Molecular Insights into the Genesis of Heat Hardening in Marine Bivalves

Heat hardening induces complex biochemical reprogramming that enhances thermal resilience in marine bivalves. Despite this technique's promising results in marine animals, the molecular basis of heat hardening is far from understood. This study elucidates the molecular mechanisms underlying the hardening process in <i>Mytilus galloprovincialis</i> exposed to a 4-day sublethal heat treatment. Induction of <i>hsf-1</i>, <i>hsp70</i>, and <i>hsp90</i> genes revealed the activation of the heat shock response and proteostasis machinery, ensuring proper protein folding and preventing oxidative and proteotoxic stress. Simultaneous upregulation of mitochondrial (<i>atpase6</i>, <i>cox1</i>, <i>nadh</i>) and glycolytic (<i>pk</i>, <i>cs</i>) genes reflects enhanced oxidative phosphorylation and glycolytic flux, maintaining ATP supply and metabolic flexibility under elevated temperatures. Increased <i>hif-1α</i> expression suggests transient hypoxia signaling, coordinating oxygen utilization with redox control. Reinforcement of antioxidant defenses, together with elevated autophagy-related transcription, denotes a shift toward oxidative stress mitigation and damaged organelle clearance. Balanced expression of pro- (<i>bax</i>) and anti-apoptotic (<i>bcl-2</i>) factors, along with <i>nf-κb</i> modulation, supports tight regulation of cell survival and inflammatory responses. These findings underscore a highly integrated biochemical network linking proteostasis, intermediary metabolism, redox balance, and antioxidant defense with cellular quality control, which together underpin the physiological plasticity of heat-hardened <i>M. galloprovincialis</i>, enhancing survival under transient thermal stress.