Abstract
The present study highlights for the first time the interplay between model nanoplastics, such as the carboxyl-modified polystyrene nanoparticles (PS-COOH, 60 nm) NPs and the coelomocytes of the sea urchin Paracentrotus lividus, a benthic grazer widely distributed in Mediterranean coastal area, upon acute in vitro exposure (4 h) (5 and 25 μg mL–1). Insight into PS-COOH trafficking (uptake and clearance) and effects on immune cell functions (i.e., cell viability, lysosomal membrane stability, and phagocytosis) are provided. Dynamic Light Scattering analysis reveals that PS NP suspensions in CF undergo a quick agglomeration, more pronounced for PS-COOH (608.3 ± 43 nm) compared to PS-NH2 (329.2 ± 5 nm). However, both PS NPs are still found as nano-scale agglomerates in CF after 4 h of exposure, as shown by the polydispersity index > 0.3 associated with the presence of different PS NP size populations in the CF. The observed changes in ζ-potential upon suspension in CF (–11.1 ± 3 mV and –12.1 ± 4 mV for PS-COOH and PS-NH2, respectively) confirm the formation of a bio-corona on both PS NPs. Optical fluorescence microscopy and fluorimetric analyses using fluorescently labeled PS-COOH (60 nm) reveal a fast uptake of PS-COOH primarily by phagocytes within 1 h of exposure. Upon transfer to PS NP-free CF, a significant decrease in fluorescence signal is observed, suggesting a fast cell clearance. No effect on cell viability is observed after 4 h of exposure to PS-COOH, however a significant decrease in lysosomal membrane stability (23.7 ± 4.8%) and phagocytic capacity (63.43 ± 3.4%) is observed at the highest concentration tested. Similarly, a significant reduction in cell viability, lysosomal membrane stability and phagocytosis is found upon exposure to PS-NH2 (25 μg mL–1), which confirms the important role of surface charges in triggering immunotoxicity. Overall, our results show that, although being quickly internalized, PS-COOH can be easily eliminated by the coelomocytes but may still be able to trigger an immune response upon long-term exposure scenarios. Taking into account that sediments along Mediterranean coasts are a sink for micro- and nanoplastics, the latter can reach concentrations able to exceed toxicity-thresholds for marine benthic species.
Highlights
Being a semi-enclosed and convective basin, the Mediterranean Sea is a high plastic accumulation region, from where plastic debris can hardly elude (Cózar et al, 2015; Suaria et al, 2016; Macias et al, 2019)
Such behavior is typically more pronounced for anionic PS nanoparticles (PS NPs) than cationic ones (Corsi et al, 2020 and references within), it still occurs with the formation of agglomerates of PS-NH2 with the increase in time of exposure (Varó et al, 2019; Eliso et al, 2020) and decrease in temperature of the media (Bergami et al, 2019)
The observed agglomeration of PS NPs in sea urchin CF can be ascribed to osmolarity and ion composition of the CF, which is in balance with the surrounding seawater, since sea urchins are considered osmoconformers, some sea urchin species can sustain positive gradients for the most relevant ions (Mg2+, Ca2+, and K+) in case of reduced salinity (Freire et al, 2011; Santos et al, 2013)
Summary
Being a semi-enclosed and convective basin, the Mediterranean Sea is a high plastic accumulation region, from where plastic debris can hardly elude (Cózar et al, 2015; Suaria et al, 2016; Macias et al, 2019). The resulting behavior of nanoplastics in dynamic and stochastic systems, such as in seawater and in biological milieus, depends on nano-specific properties (e.g., surface charge, size, chemical composition, and functionalization) and those of the receiving environment (e.g., pH, ionic strength, natural organic matter content, and hydrodynamic conditions) (Corsi et al, 2020). The authors showed that such bio-corona was dominated by the toposome precursor protein (TPP), a modified iron-less Ca2+ binding transferrin known as a biotic and environmental stress target (Castellano et al, 2018) This latest study has been conducted on the Mediterranean Sea urchin Paracentrotus lividus, a key species in the organization of benthic communities across Mediterranean coastal areas, controlling shallow macroalgal assemblages either as opportunistic generalist herbivore or as prey for various taxa (Barnes et al, 2002; Boudouresque and Verlaque, 2020). Following short-term exposure (4 h), the results obtained revealed a low acute toxicity of PS-COOH in comparison with PS-NH2, as well as a fast uptake of PS-COOH by phagocytes and their sequestration into lysosomal compartments
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