Spontaneous flows and interfacial instabilities in oxygen-sensitive living active matter
Active fluids generate motion and stress internally, but in living systems their activity is often regulated by environmental fields that organisms consume or produce. How such fields localise active stresses and create flow-generating interfaces remains unclear. Here we show that oxygen organises suspensions of the flagellated microswimmer \textit{Euglena gracilis} into an annular living interface. In circular chambers with an air-exposed periphery, a labyrinthine bioconvective pattern and an annular cellular accumulation emerge nearly simultaneously, whereas sealing the periphery suppresses the annulus. The accumulation then sharpens, develops finite-wavelength protrusions and forms a long-lived, collectively rotating corona. An oxygen-coupled polar active-fluid model qualitatively recapitulates this progression: oxygen transport, cellular consumption and oxygen-regulated swimming and reorientation assemble and position the annulus, whereas dipolar active stresses destabilise it and generate collective flow. These results show how a metabolically shaped chemical field can create and activate a living interface, linking taxis, bioconvection and active interfacial hydrodynamics.