When nanoplastics (NPs) meet algae: Heteroaggregates exacerbate bioaccumulation, immunotoxicity, and microbial dysbiosis in the green mussel (Perna viridis).

Heteroaggregates (HAs) formed by nanoplastics (NPs) and microalgae occur ubiquitously in natural aquatic systems. However, their influence on the toxicokinetics and biological effects of NPs in marine mussels remains largely unknown. Here, the green mussels (Perna viridis) were exposed to NPs and HAs at their environmentally relevant concentrations for 21 d, followed by a 7-d depuration phase. The effects on toxicokinetics, immunological responses, and microbiota of digestive gland were evaluated. The results showed that HAs increased the uptake rate constant in digestive gland by 5.5-fold and tissue accumulation of NPs by 2.5-fold compared to NPs alone, resulting in higher NPs burdens after depuration. Meanwhile, HAs exacerbated NPs-induced immunotoxicity, including increased hemocyte mortality and ROS production, and decreased phagocytosis and lysosomal membrane stability. Moreover, HAs led to more pronounced dysbiosis of microbiota in digestive gland than NPs alone, reducing fungal diversity by 56 % and enriching opportunistic fungal pathogens such as Fusarium, while bacterial communities showed minor shifts. This study has provided critical evidence that HAs act as a "Trojan horse," exacerbating NPs risks. This study highlights the necessity of adding the naturally occurring HAs into the ecological risk assessment framework of NPs, especially for benthic filter-feeding organisms.