Effects of the Interaction of Salinity and Rare Earth Elements on the Health of Mytilus galloprovincialis: The Case of Praseodymium and Europium

Abstract

The growing use of products containing rare earth elements (REEs) may lead to higher environmental emissions of these elements, which can potentially enter aquatic systems. Praseodymium (Pr) and europium (Eu) are widely used REEs with various applications. However, their ecotoxicological impacts remain largely unexplored, with poorly understood risks to wildlife. Moreover, organisms also face environmental stressors like salinity fluctuations, and the nature of the interaction between salinity variations and contaminants is not yet clear. Therefore, this study aimed to evaluate the influence of salinity shifts on the impacts of Pr and Eu on adult mussels and the sperm of the species Mytilus galloprovincialis after 28 days and 30 min of exposure, respectively. To do so, biochemical and histopathological alterations were evaluated in adults, while biochemical and physiological changes were analysed in sperm. Additionally, the Integrated Biological Index (IBR) was calculated to understand the overall impact of each treatment. The results showed that adult mussels were most affected when exposed to the combination of high salinity and each element, which altered the behaviour of defence mechanisms causing redox imbalance and cellular damage. On the other hand, sperm demonstrated sensitivity to specific REE-salinity combinations, particularly Pr at lower salinity and Eu at higher salinity. These specific treatments elicited changes in sperm motility and velocity: Pr 20 led to a higher production of O₂^- and a decrease in velocity, while Eu 40 resulted in reduced motility and an increase in irregular movement. At both lower and higher salinity levels, exposure to Eu caused similar sensitivities in adults and sperm, reflected by comparable IBR scores. In contrast, Pr exposure induced greater alterations in sperm than in adult mussels at lower salinity, whereas the reverse was observed at higher salinity. These findings suggest that reproductive success and population dynamics could be modulated by interactions between salinity levels and REE pollution, highlighting the need for further investigation into how REEs and environmental factors interact. This study offers valuable insights to inform policymakers about the potential risks of REE contamination, emphasising the importance of implementing environmental regulations and developing strategies to mitigate the impact of these pollutants.

Keywords: adult mussels; biochemistry; histopathology; rare earth elements; salinity; sperm.

Figure 1

(A) Centroid ordination diagram (PCO) based on all biological responses measured in adults and sperm of Mytilus galloprovincialis exposed to control (CTL), praseodymium (Pr), and europium (Eu) at salinities of 20, 30, and 40. Spearman correlation vectors were superimposed as supplementary variables (r > 0.7): ETS, PROT, GLY, CAT, GPx, CbEs-pNPB, GSH/GSSG, LPO (adults), O₂⁻, % MOT, and VCL; (B) Integrated Biological Response (IBRv2) score divided by the number of parameters measured in adults and sperm of Mytilus galloprovincialis exposed to control (CTL), praseodymium (Pr), and europium (Eu) at salinities of 20 and 40.

Figure 2

(A) Electron transport system (ETS) activity, expressed in nmol per min per g of fresh weight (FW); (B) protein (PROT) content, expressed in mg per g of FW; (C) glycogen (GLY) content, expressed in mg per g of FW; (D) superoxide dismutase (SOD) activity, expressed in U per g of FW, where U is a reduction of 50% in nitroblue tetrazolium (NBT) levels; (E) catalase (CAT) activity, expressed in nmol of formaldehyde per min per g of FW; (F) glutathione peroxidase (GPx) activity, expressed in nmol per min per g of FW; (G) carboxylesterase with p-nitrophenyl acetate (CbE-pNPA) activity, expressed in nmol per min per g of FW; (H) carboxylesterases with p-nitrophenyl butyrate (CbEs-pNPB), expressed in nmol per min per g of FW; (I) glutathione S-transferases (GSTs) activity, expressed in nmol per min per g of FW; (J) reduced-to-oxidized glutathione (GSH/GSSG) ratio; (K) lipid peroxidation (LPO) levels, expressed in nmol of malondialdehyde (MDA) per g of FW; (L) protein carbonylation (PC) levels, expressed in nmol per g of FW, in Mytilus galloprovincialis exposed to control (CTL), praseodymium (Pr), and europium (Eu) at salinities of 20, 30, and 40 for 28 days. Results are means with standard deviations. Significant differences (p < 0.05) among salinities are denoted by different letters (bold italic lowercase letters stand for CTL treatments, uppercase letters stand for Pr treatments, and lowercase letters stand for Eu treatments).

Figure 3

Histopathological index (Ih) in (A) gills; (B) digestive tubules of Mytilus galloprovincialis exposed to control (CTL), praseodymium (Pr), and europium (Eu) at salinities of 20, 30, and 40 for 28 days. Results are means with standard deviations. Significant differences (p < 0.05) among salinities are denoted by different letters (bold italic lowercase letters stand for CTL treatments, uppercase letters stand for Pr treatments, and lowercase letters stand for Eu treatments).

Figure 4

Representative micrographs of the histopathological alterations observed in (A) gills and (B) digestive tubules of Mytilus galloprovincialis exposed to control (CTL), praseodymium (Pr), and europium (Eu) at salinities of 20, 30, and 40 for 28 days and stained with haematoxylin. Alterations: lipofuscin aggregates (*); enlargement of the central vessel (double-headed arrow); haemocyte infiltration (red circle); loss of cilia (single arrow) atrophy (a); necrosis (n). Scale bar: 50 µm.

Figure 5

(A) Superoxide-anion-derived reactive oxygen species (ROS (O₂⁻)) production, expressed in arbitrary units of fluorescence intensity (a.u.); (B) hydrogen-peroxide-derived reactive oxygen species (ROS (H₂O₂)) production, expressed in a.u.; (C) lipid peroxidation (LPO) levels, expressed as the fluorescence intensity ratio (FIR); (D) motility, expressed as percentage (%) of motility; (E) curvilinear velocity (VLC), expressed in µm per s; (F) linearity (LIN); and (G) wobble (WOB) in sperm of the species Mytilus galloprovincialis exposed to control (CTL), praseodymium (Pr), and europium (Eu) at salinities of 20, 30, and 40 for 30 min. Results are means with standard deviations. Significant differences (p < 0.05) among salinities are denoted by different letters (bold italic lowercase letters stand for CTL treatments, uppercase letters stand for Pr treatments, and lowercase letters stand for Eu treatments).