Light effects on the multicellular magnetotactic prokaryote ‘Candidatus Magnetoglobus multicellularis’ are cancelled by radiofrequency fields: the involvement of radical pair mechanisms

Low-level EMF effects on wildlife and plants: What research tells us about an ecosystem approach

There is enough evidence to indicate we may be damaging non-human species at ecosystem and biosphere levels across all taxa from rising background levels of anthropogenic non-ionizing electromagnetic fields (EMF) from 0 Hz to 300 GHz. The focus of this Perspective paper is on the unique physiology of non-human species, their extraordinary sensitivity to both natural and anthropogenic EMF, and the likelihood that artificial EMF in the static, extremely low frequency (ELF) and radiofrequency (RF) ranges of the non-ionizing electromagnetic spectrum are capable at very low intensities of adversely affecting both fauna and flora in all species studied. Any existing exposure standards are for humans only; wildlife is unprotected, including within the safety margins of existing guidelines, which are inappropriate for trans-species sensitivities and different non-human physiology. Mechanistic, genotoxic, and potential ecosystem effects are discussed.

Magnetoreception in multicellular magnetotactic prokaryotes: a new analysis of escape motility trajectories in different magnetic fields

Magnetotactic microorganisms can be found as unicellular prokaryotes, as cocci, vibrions, spirilla and rods, and as multicellular organisms. Multicellular magnetotactic prokaryotes are magnetotactic microorganisms composed by several magnetotactic bacteria organized almost in a spherical helix, and one of the most studied is Candidatus Magnetoglobus multicellularis. Several studies have shown that Ca. M. multicellularis displays forms of behavior not well explained by magnetotaxis. One of these is escape motility, also known as "ping-pong" motion. Studies done in the past associated the "ping-pong" motion to some magnetoreceptive behavior, but those studies were never replicated. In the present manuscript a characterization of escape motility trajectories of Ca. M. multicellularis was done for several magnetic fields, considering that this microorganism swims in cylindrical helical trajectories. It was observed that the escape motility can be separated into three phases: (I) when the microorganism jumps from the drop border, (II) where the microorganism moves almost perpendicular to the magnetic field and (III) when the microorganism returns to the drop border. The total time of the whole escape motility, the time spent in phase II and the displacement distance in phase I decreases when the magnetic field increases. Our results show that the escape motility has several characteristics that depend on the magnetic field and cannot be understood by magnetotaxis, with a magnetoreceptive mechanism being the best explanation.

U-turn time and velocity dependence on the wavelength of light: multicellular magnetotactic prokaryotes of different sizes behave differently

'Candidatus Magnetoglobus multicellularis' is a multicellular magnetotactic prokaryote found in the Araruama lagoon in Rio de Janeiro, Brazil. This microorganism shows a photokinesis that depends on the incident light wavelength, but that dependence can be canceled by the presence of radio-frequency (RF) electromagnetic fields. The present manuscript has as its aim to study the effect of light wavelength and RF fields on the U-turn time of 'Candidatus Magnetoglobus multicellularis', a behavior more related to magnetotaxis. As the experiments were performed during the night, the microorganisms were greater in size than normal, indicating that they were in the process of division. Our results show that when normal in size, the microorganism's U-turn time is modified by the light wavelength (lower for blue light than for green and red light), but RF fields do not affect that U-turn time dependence on the light wavelength. For the microorganism in the process of division, we describe for the first time how the photokinesis and U-turn time dependence on the light wavelength disappear. It is proposed that methyl-accepting chemotaxis proteins are involved in that light wavelength dependence for the U-turn time, but still more studies are necessary to understand how RF fields cancel the photokinesis light wavelength dependence, but do not affect the dependence of the U-turn time.

Symbiotic magnetic sensing: raising evidence and beyond

The identity of a magnetic sensor in animals remains enigmatic. Although the use of the geomagnetic field for orientation and navigation in animals across a broad taxonomic range has been well established over the past five decades, the identity of the magnetic-sensing organ and its structure and/or apparatus within such animals remains elusive-'a sense without a receptor'. Recently, we proposed that symbiotic magnetotactic bacteria (MTB) may serve as the underlying mechanism behind a magnetic sense in animals-'the symbiotic magnetic-sensing hypothesis'. Since we first presented this hypothesis, both criticism and support have been raised accordingly. Here we address the primary criticisms and discuss the plausibility of such a symbiosis, supported by preliminary findings demonstrating the ubiquity of MTB DNA in general, and specifically in animal samples. We also refer to new supporting findings, and discuss host adaptations that could be driven by such a symbiosis. Finally, we suggest the future research directions required to confirm or refute the possibility of symbiotic magnetic-sensing. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Bacteriophytochrome from Magnetospirillum magneticum affects phototactic behavior in response to light

