Temperature, <scp>pH</scp>, and diet interactively affect biosynthesis of polyunsaturated fatty acids in a benthic harpacticoid copepod

Abstract Greenhouse gas emissions lead to ocean warming and acidification, negatively impacting marine organisms and their functioning, including long‐chain polyunsaturated fatty acid (LC‐PUFA) production by marine microalgae. Copepods, primary consumers of microalgae, possess a unique capacity for endogenous LC‐PUFA biosynthesis, possibly enabling them to cope with reduced dietary LC‐PUFA availabilities. However, this capacity may be itself impacted by changing oceanographic conditions. In this study, we conducted a laboratory experiment to evaluate the combined effects of warming (+3°C), acidification (−0.4 pH), and dietary LC‐PUFA deficiency on the fatty acid composition and LC‐PUFA biosynthesis (measured by quantitative RT‐PCR) of the benthic harpacticoid copepod Platychelipus littoralis (Brady, 1880). We hypothesized increased LC‐PUFA biosynthesis under all drivers compensating for LC‐PUFA reductions. Lipid profiles of copepods exposed to multiple stressors contained shorter‐chained and more saturated fatty acids. While copepods maintained base‐line relative concentrations of the physiologically important LC‐PUFA docosahexaenoic acid (DHA) on an LC‐PUFA deficient diet at ambient temperatures, DHA concentrations decreased significantly with higher temperatures. Expression of the DHA biosynthesis genes Δ4 front‐end desaturase and elovl1a increased under dietary LC‐PUFA deficiency but did not exceed base‐line levels when simultaneously exposed to acidification. Expression of Δ4 front‐end desaturase and multiple elongases correlated positively with C 18 precursor concentrations and negatively with those of LC‐PUFAs such as DHA, indicating their role as LC‐PUFA biosynthesis enzymes. Overall, our findings suggest that ocean warming and acidification may impede benthic copepods' LC‐PUFA biosynthesis capacity under reduced dietary inputs, limiting their contribution toward global LC‐PUFA availability for higher trophic levels.