Polystyrene nanopaticles and their impact on marine ecosystems: accumulation, disposal and toxicity in Mediterranean and Antarctic marine species

Plastic pollution has been recognised as one of the major global threats for marine ecosystems. In the Mediterranean basin, plastic debris are present at concentrations comparable to the subtropical ocean gyres. Microplastics (< 5 mm) also reached the most remote environment on Earth, being reported in Antarctic surface waters and sediments. The limited knowledge on the biological impacts of the smallest fraction of plastic debris, defined as nanoplastics (< 1 μm), is of primary concern, due to their high surface reactivity and nanometric size that allow to easily interact with biological surfaces at the cellular level. Within this thesis, the bioaccumulation and toxicity of nanoplastics in Mediterranean and Antarctic marine organisms have been investigated. Polystyrene nanoparticles (PS NPs), having a nominal diameter of approx. 50 nm and different surface charge, were adopted as nanoplastics as anionic carboxylated (PS-COOH) and cationic amino-modified (PS-NH2). The characterization of their behaviour in natural sea water media was combined with cellular/whole-animal bioassays in order to define uptake, disposition and mechanisms of action under controlled laboratory conditions. The four chapters of the thesis report findings on model organisms including zooplanktonic and benthic species from the Mediterranean Sea and Southern Ocean around Antarctica, as sea urchin embryos (Paracentrotus lividus), green microalga (Dunaliella tertiolecta), brine shrimp (Artemia franciscana) and Antarctic sea urchin (Sterechinus neumayeri) and krill (Euphausia superba) respectively. Overall, we proved that NP surface charge (anionic vs cationic) drives the behaviour of PS NPs in natural sea water in terms of stability and aggregation, which differs in Mediterranean and Antarctic sea waters. Anionic PS-COOH formed large micro-sized aggregates whereas cationic PS-NH2 resulted better dispersed in Mediterranean sea waters. In Antarctic sea waters, such difference in aggregation was reduced, with PS-COOH reaching nano-sized aggregates and PS-NH2 approaching the nominal size. A similar pattern of biodistribution and toxicity was observed in marine organisms from Mediterranean Sea and Antarctica, with respect to NP surface charge. Anionic PS-COOH were mostly ingested, accumulated and excreted with no or limited effect, while PS-NH2 revealed a less clear disposition but cause high toxicity, hinder embryo development and larval growth, disrupt physiology and organism’s survival. For this latter NP, several markers of stress, were identified through up-regulation of apoptotic-related genes (caspase8, hsp60, hsp70 and p-38 mapk) in Mediterranean sea urchin embryos and moulting-related genes (clap, cstb and cb6) in brine shrimp larvae and Antarctic krill juveniles. In Antarctic sea urchin coelomocytes, both PS NPs were able to reduce the cellular phagocytic capacity, with PS-COOH inducing the strongest gene modulation in contrast to a threshold in cell defence ability upon PS-NH2 exposure. Overall, ecotoxicological data constitute a comprehensive baseline on the biological effects of nanoplastics in Mediterranean and Antarctic marine species, motivating future research using more realistic exposure scenarios towards environmental-like conditions and more specific NP-biological interactions (i.e. eco and bio-coronas).