Polystyrene microplastics at environmentally realistic concentrations exacerbate diatom blooms caused by phosphorus pollution: Rethinking coastal eutrophication

Algal blooms are serious and frequent environmental events occurring on coasts and are mainly attributed to eutrophication (e.g., phosphorus (P) pollution). As pervasive xenobiotics, microplastics (MPs) potentially influence algal growth. Although these pollutants co-occur on coasts, the combined effects of MPs and P pollution on algal blooms remain poorly understood. We found that polystyrene (PS) MPs at environmentally realistic concentrations (1-10 μg/L) had no adverse effect on diatoms. In the presence of P pollution, however, 10 μg/L PS MPs significantly increased the accumulation of cell-associated organic phosphorus by 11 % and the cell density by 13 % at 120 h. PS MPs accelerated P transport from extracellular polymeric substances (EPS) to intracellular pools, where the P content in the EPS (EPS-P) of P+MP-treated cells was 41 % lower than that in P-treated cells, which increased P pollution and subsequently drove diatom blooms. This shift was further supported by the strong inverse correlation between growth rates and the EPS-P content. The diatoms in the P+MPs group presented denser thylakoid membranes and greater photosynthetic activity. The genes related to phosphorus transporters, photosynthesis, carbon assimilation, and cell division were upregulated upon P+MPs treatment. These findings indicate that PS MPs at environmentally realistic concentrations exacerbate P-driven diatom growth, highlighting the need to integrate MP pollution into predictions and warnings of diatom blooms on coasts.