Multiscale active transport driven by gravitactic bioconvection promotes resilience in algal blooms

Abstract

Algal blooms are frequently dominated by motile species1,2 whose vertical migration enhances resource acquisition and bloom development3,4. Yet bloom conditions present a paradox: high cell densities intensify nutrient depletion5 and self-shading6, making individual swimming increasingly costly under severe resource limitation. How motile blooms persist and remain resilient under such stress remains unresolved7, particularly as climate-driven warming strengthens stratification and resource scarcity8,9. Here we show that the red-tide-forming phytoplankton Heterosigma akashiwo overcomes bloom-induced constraints through bioconvection, a self-generated active flow that emerges above a critical cell density (>1.5x105 cells/ml). Using a custom ocean-on-chip platform that recapitulates bloom-relevant constraints, we identify an optimal synergy of cell concentration, swimming speed and gravitactic stability that promotes the formation of persistent bioconvective plumes. At constant cell density, plume onset is governed by two phenotypic traits-- vertical swimming velocity and reorientation time--demonstrating that collective transport is governed by the biophysical traits of single cells. We show that bioconvection drives ecologically relevant multiscale transport, enhancing exchange of molecules and micro-cargo across stratified interfaces, mimicking transport of nutrients, extracellular vesicles10 and co-existing species in a bloom environment11. By enabling cells to hitch a hike on self-generated flows when active propulsion becomes energetically prohibitive, bioconvection-mediated transport improves nutrient delivery, restores photosynthetic performance, reverses lipid accumulation associated with nutrient-stress, and facilitates recovery of cellular motility to ultimately mitigate resource limitations. Our findings identify bioconvection as a population-level adaptive mechanism that sustains algal blooms, and reveal a previously unrecognised role of collective microbial motion in bloom persistence under ecological stresses. One sentence summarySelf-organised bioconvection drives multiscale transport and resilience in algal blooms.