Phytochromes are a class of photoreceptors found in plants and in some fungi, cyanobacteria, and photoautotrophic and heterotrophic bacteria. Although phytochromes have been structurally characterized in some bacteria, their biological and ecological roles in magnetotactic bacteria remain unexplored. Here, we describe the biochemical characterization of recombinant bacteriophytochrome (BphP) from magnetotactic bacteria Magnetospirillum magneticum AMB-1 (MmBphP). The recombinant MmBphP displays all the characteristic features, including the property of binding to biliverdin (BV), of a genuine phytochrome. Site-directed mutagenesis identified that cysteine-14 is important for chromophore covalent binding and photoreversibility. Arginine-240 and histidine-246 play key roles in binding to BV. The N-terminal photosensory core domain of MmBphP lacking the C-terminus found in other phytochromes is sufficient to exhibit the characteristic red/far-red-light-induced fast photoreversibility of phytochromes. Moreover, our results showed MmBphP is involved in the phototactic response, suggesting its conservative role as a stress protectant. This finding provided us a better understanding of the physiological function of this group of photoreceptors and photoresponse of magnetotactic bacteria.

Light regulation of resistance to oxidative damage and magnetic crystal biogenesis in Magnetospirillum magneticum mediated by a Cys-less LOV-like protein

Light-oxygen-voltage (LOV) proteins are ubiquitous photoreceptors that can interact with other regulatory proteins and then mediate their activities, which results in cellular adaptation and subsequent physiological changes. Upon blue-light irradiation, a conserved cysteine (Cys) residue in LOV covalently binds to flavin to form a flavin-Cys adduct, which triggers a subsequent cascade of signal transduction and reactions. We found a group of natural Cys-less LOV-like proteins in magnetotactic bacteria (MTB) and investigated its physiological functions by conducting research on one of these unusual LOV-like proteins, Amb2291, in Magnetospirillum magneticum. In-frame deletion of amb2291 or site-directive substitution of alanine-399 for Cys mutants impaired the protective responses against hydrogen peroxide, thereby causing stress and growth impairment. Consequently, gene expression and magnetosome formation were affected, which led to high sensitivity to oxidative damage and defective phototactic behaviour. The purified wild-type and A399C-mutated LOV-like proteins had similar LOV blue-light response spectra, but Amb2291^A399C exhibited a faster reaction to blue light. We especially showed that LOV-like protein Amb2291 plays a role in magnetosome synthesis and resistance to oxidative stress of AMB-1 when this bacterium was exposed to red light and hydrogen peroxide. This finding expands our knowledge of the physiological function of this widely distributed group of photoreceptors and deepens our understanding of the photoresponse of MTB. KEY POINTS: • We found a group of Cys-less light-oxygen-voltage (LOV) photoreceptors in magnetotactic bacteria, which prompted us to study the light-response and biological roles of these proteins in these non-photosynthetic bacteria. • The Cys-less LOV-like protein participates in the light-regulated signalling pathway and improves resistance to oxidative damage and magnetic crystal biogenesis in Magnetospirillum magneticum. • This result will contribute to our understanding of the structural and functional diversity of the LOV-like photoreceptor and help us understand the complexity of light-regulated model organisms.

The swimming orientation of multicellular magnetotactic prokaryotes and uncultured magnetotactic cocci in magnetic fields similar to the geomagnetic field reveals differences in magnetotaxis between them

Magnetotactic bacteria have intracellular chains of magnetic nanoparticles, conferring to their cellular body a magnetic moment that permits the alignment of their swimming trajectories to the geomagnetic field lines. That property is known as magnetotaxis and makes them suitable for the study of bacterial motion. The present paper studies the swimming trajectories of uncultured magnetotactic cocci and of the multicellular magnetotactic prokaryote 'Candidatus Magnetoglobus multicellularis' exposed to magnetic fields lower than 80 μT. It was assumed that the trajectories are cylindrical helixes and the axial velocity, the helix radius, the frequency and the orientation of the trajectories relative to the applied magnetic field were determined from the experimental trajectories. The results show the paramagnetic model applies well to magnetotactic cocci but not to 'Ca. M. multicellularis' in the low magnetic field regime analyzed. Magnetotactic cocci orient their trajectories as predicted by classical magnetotaxis but in general 'Ca. M. multicellularis' does not swim following the magnetic field direction, meaning that for it the inversion in the magnetic field direction represents a stimulus but the selection of the swimming direction depends on other cues or even on other mechanisms for magnetic field detection.

