background in our natural magnetotactic bacteria samples. While greigite (cubic Fe3S4; Raman lines of 253 and 351 cm(-1)) is often found in the Deltaproteobacteria class, it is not present in our samples. In intracellular sulfur globules of Candidatus Magnetobacterium bavaricum (Nitrospirae), we identified the sole presence of cyclo-octasulfur (S8: 151, 219, 467 cm(-1)), using green (532 nm), red (638 nm) and near-infrared excitation (785 nm). The Raman-spectra of phosphorous-rich intracellular accumulations point to orthophosphate in magnetic vibrios and to polyphosphate in magnetic cocci. Under green excitation, the cell envelopes are dominated by the resonant Raman lines of the heme cofactor of the b or c-type cytochrome, which can be used as a strong marker for label-free live-cell imaging of bacterial cytoplasmic membranes, as well as an indicator for the redox state.

Exposure to static magnetic field stimulates quorum sensing circuit in luminescent Vibrio strains of the Harveyi clade

In this study, the evidence of electron-dense magnetic inclusions with polyhedral shape in the cytoplasm of Harveyi clade Vibrio strain PS1, a bioluminescent bacterium living in symbiosis with marine organisms, led us to investigate the behavior of this bacterium under exposure to static magnetic fields ranging between 20 and 2000 Gauss. When compared to sham-exposed, the light emission of magnetic field-exposed bacteria growing on solid medium at 18°C ±0.1°C was increased up to two-fold as a function of dose and growth phase. Stimulation of bioluminescence by magnetic field was more pronounced during the post-exponential growth and stationary phase, and was lost when bacteria were grown in the presence of the iron chelator deferoxamine, which caused disassembly of the magnetic inclusions suggesting their involvement in magnetic response. As in luminescent Vibrio spp. bioluminescence is regulated by quorum sensing, possible effects of magnetic field exposure on quorum sensing were investigated. Measurement of mRNA levels by reverse transcriptase real time-PCR demonstrated that luxR regulatory gene and luxCDABE operon coding for luciferase and fatty acid reductase complex were significantly up-regulated in magnetic field-exposed bacteria. In contrast, genes coding for a type III secretion system, whose expression was negatively affected by LuxR, were down-regulated. Up-regulation of luxR paralleled with down-regulation of small RNAs that mediate destabilization of luxR mRNA in quorum sensing signaling pathways. The results of experiments with the well-studied Vibrio campbellii strain BB120 (originally classified as Vibrio harveyi) and derivative mutants unable to synthesize autoinducers suggest that the effects of magnetic fields on quorum sensing may be mediated by AI-2, the interspecies quorum sensing signal molecule.

Zebrafish respond to the geomagnetic field by bimodal and group-dependent orientation

A variety of animals use Earth's magnetic field as a reference for their orientation behaviour. Although distinctive magnetoreception mechanisms have been postulated for many migrating or homing animals, the molecular mechanisms are still undefined. In this study, we found that zebrafish, a model organism suitable for genetic manipulation, responded to a magnetic field as weak as the geomagnetic field. Without any training, zebrafish were individually released into a circular arena that was placed in an artificial geomagnetic field, and their preferred magnetic directions were recorded. Individuals from five out of the seven zebrafish groups studied, groups mostly comprised of the offspring of predetermined pairs, showed bidirectional orientation with group-specific preferences regardless of close kinships. The preferred directions did not seem to depend on gender, age or surrounding environmental factors, implying that directional preference was genetically defined. The present findings may facilitate future study on the molecular mechanisms underlying magnetoreception.

Potential interactions between diadromous fishes of U.K. conservation importance and the electromagnetic fields and subsea noise from marine renewable energy developments

