Collective Response and Navigation Dynamics of Magnetospirillum gryphiswaldense under Rotating Magnetic Fields

Magnetotactic bacteria (MTB) are aquatic microorganisms that biomineralize magnetic nanoparticles called magnetosomes, organizing them into chains that enable navigation along geomagnetic field lines. Their self-propulsion, magnetic responsiveness, and preference for low-oxygen environments make them promising candidates as biohybrid microrobots for biomedical and environmental applications. However, controlling large populations of MTB simultaneously remains a significant challenge. This study analyzes over 30 000 trajectories of Magnetospirillum gryphiswaldense (MSR-1) under rotating magnetic fields (RMF) of varying strengths (0 - 1000 µT) and frequencies $\left(0.1-0.5\mathrm{Hz}\right)$ to characterize individual and collective motility behaviors. Trajectories shift from rectilinear to circular with increasing field strength, while collective alignment emerges above 250 µT. At 1000 µT and $0.1\mathrm{Hz}$ , up to 29.3% of bacteria align in the NorthSeeker (NS) direction and 23% in the South-Seeker (SS) direction. Angular dispersion decreases from ≈ 38.2° to ≈14.7° with increasing field strength, whereas higher RMF frequencies significantly reduce alignment. Swimming velocity remains stable across most conditions, showing a robust bimodal distribution centered near 21.5 and 45 µm s^-1, with a deviation only under highest tested RMF condition. These findings reveal a collective dynamic dependent on the frequency and strength of the magnetic field and highlight that the individual response cannot be straightforwardly translated to collective dynamics.