Thermal stress differentially regulates eicosanoid and fatty acid metabolism in gills of the blue mussel (Mytilus edulis).

Thermal stress significantly affects bivalve aquaculture by triggering physiological disturbances and high mortality events. The objective of this study was to investigate how acute and repeated thermal stress alters the eicosanoid metabolism and the highly unsaturated fatty acid (HUFA) composition in gill tissues of the blue mussel, Mytilus edulis. Mussels were exposed either to a single acute stress event (temperature raised from 12 °C to 25 °C for 1 h) or repetitive daily stress cycles over one week (daily temperature fluctuations between 12 °C and 25 °C). Using GC-FID and LC-QTOF-MS, fatty acids and lipid mediators derived from arachidonic acid (ARA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) were quantified. Acute stress triggered significant increases in both enzymatically (Lipoxygenase, LOX; cyclooxygenase, COX) and non-enzymatically (ROS-mediated) produced ARA metabolites, including HETEs, prostaglandin E2 (PGE2), and isoprostanes (15-deoxy-Δ12,14-PGJ2). Acute exposure also elevated EPA- and DHA-derived metabolites with recognized anti-inflammatory and protective roles, e.g., hydroxyeicosapentaenoic acids and hydroxydocosahexaenoic acids (HEPEs and HDHAs). In contrast, repeated stress induced a decrease in most measured metabolites, indicating acclimatization and potential metabolic conservation. Despite notable changes in lipid mediator profiles, repeated stress did not significantly alter total ARA, EPA, or DHA proportions, suggesting selective modulation of lipid pathways rather than broad depletion of lipid substrates. In conclusion, acute thermal stress markedly activates both pro-inflammatory and protective lipid pathways in mussel gills, while repeated thermal stress led to a decrease in the levels of certain metabolites, possibly associated with stress acclimatization and energy conservation. These findings highlight the value of eicosanoids and related lipid metabolites as potential biomarkers of thermal stress responses in marine invertebrates.