Membrane lipid metabolism, heat shock response and energy costs mediate the interaction between acclimatization and heat-hardening response in the razor clam Sinonovacula constricta.

Thermal plasticity on different time scales, including acclimation/acclimatization and heat-hardening response - a rapid adjustment for thermal tolerance after non-lethal thermal stress, can interact to improve the resilience of organisms to thermal stress. However, little is known about physiological mechanisms mediating this interaction. To investigate the underpinnings of heat-hardening responses after acclimatization in warm seasons, we measured thermal tolerance plasticity, and compared transcriptomic and metabolomic changes after heat hardening at 33 or 37°C followed by recovery of 3 or 24 h in an intertidal bivalve Sinonovacula constricta. Clams showed explicit heat-hardening responses after acclimatization in a warm season. The higher inducing temperature (37°C) caused less effective heat-hardening effects than the inducing temperature that was closer to the seasonal maximum temperature (33°C). Metabolomic analysis highlighted the elevated content of glycerophospholipids in all heat-hardened clams, which may help to maintain the structure and function of the membrane. Heat shock proteins (HSPs) tended to be upregulated after heat hardening at 37°C but not at 33°C, indicating that there was no complete dependency of heat-hardening effects on upregulated HSPs. Enhanced energy metabolism and decreased energy reserves were observed after heat hardening at 37°C, suggesting more energy costs during exposure to a higher inducing temperature, which may restrict heat-hardening effects. These results highlight the mediating role of membrane lipid metabolism, heat shock responses and energy costs in the interaction between heat-hardening response and seasonal acclimatization, and contribute to the mechanistic understanding of evolutionary change and thermal plasticity during global climate change.