Interactive effects of low-density polyethylene microplastics and benzo[a]pyrene on the growth of the freshwater microalgae Raphidocelis subcapitata.

Although microplastics (MPs) and polycyclic aromatic hydrocarbons (PAHs) are widespread in aquatic environments, their combined effects in freshwater systems remain poorly understood. This gap is relevant because green algae, frequently exposed to MPs, play an important role in PAHs bioremediation. This study evaluated how MPs influence the toxicity of benzo[a]pyrene (BaP) in the green microalga Raphidocelis subcapitata. A full-factorial design was used to expose algal cells to three concentrations of low-density polyethylene MPs (5, 50, and 500 mg L⁻¹), tested alone or in combination with the BaP median effective concentration (EC50-72 h) determined for this species. Experiments lasted 72 ± 2 h under controlled temperature, agitation, and light. The isolated and combined effects of MPs and BaP were analyzed using Generalized Linear Models. The EC50 of BaP was 21 µg L⁻¹ (20-25 µg L⁻¹) and the range provided refers to 95% confidence interval (CI95%). When tested separately, MPs at 5 and 50 mg L⁻¹ reduced algal density, likely due to additives associated with the particles. No inhibitory effect was observed at 500 mg L⁻¹, possibly because the particles served as a surface for biofilm formation, supporting growth. In combined exposures, the presence of MPs at 50 and 500 mg L⁻¹ reduced BaP toxicity, consistent with increased sorption of hydrophobic compounds to plastic surfaces and reduced BaP availability in the medium. In contrast, treatments combining BaP with 5 mg L⁻¹ MPs showed greater toxicity than BaP alone, suggesting that this MP concentration was insufficient to remove a meaningful fraction of BaP from the water, allowing interaction with cellular membranes and growth inhibition. These results indicate that the influence of MPs on BaP toxicity is concentration-dependent and nonlinear, suggesting that MPs can either enhance or mitigate BaP toxicity depending on contamination levels. Such interactions may alter the environmental bioavailability of PAHs in freshwater ecosystems, highlighting the importance of considering MP concentrations in ecological risk assessments.