Controlled bioconvection of magnetotactic bacteria

Magnetotactic bacteria (MTB) synthesize magnetic nanoparticles (magnetosomes) in their cell membrane, and use earth magnetic field to efficiently navigate for resources in oceans and swamps. In this work, we demonstrate that active cell motility feeds back into the ambient Poiseuille flow to drive the formation of bioconvective cells when MTB are directed upstream. Dense suspensions of MTB are seeded with fluorescent tracers, and video microscopy simultaneously captures both the active bacterial and passive tracer motion, when MTB are driven upstream by a Helmholtz coil through a microfluidic channel. Experiments are complemented by numerical simulations to decipher the relative roles of the rotlet exerted by the external magnetic field on the MTB (passive) and the recirculating convective cells resulting from MTB self-propulsion (active) in the instability. Such collective behaviors in driven active matter systems may elucidate novel survival mechanisms in MTB ecosystems and have the potential to impact bioremediation and drug delivery applications that employ MTB.