Toxicity of thallium to the water flea (Daphnia magna) and rainbow trout (Oncorhynchus mykiss).

Abstract

Thallium (Tl) is a trace metal that can be found at elevated concentrations in waters associated with the mining and refinement of coal and base metals. Although the effects of Tl on aquatic biota are poorly studied, the Canadian Council of Ministers of the Environment (CCME) has established a guideline value of 0.8 µg L-1 total Tl in fresh water. However, this value is based primarily on effects of Tl to an aquatic plant, and little is known regarding how protective this guideline is to freshwater animals. The goal of this thesis was to use two regulatory model freshwater species, the water flea Daphnia magna and the rainbow trout (Oncorhynchus mykiss) to expand mechanistic understanding of waterborne Tl toxicity and determine whether the current guideline value is likely to be protective of aquatic animals. An acute exposure of Daphnia to Tl established a median lethal Tl concentration of 1860 µg L-1, and the use of asymmetrical flow field-flow fractionation suggests that most Tl in natural waters is in the ionic form (Tl+), even in the presence of binding ligands such as dissolved organic matter. Over a 21-d chronic exposure, complete mortality of D. magna occurred between Tl concentrations of 702 and 1112 µg L-1, with median effect concentrations of 1.6 µg Tl L-1 for growth and 11.1 µg Tl L-1 for reproduction (all concentrations as dissolved Tl). A study examining effects of Tl on daphnid behaviour showed that the presence of Tl significantly affected phototaxis, an effect that was mediated by sensory impairment, indicating that neurotoxicity may be a conserved mode of Tl toxicity between mammals and aquatic biota. The interaction between Tl and potassium (K+), known to be a key mediator of toxicity in mammals, was shown to also persist in D. magna. Elevated water K+ protected against Tl toxicity, the presence of Tl impaired K+ uptake, and pharmaceutical agents thought to block K+ channels modified the effects of Tl on whole-body K+ concentration. The acute (96-h) and sub-chronic (28-d) exposure of rainbow trout to waterborne Tl resulted in the significant accumulation of Tl in all measured tissues (plasma, gill, muscle, otolith). However, in all soft tissues accumulation decreased with increased exposure time, indicating the presence of mechanisms that act to reduce Tl body burden. Thallium acutely stimulated renal proton ATPase activity, but sub-chronically inhibited branchial sodium/potassium ATPase activity, indicating the potential for ionoregulatory disruption in rainbow trout. Conversely, despite evidence of such a mechanism of Tl toxicity in mammals, there were no significant effects of waterborne Tl exposure on any measured oxidative stress endpoint in rainbow trout. These data indicate that CCME water quality guidelines for Tl in freshwater environments are likely to be protective against Tl toxicity in freshwater animals. However, this thesis does indicate that there may be some risk from human consumption of fish sourced from waters that have highly elevated Tl concentrations.