Daphnia magna bioconverts eicosapentaenoic acid continuously and independently of dietary supply.

Freshwater food webs are increasingly exposed to eutrophication and warming, which can shift phytoplankton community composition and reduce the availability of long-chain polyunsaturated fatty acids to higher trophic levels. We investigated whether the model freshwater crustacean Daphnia magna can compensate for a dietary lack of eicosapentaenoic acid (EPA) through endogenous EPA synthesis from its precursors, and whether this response depends on food quality, temperature, and genetic background. Two D. magna strains, isolated from contrasting temperate regions, were exposed to four diets ranging from edible cyanobacteria-dominated to cryptophyte-enriched communities at 15, 20, and 25 °C. Using deuterium labelling, compound-specific hydrogen isotope analysis, fatty acid quantification and fitness measurements, we assessed EPA bioconversion and its consequences for consumer performance. Cyanobacteria-rich food reduced biomass accrual, delayed reproduction, decreased body size, and lowered offspring production. Despite these pronounced fitness effects, endogenous EPA production remained stable across all dietary treatments. Instead, bioconversion was primarily governed by water temperature, reaching a maximum at approximately 20 °C, and was slightly higher in the cold-adapted strain, consistent with underlying biochemical constraints on metabolic processes. Slight differences between the two strains suggest an important contribution of genetic background in determining EPA biosynthesis. These results indicate that endogenous EPA synthesis in D. magna was not actively regulated but was constantly active and potentially buffered against low dietary EPA levels, but did not fully compensate for poor food quality.