Collective Response and Navigation Dynamics of <i>Magnetospirillum gryphiswaldense</i> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mo>(</mml:mo> <mml:mrow><mml:mn>0.1</mml:mn> <mml:mo>-</mml:mo> <mml:mn>0.5</mml:mn> <mml:mspace></mml:mspace> <mml:mi>Hz</mml:mi></mml:mrow> <mml:mo>)</mml:mo></mml:mrow> <mml:annotation>$( {0.1 - 0.5 \; {\mathrm{Hz}}} )$</mml:annotation></mml:semantics> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mn>0.1</mml:mn> <mml:mspace></mml:mspace> <mml:mi>Hz</mml:mi></mml:mrow> <mml:annotation>$0.1\;{\mathrm{Hz}}$</mml:annotation></mml:semantics> </mml:math> , 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.