Genomic evidence of the illumination response mechanism and evolutionary history of magnetotactic bacteria within the Rhodospirillaceae family

BACKGROUND

Magnetotactic bacteria (MTB) are ubiquitous in natural aquatic environments. MTB can produce intracellular magnetic particles, navigate along geomagnetic field, and respond to light. However, the potential mechanism by which MTB respond to illumination and their evolutionary relationship with photosynthetic bacteria remain elusive.

RESULTS

We utilized genomes of the well-sequenced genus Magnetospirillum, including the newly sequenced MTB strain Magnetospirillum sp. XM-1 to perform a comprehensive genomic comparison with phototrophic bacteria within the family Rhodospirillaceae regarding the illumination response mechanism. First, photoreceptor genes were identified in the genomes of both MTB and phototrophic bacteria in the Rhodospirillaceae family, but no photosynthesis genes were found in the MTB genomes. Most of the photoreceptor genes in the MTB genomes from this family encode phytochrome-domain photoreceptors that likely induce red/far-red light phototaxis. Second, illumination also causes damage within the cell, and in Rhodospirillaceae, both MTB and phototrophic bacteria possess complex but similar sets of response and repair genes, such as oxidative stress response, iron homeostasis and DNA repair system genes. Lastly, phylogenomic analysis showed that MTB cluster closely with phototrophic bacteria in this family. One photoheterotrophic genus, Phaeospirillum, clustered within and displays high genomic similarity with Magnetospirillum. Moreover, the phylogenetic tree topologies of magnetosome synthesis genes in MTB and photosynthesis genes in phototrophic bacteria from the Rhodospirillaceae family were reasonably congruent with the phylogenomic tree, suggesting that these two traits were most likely vertically transferred during the evolution of their lineages.

CONCLUSION

Our new genomic data indicate that MTB and phototrophic bacteria within the family Rhodospirillaceae possess diversified photoreceptors that may be responsible for phototaxis. Their genomes also contain comprehensive stress response genes to mediate the negative effects caused by illumination. Based on phylogenetic studies, most of MTB and phototrophic bacteria in the Rhodospirillaceae family evolved vertically with magnetosome synthesis and photosynthesis genes. The ancestor of Rhodospirillaceae was likely a magnetotactic phototrophic bacteria, however, gain or loss of magnetotaxis and phototrophic abilities might have occurred during the evolution of ancestral Rhodospirillaceae lineages.

Effect of applied magnetic fields on motility and magnetotaxis in the uncultured magnetotactic multicellular prokaryote 'Candidatus Magnetoglobus multicellularis'

Magnetotactic bacteria are found in the chemocline of aquatic environments worldwide. They produce nanoparticles of magnetic minerals arranged in chains in the cytoplasm, which enable these microorganisms to align to magnetic fields while swimming propelled by flagella. Magnetotactic bacteria are diverse phylogenetically and morphologically, including cocci, rods, vibria, spirilla and also multicellular forms, known as magnetotactic multicellular prokaryotes (MMPs). We used video-microscopy to study the motility of the uncultured MMP 'Candidatus Magnetoglobus multicellularis' under applied magnetic fields ranging from 0.9 to 32 Oersted (Oe). The bidimensional projections of the tridimensional trajectories where interpreted as plane projections of cylindrical helices and fitted as sinusoidal curves. The results showed that 'Ca. M. multicellularis' do not orient efficiently to low magnetic fields, reaching an efficiency of about 0.65 at 0.9-1.5 Oe, which are four to six times the local magnetic field. Good efficiency (0.95) is accomplished for magnetic fields ≥10 Oe. For comparison, unicellular magnetotactic microorganisms reach such efficiency at the local magnetic field. Considering that the magnetic moment of 'Ca. M. multicellularis' is sufficient for efficient alignment at the Earth's magnetic field, we suggest that misalignments are due to flagella movements, which could be driven by photo-, chemo- and/or other types of taxis.

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.