In the particular case of metals

In the particular case of metals, several authors showed that AChE inhibition is somewhat contradictory and even questionable (Frasco et al., 2005, Nunes et al., 2005, Sáenz et al., 2010). Uranium is a particular metal, and one that has been shown to complex with several biologically important proteins, including acetylcholinesterase (Coppin et al., 2015). Data obtained by previous studies (Bussy et al., 2006, Bensoussan et al., 2009, Lestaevel et al., 2013) clearly showed that uranium-exposed rodents evidenced significant impairment of cholinesterasic activity in specific areas of the brain. On the other hand, significant increases of cholinesterase activity were reported by Barillet et al. (20007) following exposure to distinct uranium isotopes. Despite these contradictory evidences, the toxicological mechanisms by which metals affect cholinesterase activity is still unclear, and it seems that mechanisms of different nature may be involved (Nunes, 2011). For this reason, in vivo studies are needed to ascertain the potential use of such biomarker to assess a scenario of environmental contamination by metallic species, such as uranium. The main sources of uranium pollution are industrial, extractive activities (including mining). For example, in the central area of Portugal, extensive mining activities were carried out, resulting in circa 13 million tons of solid residues being released/deposited adjacent to mining areas (Carvalho et al., 2007). Among all metallic species found in uranium ore mining areas of the central Portugal, uranium might hypothetically be the most hazardous, due to the combination of radioactivity and chemical toxicity (Sheppard et al., 2005, Antunes et al., 2008). Additionally, uranium seems to move effortlessly from the soil compartment to the aquatic environment (Baykara and Dogru, 2006). Other features that increase its environmental importance are its prompt BTL105 (Clulow et al., 1998, Anke et al., 2007, Neves et al., 2012a, Neves et al., 2012b), bioaccumulation (Barillet et al., 2007), and potential to be biomagnified along the food chain (Simon and Garnier-Laplace, 2005). Taking this into consideration, the evaluation of uranium exposure and chemical effects (i.e., not related to radioactivity) in aquatic biota is much needed. However, the number of studies concerning the exposure of aquatic organisms to uranium and its consequent effects are still scarce, and are usually related to bioaccumulation, mostly in fish (Clulow et al., 1998, Labrot et al., 1998, Rapiejko et al., 2001, Carvalho et al., 2007, Gagnaire et al., 2011, Strømman et al., 2013). The few existing studies focused specifically on the aquatic ecotoxicity of uranium are limited to assessing endpoints such as survival (Holdway, 1992, Cheng et al., 2010), growth (Cheng et al., 2010), metabolism (Augustine et al., 2012), and antioxidant defence (Barillet et al., 2011). Bearing this in mind, the present work intended to study the effect of waterborne uranium on the cholinesterasic activity of several freshwater species: a planktonic filter-feeder, Daphnia magna (crustacean); a benthic filter-feeder Corbicula fluminea (mollusc); and a pelagic vertebrate, Carassius auratus (fish). These species were selected on the basis of their role as model organisms, and on the fact they work as surrogates of native species. Our approach was to roughly simulate a uranium effluent representative of the aquatic system of Cunha Baixa mine (Mangualde, Portugal) in the form of growing concentrations of the soluble uranyl ion (UO22+), mimicking a worst-case environmental scenario. Cunha Baixa is nowadays the location of a deactivated uranium mine, in which metals were extracted (mostly uranium ore) from 1967 to 1993 (Lourenço et al., 2010). Poor ore, mine tailings and muds were buried or left abandoned in the main mining area (Neves et al., 1997; Santos Oliveira and Ávila, 2001). The activities carried out in the vicinity of the mines resulted in widespread contamination of agricultural landscape and aquatic systems (Lourenço et al., 2010), especially with metals but also with radionuclides (Antunes et al., 2007, Machado, 2008), and uranium is particularly relevant.