What do we know about the ecotoxicological implications of the rare earth element gadolinium in aquatic ecosystems?

Gadolinium (Gd) is one of the most commercially exploited rare earth elements, commonly employed in magnetic resonance imaging as a contrast agent. The present review was performed aiming to identify the Gd concentrations in marine and freshwater environments. In addition, information on Gd speciation in the environment is discussed, in order to understand how each chemical form affects its fate in the environment. Biological responses caused by Gd exposure and its bioaccumulation in different aquatic invertebrates are also discussed. This review was devoted to aquatic invertebrates, since this group of organisms includes species widely used as bioindicators of pollution and they represent important resources for human socio-economic development, as edible seafood, fishing baits and providing food resources for other species. From the literature, most of the published data are focused on freshwater environments, revealing concentrations from 0.347 to 80 μg/L, with the highest Gd anomalies found close to highly industrialized areas. In marine environments, the published studies identified a range of concentrations between 0.36 and 26.9 ng/L (2.3 and 171.4 pmol/kg), reaching 409.4 ng/L (2605 pmol/kg) at a submarine outfall. Concerning the bioaccumulation and effects of Gd in aquatic species, most of the literature regards to freshwater species, revealing concentration ranging from 0.006 to 0.223 μg/g, with high variability in the bioaccumulation extent according to Gd complexes chemical speciation. Conversely, no field data concerning Gd bioaccumulation in tissues of marine species have been published. Finally, impacts of Gd in invertebrate aquatic species were identified at different biological levels, including alterations on gene expression, cellular homeostasis, shell formation, metabolic capacity and antioxidant mechanisms. The information here presented highlights that Gd may represent an environmental threat and a risk to human health, demonstrating the need for further research on Gd toxicity towards aquatic wildlife and the necessity for new water remediation strategies.