Nanoplastics Stimulate Colony Formation and Bloom in <i>Phaeocystis globosa</i>

The genus <i>Phaeocystis</i> is a globally distributed harmful alga that adversely impacts marine ecosystems. Nanoplastics (NPs) are ubiquitous in marine environments and often co-occur with <i>Phaeocystis</i> blooms. However, the effect of NPs on the colony formation and bloom dynamics of <i>Phaeocystis</i> remains poorly understood. Here, we conducted both indoor and outdoor mesocosm experiments to demonstrate that NPs significantly increased the total cell density, colonial diameter, and colonial density of <i>P. globosa</i>. These enhancements in colonial density and diameter were primarily driven by elevated levels of extracellular polysaccharides as well as key substrates involved in polysaccharide synthesis, including glucose 1-phosphate, adenosine diphosphate glucose, and uridine diphosphate glucose. Concurrently, the accumulation of extracellular polysaccharides was accompanied by the significant upregulation of ten genes associated with N-glycan synthesis. Furthermore, assuming no other environmental changes, our projections indicated that if 0.1% of microplastics (MPs) degrade into NPs, colonial cell densities of <i>Phaeocystis</i> could increase by more than 5% at 242 global locations, with potential maxima approaching 80%. These findings indicate that the ongoing rise in MPs pollution could exacerbate <i>Phaeocystis</i> blooms by enhancing colony formation, posing substantial risks to marine ecosystems.