Chronic toxicity and accumulation of uranium in the aquatic invertebrate Chironomus tentans
Muscatello, Jorgelina Rosa
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Northern Saskatchewan is home to several active uranium (U) mines. Discharges of U to the aquatic environment during mining operations may produce measurable increases in U levels that could pose a potential hazard for aquatic organisms. Aqueous U toxicity has been investigated in a number of aquatic invertebrates, but limited data are available on the effects of U exposure on benthic macroinvertebrates. The objective of this study was to evaluate the toxicity and accumulation of U over the life cycle of the aquatic invertebrate, Chironomus tentans, and to determine the rates of uptake and depuration for U in C. tentans larvae. Chronic U toxicity was evaluated by exposing the animals to three different U concentrations (40, 200, and 1000 µg/L) and an untreated control. Dry weight and U tissue concentration were measured at all life stages and for the exuvia remaining after adult emergence. In addition, some C. tentans adults collected from the treatments were mated with un-exposed adults from an in-house colony to evaluate hatching and mating success. After 10 d of U exposure, C. tentans larvae showed a significant decrease in growth (as dry weight) at mean U concentrations ≥ 157 µg/L. The no observable effect concentration and lowest observable effect concentration for growth in U exposed C. tentans larvae were 39 and 157 µg/L, respectively. Ten-day larval growth retardations correlated strongly with emergence reductions (r² = 0.88) and also led to significant delays in time to adult emergence. Reductions of approximately 30 to 40 % in larval growth corresponded to decreases of 40 to 60 % in adult emergence. This strongly indicates that reductions in C. tentans emergence can be predicted by 10-d growth data. Furthermore, the effects of U on larval growth were observed not only in the directly exposed F₀ larvae, but also in the unexposed F₁ generation larvae. Therefore, U exposure during the paternal generation can significantly influence the growth and reproductive potential of the next generation. Uranium that accumulated during C. tentans immature stages was partially excreted (approximately 50%) during metamorphosis to the adult stage. The process of U elimination can be explained both by physiological changes taking place during metamorphosis, as well as by the shedding of the exoskeleton when molting from the pupal to the adult stage. However, the elimination of U was not complete and, as a result, transfer of U to the adult midges was observed. In order to investigate U uptake and depuration, 10-d old C. tentans larvae were exposed to 300 µg U/L. After 9 d, larvae were transferred to clean water to calculate the U depuration rate. Animal samples were collected every 3 d to evaluate U tissue concentration and dry weight. Steady state conditions during U exposure were approached in 9-11 d. However, accumulated U was rapidly depurated (within 3 d) when larvae were transferred to U free water. The calculated uptake (Kᵤ) and depuration rate constants (Kd) were 20.3 and 0.36, respectively. A separate experiment that measured U uptake in dead and live larvae revealed that accumulation of U in C. tentans larvae could be explained by a passive mechanisms of uptake coupled with an active mechanisms of U depuration.