Impact of heatwaves and environmental ammonia on energy metabolism, nitrogen excretion, and mRNA expression of related genes in the indicator model system Daphnia magna.

ABSTRACT

Due to increasing anthropogenic impacts, heatwaves and prolonged exposure to elevated concentrations of ammonia (HEA) may occur in aquatic environments as a single stressor or a combination thereof, potentially impacting the physiology of exposed animals. In the current study, common water fleas Daphnia magna were exposed for one week to either a 5°C increase in temperature, an increase of 300 µmol l-1 total environmental ammonia, or to both of these stressors simultaneously. Exposure to elevated temperature caused a decrease in MO2, ammonia excretion rates, a downregulation of mRNA coding for key Krebs cycle enzymes and the energy consuming Na+/K+-ATPase and V-type H+-ATPase, as well as the energy distributing crustacean hyperglycemic hormone Rh-protein. High environmental ammonia inflicted a lesser inhibitory effect on the energy metabolism of Daphnia, but initiated ammonia detoxification processes via urea synthesis evident by elevated urea excretion rates and a mRNA upregulation of arginase. Effects observed under the combined stressors resembled largely the effects seen after acclimation to elevated temperature alone, potentially due to the animals' capability to efficiently detoxify critical ammonia loads. The observed physiological effects and potential threats of the environmental stressor are discussed in detail.