Adverse multigeneration combined effects of nano-sized plastics and mercury on growth and reproduction in a planktonic copepod Pseudodiaptomus annandalei.

Nano-plastics (NPs) and heavy metals have attracted growing scientific attention because of both pollutants' wide distribution and ecotoxicity. However, the long-term combined toxicity of NPs and mercury (Hg) on planktonic copepods, a crucial presence in marine environments, is unknown. Here, our study aimed to investigate the multigenerational phenotypic responses of the planktonic copepod Pseudodiaptomus annandalei to polystyrene NPs (about 50 nm) and Hg (alone or combined) at environmentally realistic concentrations (23 μg/L for NPs and 1 μg/L for Hg), and the underlying molecular mechanisms were explored. Despite the insignificant effect on survival, NPs could threaten the development and reproduction of P. annandalei, being ascribed to down-regulated genes in ingestive and reproductive functions. Hg exposure revealed inhibition of reproduction probably as an energy trade-off strategy. Importantly, in combined NPs and Hg, development and reproduction were further negatively impacted, even relative to NPs or Hg alone. Correspondingly, combined NPs and Hg presented the most pronounced transcriptomic response with a series of changes in cell functions and down-regulation of key genes in the DNA replication pathway and reproductive function as compared to NPs or Hg alone. The findings indicated adverse combined effects of NPs and Hg on P. annandalei under multigenerational scenarios, being a greater ecological risk for planktonic copepod than NPs or Hg alone. This study provides molecular insights into the long-term toxicity of combined NPs and Hg to planktonic copepods, underlining the increased risk in the population sustainability of marine zooplankton facing co-existing plastics and Hg pollution.