SPECIES AND LIFE-STAGE SPECIFIC DIFFERENCES IN THE SENSITIVITY OF RAINBOW TROUT AND WHITE STURGEON TO CADMIUM AND COPPER: A MECHANISTIC STUDY
Marked differences in the sensitivity among fish life-stages and species have been observed for cadmium (Cd) and copper (Cu), two contaminants of particular concern in Canadian surface waters. White sturgeon (Acipenser transmontanus) are highly sensitive to metals such as Cu. Moreover, white sturgeon have also shown significant life-stage-specific differences in sensitivity to metals. When compared to rainbow trout (Oncorhynchus mykiss), white sturgeon are more sensitive to waterborne acute Cu exposure, whereas the trend in sensitivity is reversed with Cd exposure. The mechanisms of life-stage- and species-specific differences in sensitivity among fish species are not well understood, especially for data-poor, non-model species such as white sturgeon. Hence, the overall objective of this study was to characterize the physiological, biochemical and molecular drivers of species-specific differences in the sensitivity of white sturgeon and rainbow trout to Cd and Cu over three different early life-stages. During acute waterborne exposure to Cd and Cu, the most important mechanisms of toxicity are the disruption of calcium (Ca) and sodium (Na) homeostasis, respectively. Therefore, I compared the effect of Cd and Cu on uptake and whole-body levels of Ca and Na, respectively, between rainbow trout and white sturgeon across multiple early life-stages. I showed that the greater sensitivity of white sturgeon to Cu was explained by greater reduction of its sodium Na uptake compared to rainbow trout, when exposed to same waterborne Cu concentration. Similarly, higher sensitivity of rainbow trout to Cd was explained by significantly higher reduction in Ca uptake relative to white sturgeon. With both Cu and Cd, reduction in whole-body ion uptake resulted in reduction of whole-body levels of Na and Ca, respectively, and the response level was significantly higher in the more sensitive species. Additionally, I analysed Cd/Cu accumulation, Cd/Cu induced oxidative stress, metallothionein and heat shock protein responses and compared these parameters between rainbow trout and white sturgeon across multiple early life-stages. Species-specific differences in these parameters were evaluated because they are known to be important pathways through which Cd and Cu cause toxicity. In larval and swim-up life-stages, white sturgeon showed greater Cu-induced oxidative damage (lipid hydroperoxide (LPO) induction) than in rainbow trout, which explained the greater sensitivity of white sturgeon to Cu in early life-stages. Similarly, exposure to Cd showed that rainbow trout, the more sensitive species to Cd, had significantly greater LPO induction at the swim-up and juvenile life-stages as compared to LPO levels in the respective life-stages of white sturgeon. Mechanisms such as antioxidant enzymes, antioxidant molecules, metal accumulation and metallothionein could only partially explain the patterns observed in Cd or Cu induced LPO levels with no consistent response across life-stages. Regardless of the underlying mechanism, LPO seems to be a good indicator of species-specific differences in the sensitivity to Cd and Cu between rainbow trout and white sturgeon in selective life-stages. In conclusion, my study showed that ion physiology parameters such as ion uptake and whole-body ion levels are good indicators of species-specific differences in the sensitivity of rainbow trout and white sturgeon to Cd and Cu, and possibly other metals as well. In addition, LPO levels in the gills also generally explained the species-specific differences in sensitivity to Cd and Cu between rainbow trout and white sturgeon. The parameters identified in this study could be good surrogates for testing species-specific differences in the sensitivity to metals. Moreover, the parameters identified in this study are also good candidates for developing predictive approaches for toxicity testing of metals, after further characterization in more species.
cadmium, copper, sensitivity
Doctor of Philosophy (Ph.D.)