| Summary: | Hypoxia is a global and increasingly important stressor in aquatic ecosystems, with major impacts on biodiversity worldwide. Hypoxic waters are often contaminated with a wide range of chemicals but knowledge is limited about the interactions between hypoxia and chemical stressors. Therefore, during this PhD I set out to investigate how the concentration of oxygen in the water influences chemical toxicity in fish. My work focused on two groups of chemicals: toxic metals, of which copper was used as an example, and anti-androgenic chemicals. Copper is an essential metal, widespread in the aquatic environment, but it can become toxic to aquatic organisms when environmental levels become too high. Copper and hypoxia are likely to co-occur but despite this our understanding of the interactions between these two stressors is limited. To address this, I performed a series of experiments using both the zebrafish (Danio rerio) and the three-spined stickleback (Gasterosteus aculeatus) as fish models. I first investigated the effects of hypoxia on copper toxicity to zebrafish embryos during development. Copper toxicity was reduced by over 2-fold under hypoxia compared to normoxia during the first day of development, and I demonstrated that this protective effect was associated with the activation of the hypoxia inducible factor pathway. In contrast, hypoxia increased copper toxicity in hatched larvae, which was deduced to be associated with differential copper uptake. To test if the interactions between copper and low oxygen observed for the zebrafish also occurred for other fish species, exposures were conducted in a species with a lower tolerance to hypoxia, the three-spined stickleback. The results obtained showed that hypoxia suppressed copper toxicity prior to hatching, but after hatching this effect was reversed, similarly to that observed for the zebrafish. This suggests a potential conserved effect of hypoxia on copper toxicity during embryogenesis across fish species. To investigate if life stage influences the interactions between low oxygen and copper toxicity, the effects of combined exposures were assessed in the adult male three-spined stickleback. The critical oxygen level (Pcrit) was determined to allow appropriate experimental design. The combined exposures to copper and low oxygen resulted in a decreased ability to acclimate to low oxygen. Fish were able to lower their Pcrit in response to low oxygen conditions when exposed to hypoxia alone but not when exposed to hypoxia in combination with copper. Together, these datasets support the hypothesis that the life stage influences the effects of the combined exposure, as hypoxia protects from copper toxicity during early embryogenesis but increases copper toxicity in hatched embryos and in adults. I then investigated whether hypoxia can affect the toxicity of another widespread group of pollutants, anti-androgenic chemicals. Male three-spined sticklebacks were exposed to three anti-androgenic chemicals, flutamide, linuron and fenitrothion, under different air saturations. Each chemical had a unique transcriptional response alone and in combination with reduced oxygen saturation. Under both air saturations, spiggin transcription was strongly inhibited by exposure to flutamide. In contrast, exposure to fenitrothion did not result in a significant effect on spiggin transcription. Interestingly, linuron strongly inhibited spiggin under 100% air saturation, but this effect was absent under low air saturation, potentially as a result of interactions between the hypoxia inducible factor pathway and the aryl hydrocarbon receptor pathway. This work illustrates the potential mechanisms responsible for interactions between reduced oxygen and chemical toxicity, especially for aryl hydrocarbon receptor agonists, and highlights how hypoxia can modify the effects of a variety of chemicals with diverse modes of action. My research highlights the importance of considering the interactions between multiple stressors, and the need to take into account the type of chemical, life stage, and the species tolerance. Understanding these interactions is essential to facilitate the accurate prediction of the consequences of exposure to complex stressors in a rapidly changing environment.
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