  Highly concentrated active agents tend to exhibit turbulent flows,
reminiscent of classical hydrodynamic turbulence, which has attracted
considerable attention lately. Controlling the so-called active turbulence has
long been a challenge, and the influence of external fields on such chaotic
self-organization remains largely unexplored. Here we report on active
turbulence of Bacillus subtilis bacteria controlled by a uniform magnetic field
via a magnetizable medium based on magnetic nanoparticles. The rod-shaped
bacteria act as non-magnetic voids in the otherwise magnetic medium, allowing
magnetic torques to be generated on their bodies. This leads to an externally
controllable nematic alignment constraint that further controls bacterial
turbulence into a nematic state. The nematic orientational ordering in the
direction parallel to the magnetic field is accompanied by transverse flows
owing to active stress by dipole pushers, which induce undulation of the
nematic state. Remarkably, the typical length of the undulation is almost
independent of the magnetic field strength. Our theoretical model based on the
hydrodynamic equations for suspensions of self-propelled particles predicts the
intrinsic length scale of hydrodynamic instability independent of the magnetic
field. Our findings suggest that magnetic torques are a powerful approach for
controlling both individual agents and their collective states in active
systems.
