On the backward excursions in the free-swimming magnetotactic multicellular prokaryote 'Candidatus Magnetoglobus multicellularis'

Magnetotactic bacteria align to magnetic field lines while swimming in a behavior known as magnetotaxis. They are diverse phylogenetically and morphologically and include both unicellular and multicellular morphologies. The magnetotactic multicellular prokaryote (MMP) 'Candidatus Magnetoglobus multicellularis' has been extensively studied, even though it remains uncultured up to now. It swims back and forth along magnetic field lines, exhibiting a preferential swimming direction that is usually south-seeking, as described for most magnetotactic microorganisms from the Southern Hemisphere. In order to understand the effects of the magnetic field intensity on the backward excursions of 'Ca. M. multicellularis', we applied magnetic fields ranging from 0.09 to 3.4 mT and recorded their movements. Each microorganism was followed frame by frame generating position coordinates, which were used to calculate the frequency of reversal events, as well as the time, distance, and velocity. The velocities in forward movements before and after backward excursions are similar, but no relation was found with the velocity in backward movements. The shapes of the trajectories are distinct in forward and backward movements. In addition, the backward velocities are usually higher. The sharp changes in direction (approximately 180°) indicate that reversal of the flagella rotation direction is the probable mechanism for swimming backward. In conclusion, the backward excursions provide additional freedom of movement to the microorganism, especially when it is constrained by magnetic fields stronger than the Earth's. Backward movements integrate the 'Ca. M. multicellularis' behavioral toolbox, which includes also negative phototaxis, photokinesis, magnetotaxis and possibly helical klinotaxis.