Q. Zhang, M. Prakash
At the ocean-land interface lies a critical yet largely ignored ecosystem of tidal flats[1]. These habitats are colonized by diatoms that form thin surface biofilms, creating ecosystem-scale cellular collectives stretching meters to kilometers[2-4]. How these collectives coordinate functions across vast spatiotemporal scales remains poorly understood. This task requires both long-distance transfer and long-term storage of information[5, 6]. In cellular collectives, trigger waves have been known to provide rapid, non-attenuating signaling[7-9], while extracellular matrices allow cells to externalize structural memory[10-12]. Yet, how propagating signals and structural memory integrate to shape collective functions remains elusive. Combining field observations with laboratory reconstitution, we show that tidal-flat diatoms assemble active nematic circuits that physically wire trigger-wave routing into structural memory. Here, trigger waves recruit resting diatoms into polarized motion steered by the nematic director, and topological defects act as gates that focus or bifurcate propagating wave fronts. Memory follows a fast-read, slow-write rule: rapid signals deform the cellular nematic, while slowly evolving cell-deposited mucilage retains a hidden orientational template that guides alignment recovery. Strikingly, recurrent waves progressively remodel the nematic architecture, letting the collective reshape its future routing. Coupling rapid wave propagation to extracellular memory allows collectives to resolve the stability-plasticity dilemma, maintaining structural cohesion while adapting to recurrent stimuli. Our work reveals previously hidden adaptive nematic circuits that self-organize at ecological scales, establishing a physical framework for time-programmable active materials.