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 often regulated by environmental fields that the organisms consume or produce. Here we show that oxygen gradients organise dense suspensions of the flagellated microswimmer $\mathit{\text{Euglena}}\mathit{\text{gracilis}}$ and trigger an active interfacial instability. In circular chambers open to air at the periphery, oxygen exchange and cellular consumption generate a radial chemical gradient. Starting from an initially homogeneous suspension, cells spontaneously localise into a dense annular band through oxygen-dependent motility and bidirectional oxytaxis. This oxytactically formed ring then deforms and undergoes collective azimuthal motion, rotating as a long-lived corona of protrusions. We reproduce this sequence with an oxygen-coupled polar active-fluid model in which oxygen regulates both cell reorientation and motility, while dipolar active stresses drive the deformation and flow of the dense interface. The simulations show that oxygen taxis creates and positions the annular active interface, whereas 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.