Bio-energetic traits and hemoglobin as drivers of tolerance to combined exposure to metals and warming

Warming is increasingly recognized to magnify the toxicity of chemicals. Yet, limited understanding of the underlying physiological mechanisms constrains our ability to predict these interactions between warming and toxicants, and how these may evolve under global climate change scenarios. Here, a common-garden experiment was conducted with two resurrected subpopulations of Daphna magna, separated by ca. 40 years in time, of which the recent subpopulation evolved higher heat tolerance. Daphnia were exposed to low (150 µg L^-1) and high (300 µg L^-1) zinc levels across a large thermal gradient (12°C - 30°C). After 96-h exposure, bioenergetic traits, oxidative damage to lipids and hemoglobin levels were quantified. Both zinc levels caused stronger reductions in net energy budget at higher temperatures in both subpopulations. At the highest temperature, low zinc reduced energy consumption in the old subpopulation, whereas the recent subpopulation evolved an increased energy consumption, resulting in a greater decline in the net energy budget. Across treatment groups, survival correlated positively with the net energy budget and oxidative damage, and hemoglobin levels correlated positively with the acute heat tolerance. These findings suggest reductions in the net energy budget to be a key driver of the widespread phenomenon whereby toxicants become more lethal at higher temperatures, and reductions in hemoglobin to contribute to the toxicant-induced declines in heat tolerance. Overall, our study underscores the importance of incorporating physiological mechanisms to understand and predict the current and future sensitivity of organisms to contaminants under ongoing climate change.