Microplastic-induced modulation of interspecific interactions in microalgae under mono- and co-culture conditions

Microplastics have been shown to alter physiological processes in microalgae, key primary producers in aquatic systems. Although microalgae rarely exist in isolation, most studies evaluating microplastic toxicity have relied on monoculture systems, limiting insight into how interspecific interactions influence algal responses. In this study, we directly compared the physiological and morphological responses of two green microalgae, Chlamydomonas reinhardtii and Chlorella vulgaris, to high-density polyethylene (HDPE) microplastics (1000 mg/L; 20 μm) under mono- and co-culture conditions for six days. Algal responses were assessed using cell density, photosynthetic activity, and cell morphology as key endpoints to evaluate how interspecific interactions modulate species-specific sensitivity to microplastic exposure. HDPE microplastics consistently affected C. reinhardtii regardless of culture conditions, whereas alterations in C. vulgaris were more pronounced under co-culture conditions, indicating enhanced sensitivity under interspecific interactions. Overall, HDPE microplastics disrupted algal biomass and reshaped interspecific interactions under co-culture conditions, highlighting the importance of incorporating interspecific interactions into microplastic toxicity assessments of microalgae.