Stress response mechanisms in <i>Daphnia magna</i>: role of HSPs and MAPKs under different environmental conditions

Abstract Freshwater organisms like Daphnia magna are frequently exposed to fluctuating environmental conditions that challenge their physiological resilience. This study investigates the cellular stress responses of D. magna to acute exposure to stressors, including changes in temperature (heat and cold shock), salinity, pH and fish predation. Specifically, we analyzed the expression patterns of heat shock proteins (HSPs) and mitogen-activated protein kinases (MAPKs), two key molecular biomarkers involved in cellular stress responses. Elevated HSP90 and HSP70 levels were observed under salinity stress and fish kairomones, suggesting sustained stress activation. However, under certain conditions—such as extreme temperatures, high pH and prolonged stress exposure—downregulation of HSPs and MAPKs was detected, potentially due to energy constraints, impaired cellular machinery or stress overload. Additionally, differential MAPK phosphorylation patterns suggest a context-dependent role in regulating growth and reproduction, particularly under fish predation. These findings demonstrate the complex, stressor-specific dynamics of HSP and MAPK responses in D. magna, offering insights into their roles in organismal adaptation. The results highlight the importance of considering energy availability, exposure duration and life-history trade-offs when evaluating molecular stress responses. Further time-course and functional studies are needed to elucidate the precise roles of these proteins in stress tolerance and fitness regulation.