LC-PUFA endogenous biosynthesis is improved in Nile tilapia fed plant-oil sources at cold suboptimal temperature

Nile tilapia, though a tropical species, is cultured throughout most subtropical regions of the globe, in areas with large seasonal thermal variations. The maintenance of sustained productivity during adverse environmental conditions, such as winter, is a growing area of research effort, including nutritional interventions and seasonal specific diets. The present study aimed at evaluating how the in vivo fatty acid metabolism is modulated by the rearing temperature in juvenile Nile tilapia fed different lipid sources. Four experimental diets were tested in two trials at optimal (28 °C) and suboptimal (22 °C) temperature, and the dietary treatments were: a diet containing fish oil (FO), and therefore rich in n-3 LC-PUFA (long-chain PUFA of 20 or 22 carbons), and three plant-oil based diets rich in C₁₈ PUFA (PUFA of 18 carbons), containing formulated blends including sunflower oil (rich in 18:2n-6) and linseed oil (rich in 18:3n-3), to achieve a gradient of C₁₈ PUFA n-3/n-6 ratio. The results showed that the in vivo fatty acid metabolism was substantially modified by the diets and the environmental conditions. Among dietary treatments, differences were noted in the enzyme activities for SFA (saturated fatty acids) and MUFA (monounsaturated fatty acids) neogenesis and bioconversion. The activities of the enzymes involved in C₁₈ PUFA bioconversion towards LC-PUFA, namely elongases, Δ5Δ6-desaturase and Δ4-desaturase, were low in Nile tilapia fed FO diet, whereas in fish fed the plant-oil based diets the bioconversion towards n-6 and n-3 LC-PUFA was proportional to substrate availability. Temperature was responsible for important modifications, including a clear metabolic attempt towards increasing LC-PUFA content, and a simultaneous reduction of other classes, in fish exposed to cold suboptimal conditions. In fact, Nile tilapia fed plant-oil diets at 22 °C increased the activities of PUFA elongases and desaturases, whilst decreased activities of SFA neogenesis, Δ9 desaturation, and elongation of SFA or MUFA. Interestingly, at 22 °C, Δ4 desaturation increased with dietary 18:3n-3 increase, while at 28 °C, fish fed the diet poor in 18:3n-3 had no apparent activity of this enzyme. This study also showed a preference of enzymes towards n-3 substrates over n-6 substrates, but the possible essentiality of n-6 LC-PUFA, as well as n-3 LC-PUFA, at the lower temperature for the Nile tilapia, could not be excluded. In summary, dietary fatty acids directly influence body fatty acid accumulation, but environmental temperatures play major roles in modulating the overall in vivo fatty acid metabolism in Nile tilapia juveniles.