Multiple climate stressors drive shifts in fatty acid composition and epigenetic regulation in the benthic copepod Platychelipus littoralis

Global climate change is reshaping marine ecosystems through interacting shifts in temperature, pH and food availability, yet how these stressors jointly influence organismal plasticity remains insufficiently resolved. This study examines how warming, ocean acidification and food limitation interact to affect two important drivers of plasticity: fatty acid (FA) composition and DNA methylation in the benthic harpacticoid copepod Platychelipus littoralis. Copepods were collected in March 2025 at 51.20 °N, 3.43 °E and exposed to factorial combinations of the three stressors. Temperature consistently emerged as the dominant driver of physiological change: warming reduced total FA concentrations, increased saturation and decreased essential long-chain n-3 polyunsaturated FAs, including eicosapentaenoic acid (EPA, 20:5n-3). Acidification and food limitation had weaker main effects but produced several context-dependent interactions: under low food availability, acidification reduced EPA levels, while warming combined with either acidification or food limitation increased n-3/n-6 ratios. When all three stressors co-occurred, arachidonic acid (ARA, 20:4n-6) increased, elevating the ARA/EPA ratio, a marker of physiological stress. Global DNA methylation increased under both warming and acidification, with an antagonistic interaction between them, whereas the three-stressor combination produced the highest methylation levels. Together, these results demonstrate that warming is the primary driver of FA depletion and epigenetic modification in P. littoralis, while acidification and food limitation reshape or amplify these temperature-driven responses. These integrated biochemical and epigenetic shifts show how multiple stressors simultaneously challenge organismal plasticity through metabolic regulation, with consequences for its energetic resilience and fitness of P. littoralis under future-climate conditions.