**Acknowledgements**

the magnetic field increases, in the contrary of a Langevin curve. To explain this, they assumed that the magnetosome chain has an inclination relative to the flagellar

The movement of magnetotactic microorganisms also depends on the presence

) and increasing the velocity when illumi-

), both respectively to the velocity

) [76, 77]. Recently,

bundle. Interestingly, it has been observed that some MTB strains have the magnetosome chain with different inclinations relative to the flagellar bundle, in some cases being almost perpendicular to it, not orienting the magnetic moment to

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<sup>2</sup> of intensity, and blue light (450–490 nm filter) of about 200 W m<sup>2</sup> 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

De Melo and Acosta-Avalos [78] 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

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 bio-

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.

reactors. If high amounts of magnetosomes were achieved at low cost,

the magnetic field direction during their swimming [73].

observed when illuminated with blue light (469 nm, 0.8 W m<sup>2</sup>

with green light (517 nm, 0.46 W m<sup>2</sup>

in MTB.

*Microorganisms*

**14**

**6. Conclusions**

nated with red light (628 nm, 0.16 W m<sup>2</sup>

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).
