Abstract
The importance of incorporating kinetic approaches in order to gain information on underlying physiological processes explaining species sensitivity to environmental stressors has been highlighted in recent years. Uranium is present in the aquatic environment worldwide due to naturally occurring and anthropogenic sources, posing a potential risk to freshwater taxa in contaminated areas. Although literature shows that organisms vary widely with respect to susceptibility to U, information on toxicokinetics that may explain the variation in toxicodynamic responses is scarce. In the present work, Daphnia magna were exposed to a range of environmentally relevant U concentrations (0 - 200 µg L-1) followed by a 48 h depuration phase to obtain information on toxicokinetic parameters and toxic responses. Results showed time-dependent and concentration-dependent uptake of U in daphnia (ku=1.2 - 3.8 L g-1 day-1) with bioconcentration factors (BCFs) ranging from 1,641 - 5,204 (L kg-1), a high depuration rate constant (ke=0.75 day-1), the majority of U tightly bound to the exoskeleton (~ 50 - 60%) and maternal transfer of U (1 - 7%). Effects on growth, survivorship and major ion homeostasis strongly correlated with exposure (external or internal) and toxicokinetic parameters (uptake rates, ku, BCF), indicating that uptake and internalization drives U toxicity responses in D. magna. Interference from U with ion uptake pathways and homeostasis was highlighted by the alteration in whole-body ion concentrations, their ionic ratios (e.g., Ca:Mg and Na:K) and the increased expression in some ion regulating genes. Together, this work adds to the limited data examining U kinetics in freshwater taxa and, in addition, provides perspective on factors influencing stress, toxicity and adaptive response to environmental contaminants such as uranium.
Highlights
Uranium (U) is a naturally occurring radioactive element with three isotopes (234U, 235U and 238U) and enters aquatic environments due to the release from naturally occurring (U containing minerals) or anthropogenic sources (UNSCEAR, 2016)
After 48 h exposures, the dissolved U loss from the exposure solution ranged from 13 – 29%, with higher loss occurring at lower ambient U concentrations (i.e., 6.7 μg L− 1)
The U concentrations in the control exposures were below the limit of quantification (LOQ) (0.0015 μg L− 1) for both the uptake and depuration phases in the studies
Summary
Uranium (U) is a naturally occurring radioactive element with three isotopes (234U, 235U and 238U) and enters aquatic environments due to the release from naturally occurring (U containing minerals) or anthropogenic sources (UNSCEAR, 2016). Due to the long half-life of 238U, the most abounded U isotope, the radioactivity is relatively low, and it is assumed that adverse effects to aquatic organisms in U contaminated environments result from chemotoxicity (Shep pard et al, 2005; Zeman et al, 2008). Other evaluations of U toxicological thresholds (benchmarks) concluded that water concentrations should not exceed 3.2 – 5 μg L− 1 (Mathews et al, 2009; Sheppard et al, 2005). Large exceedance of benchmark safety thresholds in various water bodies (range: < 1 μg L− 1 to > 7 mg L− 1) suggests needs for more ac curate assessment of environmental and physiological factors modifying toxicity to evaluate risks and whether stricter regulation of U should be required (Waseem et al, 2015). Physical and chemical parameters can alter the mobility and bioavailability of U which can drive toxicokinetic
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