Tunable self-organization of swimming magnetic bacterial suspensions*

When units of active matter are brought together, the properties of the new construct are not always equal to the sum of the properties of the components. Instead, at each level, new behavior and rules (i.e., emergent properties) appear. For instance, the emergence of spontaneous order in collections of disordered biological components, from suspensions of biofilaments to bird flocks, is widespread in nature. We present population-level self-organization in a flagellated magnetic bacterial suspension of Magnetospirillum magneticum (AMB-1) that is amenable to experimental control using programmable fields. A major hindrance to the understanding of the emergent properties in a typical biological system is its complexity due to competing inter-component interactions, biological and external constraints, non-equilibrium dynamics, as well as stochasticity. Detailed computational modeling of hydrodynamics and magneto-aerotaxis in these motile AMB-1 suspensions reveals the underlying mechanisms leading to their self-organization, by producing experimentally verifiable population-level dynamics. This approach can be generalized to other species and active matter systems in general.