Hydrodynamics of microalgae in sea spray

Airborne transmission of biological matter is likely to cause numerous respiratory distress in coastal populations during certain harmful algal blooms. While we know that dissolved algal toxins aerosolize, the contribution of particle-associated toxins is still unknown. The observation of microalgae in the atmosphere suggests that cells or cell debris could be a carrier. Exchange of particulate matter between ocean and atmosphere is driven by sea spray: microplastics and bacteria aerosolize in jet droplets, following the bursting of air bubbles. However, current models for particle capture only consider surface effects, which fail to explain how large microalgal cells (5-50 um) can be entrapped in jet drops (100-800 um). Here, we demonstrate with a variety of species that microalgae are consistently captured in jet drops, and derive a new particle capture model based on the hydrodynamics of the jet. Using the enrichment factor, which measures the increase in concentration in the first jet drop relative to the bulk, we uncover two regimes. The transition between regimes strongly depends on cell size. In addition, we highlight the dynamics of particles within the jet and derive an enrichment model based on hydrodynamics rather than surface considerations. This work widens and generalizes established results on bacterial and microplastic entrapment in jet drops, resolving discrepancies found in the literature. Finally, we demonstrate that sea spray can aerosolize toxic Ostreopsis cf. ovata cells. This reveals a potential particulate pathway for airborne toxicity during harmful algal blooms and raises the question of the relative contribution of particles to that of dissolved toxins.