Spontaneous flows and interfacial instabilities in oxygen-sensitive living active matter

Active fluids generate motion and stress internally, but in living systems this activity is of-ten regulated by environmental fields that the organisms consume or produce. Here we show that oxygen gradients organise and destabilise dense suspensions of the flagellated microswimmer Euglena gracilis. In circular chambers open to air at the periphery, oxygen exchange and cellular consumption generate a radial chemical gradient. An initially homogeneous suspension spontaneously forms a dense cellular ring through oxygen-dependent motility and bidirectional oxytaxis. The ring then develops collective rotation and destabilises into a long-lived corona of protrusions. We reproduce this sequence with an oxygen-coupled polar active-fluid model in which oxygen controls both the direction and speed of cell motion, while dipolar active stresses drive the instability of the dense interface. The simulations show that oxygen taxis create the annular active interface, but the subsequent corona is an activity-driven interfacial instability. Our results reveal how a self-generated chemical gradient can position and activate a living fluid, providing a route to environmental control of active-matter flows and interfaces.