The considerable extent of construction and operation of marine renewable energy developments (MRED) within U.K. and adjacent waters will lead, among other things, to the emission of electromagnetic fields (EMF) and subsea sounds into the marine environment. Migratory fishes that respond to natural environmental cues, such as the Earth's geomagnetic field or underwater sounds, move through the same waters that the MRED occupy, thereby raising the question of whether there are any effects of MRED on migratory fishes. Diadromous species, such as the Salmonidae and Anguillidae, which undertake large-scale migrations through coastal and offshore waters, are already significantly affected by other human activities leading to national and international conservation efforts to manage any existing threats and to minimize future concerns, including the potential effect of MRED. Here, the current state of knowledge with regard to the potential for diadromous fishes of U.K. conservation importance to be affected by MRED is reviewed. The information on which to base the review was found to be limited with respect to all aspects of these fishes' migratory behaviour and activity, especially with regards to MRED deployment, making it difficult to establish cause and effect relationships. The main findings, however, were that diadromous species can use the Earth's magnetic field for orientation and direction finding during migrations. Juveniles of anadromous brown trout (sea trout) Salmo trutta and close relatives of S. trutta respond to both the Earth's magnetic field and artificial magnetic fields. Current knowledge suggests that EMFs from subsea cables may interact with migrating Anguilla sp. (and possibly other diadromous fishes) if their movement routes take them over the cables, particularly in shallow water (<20 m). The only known effect is a temporary change in swimming direction. Whether this will represent a biologically significant effect, for example delayed migration, cannot yet be determined. Diadromous fishes are likely to encounter EMFs from subsea cables either during the adult movement phases of life or their early life stages during migration within shallow, coastal waters adjacent to natal rivers. The underwater sound from MRED devices has not been fully characterized to determine its acoustic properties and propagation through the coastal waters. MRED that require pile driving during construction appear to be the most relevant to consider. In the absence of a clear understanding of their response to underwater sound, the specific effects on migratory species of conservation concern remain very difficult to determine in relation to MRED. Based on the studies reviewed, it is suggested that fishes that receive high intensity sound in close proximity to construction may be physiologically affected to some degree, whereas those at farther distances, potentially up to several km, may exhibit behaviour responses; the effect of which is unknown and will be dependent on the properties of the received sound and receptor characteristics and condition. Whether there are behavioural effects on the fishes during operation is unknown but any change to the environment and subsequent response by the fishes would need to be considered over the lifetime of the MRED. It is not yet possible to determine if effects relating to sound exposure are biologically significant. The current assumptions of limited effects are built on an incomplete understanding of how the species move around their environment and interact with natural and anthropogenic EMFs and subsea sound. A number of important knowledge gaps exist, principally whether migratory fish species on the whole respond to the EMF and the sound associated with MRED. Future research should address the principal gaps before assuming that any effect on diadromous species results in a biological effect.

Magnetite and magnetotaxis in algae

Magnetotactic algae of the genus Anisonema (Euglenophyceae) have been isolated from a coastal mangrove swamp in northeastern Brazil. The magnetotactic response is based on a permanent magnetic dipole moment per cell approximately 7 10(-10) emu. Each cell contains many magnetite (Fe(3)O(4)) particles organized in chains.

Avian magnetite-based magnetoreception: a physiologist's perspective

It is now well established that animals use the Earth's magnetic field to perform long-distance migration and other navigational tasks. However, the transduction mechanisms that allow the conversion of magnetic field variations into an electric signal by specialized sensory cells remain largely unknown. Among the species that have been shown to sense Earth-strength magnetic fields, birds have been a model of choice since behavioural tests show that their direction-finding abilities are strongly influenced by magnetic fields. Magnetite, a ferromagnetic mineral, has been found in a wide range of organisms, from bacteria to vertebrates. In birds, both superparamagnetic (SPM) and single-domain magnetite have been found to be associated with the trigeminal nerve. Electrophysiological recordings from cells in the trigeminal ganglion have shown an increase in action potential firing in response to magnetic field changes. More recently, histological evidence has demonstrated the presence of SPM magnetite in the subcutis of the pigeon's upper beak. The aims of the present review are to review the evidence for a magnetite-based mechanism in birds and to introduce physiological concepts in order to refine the proposed models.

A quantitative assessment of torque-transducer models for magnetoreception

Although ferrimagnetic material appears suitable as a basis of magnetic field perception in animals, it is not known by which mechanism magnetic particles may transduce the magnetic field into a nerve signal. Provided that magnetic particles have remanence or anisotropic magnetic susceptibility, an external magnetic field will exert a torque and may physically twist them. Several models of such biological magnetic-torque transducers on the basis of magnetite have been proposed in the literature. We analyse from first principles the conditions under which they are viable. Models based on biogenic single-domain magnetite prove both effective and efficient, irrespective of whether the magnetic structure is coupled to mechanosensitive ion channels or to an indirect transduction pathway that exploits the strayfield produced by the magnetic structure at different field orientations. On the other hand, torque-detector models that are based on magnetic multi-domain particles in the vestibular organs turn out to be ineffective. Also, we provide a generic classification scheme of torque transducers in terms of axial or polar output, within which we discuss the results from behavioural experiments conducted under altered field conditions or with pulsed fields. We find that the common assertion that a magnetoreceptor based on single-domain magnetite could not form the basis for an inclination compass does not always hold.

