Surprising speed: Rapid wavefront propagation in active nematic diatoms

Active nematics - systems of self-driven elongated particles exhibiting nematic order - are pervasive in nature; however, their roles in ecological systems remain largely unexplored. Studying mudflats and sands submerged in water, we discover large-scale active nematic phases in benthic diatoms. We replicate this active nematic order in lab conditions using pennate diatoms and demonstrate how biological activity affects orientational order. Notably, we find that fluctuations in the orientational order initiate rapid propagation waves, during which the order parameter temporarily decreases. Surprisingly, diatoms significantly increase their gliding speed during these events, while the wavefront propagates up to ten times faster than the diatoms themselves. These waves are guided by nematic patterns and active stress distributions, resulting in fingering-like structures at the wavefront. The interplay between orientational order fluctuations and diatom motility suggests feedback loops that can stabilize or destabilize the biofilm's structural states. By incorporating diatoms' unique biological activity into the framework of active nematics, our work opens avenues for understanding collective behaviors in ecological systems and offers insights for aquatic ecosystem management.