Progress in selecting marine bioindicators for nanoplastics ecological risk assessment

Nanoplastic (<1 µm) pollution in the marine environment is a cause of growing concern due to the current difficulties in measuring their occurrence in abiotic and biotic matrices, with consequent uncertainties on their ecological risk for natural communities and associated ecosystem services. Most investigations dealing with marine nano-ecotoxicity have been conducted on a bench-scale by examining the effects on single model species under short-term exposure conditions and at high concentrations (>50 mgL−1). Both negligible impacts and detrimental effects, although poorly descriptive of the real environmental exposure scenarios, have been documented on different trophic levels and ecological functionalities. Polystyrene nanospheres (<100 nm) are by far the most tested as a proxy for nanoplastics, even though the occurrence of nanoplastics composed by other polymers and shapes (i.e., irregular and fibers) has been reported in seawater column and sediments. Limited information on bioaccumulation in marine species hamper the selection of key bioindicator species following various criteria (i.e., target, highly sensitive, endangered, etc) for pollution monitoring and ecological risk assessment (ERA) purposes. A holistic approach is thus required starting from setting concentrations as environmentally relevant coupled with chronic exposure, and selecting bioindicators including those having a key role in marine ecosystem processes, functions and services, also relevant for human consumption (shellfish and seafood). The present mini-review aims to provide a framework for the selection of the best bioindicators for nanoplastic in the marine environment along with current knowledge on sources, circulation and behavior in temperate and polar environments and potential compartments/species more at risk of exposure, to support nanoplastic ERA. Less investigated ecological niches and habitats, which should deserve more attention in future studies, are also identified.