Three-dimensional movements and swimming activity of a northern elephant seal

We attached a video system and data recorder to a northern elephant seal to track its three-dimensional movements and observe propulsive strokes of the hind flippers. During 6 h of recording, the seal made 20 dives and spent 90% of the time submerged. Average dive duration, maximum depth and swimming speed were 14.9 min+/-6.1 S.D., 289 m+/-117 S.D. and 1.1 m s(-1)+/-0.12 S.D., respectively. The distance swum during a dive averaged 925 m+/-339 S.D., and the average descent and ascent angles were 41 degrees +/-18 S.D. and 50 degrees +/-21 S.D., respectively. Dive paths were remarkably straight suggesting that the seal was navigating while submerged. We identified three modes of swimming based on the interval between propulsive strokes: continuous stroking; stroke-and-glide swimming; and prolonged gliding. The seal used continuous stroking from the surface to a mean depth of 20 m followed by stroke-and-glide swimming. Prolonged gliding started at a mean depth of 60 m and continued to the bottom of dives. For dives to depths of 300 m or more, 75% of the descent time was spent in prolonged gliding and 10% in stroke-and-glide swimming, amounting to 5.9-9.6 min of passive descent per dive. Average swimming speed varied little with swimming mode and was not a good indicator of propulsive effort. It appears that the seal can use prolonged gliding to reduce the cost of transport and increase dive duration. Energetically efficient locomotion may help explain the long and deep dives that routinely exceed the theoretical aerobic dive limit in this species.

Molecular mechanism of magnet formation in bacteria

Magnetic bacteria have an ability to synthesize intracellular ferromagnetic crystalline particles consisting of magnetite (Fe3O4) or greigite (Fe3S4) which occur within a specific size range (50-100 nm). Bacterial magnetic particles (BMPs) can be distinguished by the regular morphology and the presence of an thin organic membrane enveloping crystals from abiologically formed magnetite. The particle is the smallest magnetic crystal that has a regular morphology within the single domain size. Therefore, BMPs have an unfathomable amount of potential value for various technological applications not only scientific interests. However, the molecular and genetic mechanism of magnetite biomineralization is hardly understood although iron oxide formation occurs widely in many higher animals as well as microorganisms. In order to elucidate the molecular and genetic mechanisms of magnetite biomineralization, a magnetic bacterium Magnetospirillum sp. AMB-1, for which gene transfer and transposon mutagenesis techniques had been recently developed, has been used as a model organism. Several findings and information on the BMPs formation process have been obtained within this decade by means of studies with this model organism and its related one. Biomineralization mechanism and potential availability in biotechnology of bacterial magnets have been elucidated through molecular and genetic approach.

The case for light-dependent magnetic orientation in animals

Light-dependent models of magnetoreception have been proposed which involve an interaction between the magnetic field and either magnetite particles located within a photoreceptor or excited states of photopigment molecules. Consistent with a photoreceptor-based magnetic compass mechanism, magnetic orientation responses in salamanders, flies and birds have been shown to be affected by the wavelength of light. In birds and flies, it is unclear whether the effects of light on magnetic orientation are due to a direct effect on a magnetoreception system or to a nonspecific (e.g. motivational) effect of light on orientation behavior. Evidence from shoreward-orienting salamanders, however, demonstrates that salamanders perceive a 90 degrees counterclockwise shift in the direction of the magnetic field under long-wavelength (&gt;=500 nm) light. A simple physiological model based on the antagonistic interaction between two magnetically sensitive spectral mechanisms suggests one possible way in which the wavelength-dependent effects of light on the salamander's magnetic compass response might arise. Assuming that the wavelength-dependent characteristics of the avian magnetic response can be attributed to an underlying magnetoreception system, we discuss several hypotheses attempting to resolve the differences observed in the wavelength-dependent effects of light on magnetic orientation in birds and salamanders. By considering the evidence in the context of photoreceptor- and non-photoreceptor-based mechanisms for magnetoreception, we hope to encourage future studies designed to distinguish between alternative hypotheses concerning the influence of light on magnetoreception.

Sensory bases of navigation

Navigating animals need to know both the bearing of their goal (the 'map' step), and how to determine that direction (the 'compass' step). Compasses are typically arranged in hierarchies, with magnetic backup as a last resort when celestial information is unavailable. Magnetic information is often essential to calibrating celestial cues, though, and repeated recalibration between celestial and magnetic compasses is important in many species. Most magnetic compasses are based on magnetite crystals, but others make use of induction or paramagnetic interactions between short-wavelength light and visual pigments. Though odors may be used in some cases, most if not all long-range maps probably depend on magnetite. Magnetitebased map senses are used to measure only latitude in some species, but provide the distance and direction of the goal in others.

