Microplastics induce PSTs synthesis in Alexandrium tamarense and their combined toxicity to juvenile Mytilus galloprovincialis.

To explore the ecological risks of marine shellfish under the combined stress of microplastics and toxic red tide algae, this study took polypropylene (PP), polyethylene, polyethylene terephthalate (PET) microplastics, Alexandrium tamarense and juvenile Mytilus galloprovincialis as research objects. We analyzed the effects of microplastics on the growth, photosynthetic physiology and paralytic shellfish toxins (PSTs) synthesis of A. tamarense, as well as the impacts of single and combined exposure to microplastics and A. tamarense on enzyme activities of juvenile shellfish. The results showed that the effects of the three microplastics on A. tamarense were concentration- and type-dependent, and all microplastics extremely significantly induced PSTs synthesis in a concentration-dependent manner, with 50 mg·L-1 PET-MPs exhibiting the most remarkable induction effect. The combined exposure to microplastics and A. tamarense showed synergistic/additive toxicity to juvenile shellfish: the activities of antioxidant enzymes presented a pattern of "initial induction followed by inhibition", the changes in the activities of metabolism-related enzymes were type-specific, and the amplitude of enzyme activity changes in the combined exposure groups was significantly higher than that in the single exposure groups. Principal component analysis revealed a co-regulation of superoxide dismutase and alkaline phosphatase under AT+PP stress, and acetylcholinesterase was identified as the core biomarker. Microplastics exacerbate toxicity by inducing algal toxin production and acting as toxin carriers. The combined pollution of microplastics and A. tamarense poses a dual risk to inshore shellfish aquaculture and marine ecosystems. This study provides experimental evidence for the ecological risk assessment of marine combined pollution.