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 C18 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.
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