Magnetic particles in the lateral line of the Atlantic salmon ( Salmo salar L.)

Magnetization measurements with a superconducting quantum inference device magnetometer of various tissues of the Atlantic salmon ( Salmo salar L.) have shown the presence of magnetic material associated with the lateral line. The data suggest that the material is magnetite and of a size suitable for magnetoreception. Magnetic particles were isolated from the lateral line and nerve tissue, which have characteristics suggesting that the material is magnetite and of biogenic origin. The magnetic particles and their association with the lateral line are discussed in relation to their possible role in allowing the salmon to orientate with respect to the geomagnetic field during the high-seas phase of their migration.

An iron-regulated gene, magA, encoding an iron transport protein of Magnetospirillum sp. strain AMB-1

Magnetospirillum sp. AMB-1 is a freshwater magnetic bacterium which synthesizes intracellular particles of magnetite (Fe3O4). A genomic DNA fragment required for synthesis of magnetic particles was previously isolated from a nonmagnetic transposon Tn5 mutant. We have determined the complete nucleotide sequence of this fragment. The 2975-base pair region contains two putative open reading frames. One open reading frame, designated magA, encodes a polypeptide which is homologous to the cation efflux proteins, the Escherichia coli potassium ion-translocating protein, KefC, and the putative Na+/H(+)-antiporter, NapA, from Enterococcus hirae. Northern hybridization demonstrated that the magA mRNA transcript is 1.3 kilobases in size, corresponding to the size of the magA gene. A functional promoter was located upstream from the magA gene, and the transcription in AMB-1 was regulated by environmental iron concentration. Vesicles isolated from E. coli in which the MagA protein was expressed exhibited iron accumulation ability. We consider that the MagA protein is an iron transport involved in the synthesis of magnetic particles in AMB-1.

Phylogenetic analysis of a novel sulfate-reducing magnetic bacterium, RS-1, demonstrates its membership of the delta-Proteobacteria

Most of the 16S ribosomal RNA gene of a sulfate-reducing magnetic bacterium, RS-1, was sequenced, and phylogenetic analysis was carried out. The results suggest that RS-1 is a member of the delta-Proteobacteria, and it appears to represent a new genus. RS-1 is the first bacterium reported outside the alpha-Proteobacteria that contains magnetite inclusions. RS-1 therefore disrupts the correlation between the alpha-Proteobacteria and possession of magnetite inclusions, and that between the delta-Proteobacteria and possession of greigite inclusions. The existence of RS-1 also suggests that intracellular magnetite biomineralization is of multiple evolutionary origins.

Magnetoreception in Honeybees

Magnetoreception by honeybees (Apis mellifera) is demonstrated by such activities as comb building and homing orientation, which are affected by the geomagnetic field. In other magnetoreceptive species, iron oxide crystals in the form of magnetite have been shown to be necessary for primary detection of magnetic fields. Here it is shown that trophocytes, which are apparently the only iron granule-containing cells in honeybees, contain super-paramagnetic magnetite. These cells are innervated by the nervous system, which suggests that trophocytes might be primarily responsible for magnetoreception. Electron microscopy also shows cytoskeletal attachments to the iron granule membrane.

Biophysical estimation of the environmental importance of electromagnetic fields

The rapid development of science and technology exposes living organisms to a wide range of electromagnetic fields. Some life-style and occupational conditions are associated with a level of electromagnetic fields higher than average. A series of epidemiological studies has raised concern about possible cancer risk of electromagnetic fields generated by power lines and electrical appliances. In contrast, hundreds of thousands of patients world-wide have been cured by use of electromagnetic fields. Biological effects of low-level electromagnetic radiation have become the focus of a number of studies. However, there are not enough basic scientific data related to mechanisms of action of electromagnetic fields. This paper proposes a biophysical approach to the estimation of the environmental importance of electromagnetic fields. The methods of collecting data, dosimetry, possible mechanisms of action and open problems are discussed in this review.

