Zheng Zhu and Quan-Xing Liu
The metabolism of a living organism (bacteria, algae, zooplankton) requires a continuous uptake of nutrients from the surrounding environment. However, within local-spatial scales, the nutrients are quickly used up under dense concentration of organisms. Here we report that self-spinning dinoflagellate Symbiodinium sp. (clade E) generate a microscale flows that mitigates competition and enhances the uptake of nutrients from the surrounding environment. Our experimental and theoretical results reveal that this incessant active behavior enhances transports by about 80-fold when compared to Brownian motion in living fluids. We find that the tracers ensemble probability density function for displacement is time-dependent but consisting of a Gaussian core and robust exponential tails (so-called non-Gaussian diffusion). This can be explained by interactions of far-field Brownian motions and a near-field entrainment effect along with microscale flows. The contribution of exponential tails sharply increases with algal density, and saturates at a critical density, implying the trade-off between aggregated benefit and negative competition on the spatial self-organized cells. Our work thus shows that active motion and migration of aquatic algae play a key role in diffusive transport and should be included in theoretical and numerical models on the physical and biogeochemical ecosystems.