Introduction
Organic pollutants (OPs) of anthropogenic origin are widespread in surface water globally including pharmaceuticals and personal care products (PPCPs), per- and polyfluoroalkl substances (PFASs), pesticides, microplastics (MPs), and so on. In this study, the European eel (Anguilla Anguilla Linnaeus, 1758) was selected to study the bioconcentration of OPs, given its physiological and behavioural characteristics, which make it vulnerable and sensitive to the presence of these substances (1). Furthermore, in 2007, it was placed on the International Union for Conservation of Nature’s Red List of Endangered Species, listed as "critically endangered" (2). Overfishing, water barriers hindering migration, climate change, parasitic diseases, habitat reduction and chemical pollution, among others, can be defined as main causes of the drastic reduction of its population since 1970 (1). The general aim of this research was to study the bioconcentration and depuration kinetics of a mixture of 22 OPs in muscle, liver tissue and plasma of European eel, and to evaluate the influence of MPs in them.
Materials and methods
The OPs included 10 PPCPs, 5 pesticides, 5 PFASs and 2 illicit drugs, which were vehiculated through water or food, depending on their chemical properties. The compounds of the former group are acetaminophen, atenolol, bentazone, bufotenine, caffeine, diclofenac, etoricoxib, ibuprofen, imazalil, naproxen, perfluorobutane sulfonate (PFBS), perfluorodecanoic acid (PFDA), perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA), perfluoropentanoic acid (PFPeA), salicylic acid, terbuthylazine, and vildagliptin (n=18). The compounds that were supplied through food include (n=4): chlorfenvinphos, chlorpyrifos, triclosan and 4-methoxyphencyclidine (4-MeO-PCP). The experimental study lasted 58 days including: (a) an exposure stage (days 0-28), in which the mixture of OPs and spherical MPs (polyethylene) was administered through water (10 µg/L of OPs and 0.04 g MPs per specimen), and through food (20 ng per eel/day); and (b) a depuration phase (days 29-58), in which the water from aquariums was renewed (without OPs). OPs in eels’ muscle and liver samples were extracted by QuEChERS and dispersive solid phase extraction (dSPE) whereas plasma was extracted by SPE (3). Extracts were analysed via UHPLC-MS/MS. The average weight and lengths of the silver (adult) European eels (n=144) employed were 63.3±18.3 g and 30.8±3.1 cm.
Results and discussion
The exposure through water of diclofenac, etoricoxib, imazalil, PFDA, PFOA, PFOS, terbuthylazine, and through food of chlorpyrifos, at the tested concentrations, seemed to cause their bioconcentration in the three eels’ tissues (with and without MPs). Other pollutants appeared to show a preferential tissue bioconcentration as caffeine, chlorfenvinphos (muscle), naproxen (liver), bufotenine and PFBS (plasma), triclosan (muscle, liver), and 4-MeO-PCP (liver, plasma). During the exposure stage, two different trends were observed. A first group of OPs with increasing concentrations in the three tissues (PFDA, PFOS, PFOA, chlorpyrifos, and terbuthylazine), or in some of them, and a second group with less clear or erratic trends. PFASs concentrations followed the trend plasma > liver tissue > muscle. The occurrence of MPs seemed to play an important role in the bioconcentration kinetic of the OPs analysed here. Some recent research have proved that MPs can act as vectors of pollutants in the aqueous environment (4). During the depuration stage, the OPs described two trends. In the first one, concentrations tended to decrease (chlorpyrifos, terbuthylazine), and in the second one, concentrations were similar or even tended to be higher than were those measured at the exposure phase (PFDA, PFOS, PFOA). In relation to the occurrence of MPs, concentrations reached at the end of the depuration phase tended to be higher at the group with MPs than at group without them in muscle and liver tissue, but lower in the group without them (plasma).
Conclusions
Many pollutants are widespread and measured concentrations are at a level that more than likely is causing ecotoxicological effects in European eels. Legacy substances such as PCBs and heavy metals are relatively well studied, however the impact on the eel of emerging compounds (e.g. PPCPs, PFASs, illicit drugs, etc.) which are known to pose serious and increasing problems in aquatic ecosystems, is poorly understood. Compared to other fish species, eels are challenging to study and to protect due to their catadromous and semelparous lifecycle. We have proved that pollutants are not distributed evenly among eels’ tissues and that higher concentrations can be found in plasma and liver than in muscle. Bioconcentration and depuration of OPs in eels seemed to be highly determined by the type of pollutant, and by different processes related to the combination of a mixture of chemicals including the presence of MPs. Further research is needed to clearly understand the effects and toxicity, singly and cumulatively, of multiple and different pollutants in target organs of (wild) eels, and to assess their partitioning and distribution among eel tissues.
References
1. Belpaire, C., et al., Impact of chemical pollution on Atlantic eels: Facts, research needs, and implications for management. Current Opinion Env. Science Health 2019. 26-36.
2. ICES, Joint EIFAAC/ICES/GFCM Working Group on Eels (WGEEL). ICES Scientific Reports. 4:62. 297 pp.
3. Vitale, D., et al., Determination of organic pollutants in A. anguilla by LC-MS/MS. MethodsX, 2021. 8.
4. Kang, P., et al., The unheeded inherent connections and overlap between microplastics and poly- and perfluoroalkyl substances: A comprehensive review. Science of the Total Environment, 2023. 878: p. 163028.