Evolutionary relationships among Magnetospirillum strains inferred from phylogenetic analysis of 16S rDNA sequences

We have investigated the evolutionary relationships between two facultatively anaerobic Magnetospirillum strains (AMB-1 and MGT-1) and fastidious, obligately microaerophilic species, such as Magnetospirillum magnetotacticum, using a molecular phylogenetic approach. Genomic DNA from strains MGT-1 and AMB-1 was used as a template for amplification of the genes coding for 16S rRNA (16S rDNA) by the polymerase chain reaction. Amplified DNA fragments were sequenced (1,424 bp) and compared with sequences for M. magnetotacticum MS-1 and Magnetospirillum gryphiswaldense MSR-1. Phylogenetic analysis of the aligned 16S rDNA sequences indicated that the two new magnetic spirilla, AMB-1 and MGT-1, lie within the alpha subdivision (alpha-1) of the eubacterial group Proteobacteria and are closely related to Rhodospirillum fulvum and to several endosymbiotic bacteria. Strains AMB-1, MGT-1, and MS-1 formed a cluster, termed group I, in which they were more closely related to each other than to group II, which contained M. gryphiswaldense MSR-1. Group I strains were also physiologically distinct from strain MSR-1. Sequence alignment studies allowed elucidation of genus-specific regions of the 16S rDNA, and oligonucleotide primers complementary to two of these regions were used to develop a specific polymerase chain reaction assay for detection of magnetic spirilla in natural samples.

Gene transfer in magnetic bacteria: transposon mutagenesis and cloning of genomic DNA fragments required for magnetosome synthesis

Broad-host-range IncP and IncQ plasmids have been transferred to the aerobic magnetic bacterium Aquaspirillum sp. strain AMB-1. Conjugal matings with Escherichia coli S17-1 allowed high-frequency transfer of the RK2 derivative pRK415 (4.5 x 10(-3) transconjugant per recipient cell) and the RSF1010 derivative pKT230 (3.0 x 10(-3) transconjugant per recipient). These plasmids successfully formed autonomous replicons in transconjugants and could be isolated and transformed back into E. coli, illustrating their potential as shuttle vectors. A mobilizable plasmid containing transposon Tn5 was transferred to Aquaspirillum sp. strain AMB-1 and also to the obligately microaerophilic magnetic bacterium Aquaspirillum magnetotacticum MS-1. Five nonmagnetic kanamycin-resistant mutants of Aquaspirillum sp. strain AMB-1 in which Tn5 was shown to be integrated into the chromosome were obtained. Different genomic fragments containing the mutagenized regions were cloned into E. coli. Two genomic fragments were restriction mapped, and the site of Tn5 insertion was determined. They were shown to be identical, although derived from independent transposon insertions. One of these clones was found to hybridize strongly to regions of the A. magnetotacticum MS-1 chromosome. This is the first report of gene transfer in a magnetic bacterium.

Electroreception and magnetoreception in simple and complex organisms

A considerable body of evidence now indicates that electromagnetic and geomagnetic detection systems exist in both simple, unicellular organisms and in more complex species such as avians, bees, and marine animals. A major challenge that faces researchers in this field is the identification of physiological mechanisms through which the detection of weak fields provides significant somatosensory cues for direction finding, foraging, and predation. Many of the anatomical, physiological, and biophysical approaches that are being taken in studies of this nature are described in the series of review articles that appear in this issue of Bioelectromagnetics.

Magnetite biomineralization and geomagnetic sensitivity in higher animals: an update and recommendations for future study

Magnetite, the only known biogenic material with ferromagnetic properties, has been identified as a biochemical precipitate in three of the five kingdoms of living organisms, with a fossil record that now extends back nearly 2 billion years. In the magnetotactic bacteria, protoctists, and fish, single-domain crystals of magnetite are arranged in membrane-bound linear structures called magnetosomes, which function as biological bar magnets. Magnetosomes in all three of these groups bear an overall structural similarity to each other, which includes alignment of the individual crystallographic [111] directions parallel to the long axis. Although the magnetosomes represent only a small volume fraction in higher organisms, enough of these highly energetic structures are present to provide sensitivity to extremely small fluctuations and gradients in the background geomagnetic field. Previous experiments with elasmobranch fish are reexamined to test the hypothesis that gradients played a role in their successful geomagnetic conditioning, and a variety of four-turn coil designs are considered that could be used to test the various hypotheses proposed for them.

Occurrence of magnetic material in teleosts

Fishes representing the main groups of teleosts have been investigated for magnetic material by susceptibility measurements. All the investigated species contain magnetic material. Bone samples from the skull and the vertebral column contain magnetic particles which yield a saturation magnetization in the range 10(-4) emu g-1 to 10(-3) emu g-1 in a sample. The localization of the magnetization is diffuse within the tissues connected to the bone. There are no significant differences between the amounts of magnetic material that are found in migrating or more stationary species.