Microplastics Role in Metolachlor Toxicity Modulation in the South American Decapod Palaemon argentinus: Bioaccumulation and Biochemical Responses in Sub-Chronic Bioassays

Abstract

Microplastics (MPs) are contaminants of emerging concern reported in most of the world ecosystems including aquatic ones. Due to their small size (1 pm—5 mm), they can be ingested by a wide range of organisms. However, the information about their adverse effects on freshwater invertebrates remains limited. Moreover, MPs have been identified for their capacity to bind and transport other contaminants (such as pesticides). The main goal of the present study was to assess the toxicity of polylactic acid (PLA) MPs, alone and combined with an herbicide (metolachlor, MET), through sub-chronic bioassays using a South American native species (Palaemon argentinos). Organisms were collected in quasi-pristine site and acclimated to laboratory conditions for two weeks. Adults were exposed during 7 days to following experimental treatments (n=3): Control (CTL), MPs (5 mg/L), MET (7.5 kg/L) and the mixture (MPs+MET). The type of MPs polymer was confirmed using attenuated total reflection infrared spectroscopy and the size was characterized using optical microscope (fragments, 75-270 pm mayor side). After exposure, the bioaccumulation as well as damage and defense biomarkers were measured in shrimps’ cephalothorax (CTH) and abdomen (ABD). The MET concentration in the exposure media was measured by LC-MS. CTL and MPs were below the detection limit (1.8 yg/L); MET 6.70+0.37 //g/L and MPs+MET: 6.63+0.29 yg/L. The results showed a significant bioaccumulation of fragments in organisms, with higher values in CTH compared with ABD. In addition, significantly more particles were observed in CTH from MPs and MPs+MET treatments (22.25+0.96 items) compared with CTL and MET conditions (3.25+3.20 items). The content of carbonyl groups in proteins (CP) was quantified, giving a significant increase in CTH from MPs and MPs + MET treatments compared with CTL. Regarding enzymatic activities, significant effects were observed: MET inhibited CAT activity compared to CTL; MPs inhibited AChE activity compared to CTL and MET+MPs; while MET+MPs induced the GST activity respect to all other treatments. No significant difference in BChE activity was observed. The results of this study demonstrate that MPs can bioaccumulate and induces biomolecules damages and neurotoxicity, while MET affected oxidative stress and detoxification processes. Measured biomarkers were significantly modified by the co-exposure to MPs. Our results suggest the usefulness of P. argentinos as bioindicator of MPs exposure