Combined effects of microplastics and petroleum hydrocarbons on Chlorella vulgaris in different aquatic ecosystems:...

Microplastics (MPs), polycyclic aromatic hydrocarbons (PAHs), and total petroleum hydrocarbons (TPHs) frequently coexist in aquatic ecosystems, creating complex pollution scenarios whose interaction with primary producers remains poorly understood. This study evaluates, for the first time, how extracellular polymeric substances (EPS) secreted by Chlorella vulgaris modulate interactions between polyethylene microplastics (PE-MPs) and mixtures of PAHs-TPHs across salinity gradients, and how these processes influence the physiological responses of the microalgae. Using a 2³ factorial design, EPS production and characterization were analyzed using FTIR, PAHs-TPHs removal, aggregate formation, chlorophyll, and growth. The growth of C. vulgaris increased in freshwater and marine water by 36.7% and 57.4%, respectively. In brackish water, however, it was inhibited by up to 80%. Chlorophyll decreased significantly in brackish water as PE-MP concentrations increased. EPS varied according to salinity and pollutant load, reaching a maximum of 54.5 μg mL^-1 in marine water and being positively associated with PAHs-TPHs removal. Under conditions of high salinity or high pollutant load, the decrease in EPS coincided with lower removal and growth inhibition. FTIR analyses showed changes in the functional composition of EPS, and SEM images confirmed the formation of microalgae-EPS-MPs (MEM) aggregates. This work demonstrates that C. vulgaris modulates interactions between PE-MPs and PAHs/TPHs mixtures through EPS, which act as key interfacial mediators that regulate the bioavailability and fate of these pollutants in aquatic systems, providing essential knowledge for the advancement of microalgae-based strategies for the treatment of emerging pollutants.