Arachidonic Acid and Eicosapentaenoic Acid Metabolism in Juvenile Atlantic Salmon as Affected by Water Temperature

Abstract

Salmons raised in aquaculture farms around the world are increasingly subjected to sub-optimal environmental conditions, such as high water temperatures during summer seasons. Aerobic scope increases and lipid metabolism changes are known plasticity responses of fish for a better acclimation to high water temperature. The present study aimed at investigating the effect of high water temperature on the regulation of fatty acid metabolism in juvenile Atlantic salmon fed different dietary ARA/EPA ratios (arachidonic acid, 20:4n-6/ eicosapentaenoic acid, 20:5n-3), with particular focus on apparent in vivo enzyme activities and gene expression of lipid metabolism pathways. Three experimental diets were formulated to be identical, except for the ratio EPA/ARA, and fed to triplicate groups of Atlantic salmon (Salmo salar) kept either at 10°C or 20°C. Results showed that fatty acid metabolic utilisation, and likely also their dietary requirements for optimal performance, can be affected by changes in their relative levels and by environmental temperature in Atlantic salmon. Thus, the increase in temperature, independently from dietary treatment, had a significant effect on the β-oxidation of a fatty acid including EPA, as observed by the apparent in vivo enzyme activity and mRNA expression of pparα -transcription factor in lipid metabolism, including β-oxidation genes- and cpt1 -key enzyme responsible for the movement of LC-PUFA from the cytosol into the mitochondria for β-oxidation-, were both increased at the higher water temperature. An interesting interaction was observed in the transcription and in vivo enzyme activity of Δ5fad–time-limiting enzyme in the biosynthesis pathway of EPA and ARA. Such, at lower temperature, the highest mRNA expression and enzyme activity was recorded in fish with limited supply of dietary EPA, whereas at higher temperature these were recorded in fish with limited ARA supply. In consideration that fish at higher water temperature recorded a significantly increased feed intake, these results clearly suggested that at high, sub-optimal water temperature, fish metabolism attempted to increment its overall ARA status -the most bioactive LC-PUFA participating in the inflammatory response- by modulating the metabolic fate of dietary ARA (expressed as % of net intake), reducing its β-oxidation and favouring synthesis and deposition. This correlates also with results from other recent studies showing that both immune- and stress- responses in fish are up regulated in fish held at high temperatures. This is a novel and fundamental information that warrants industry and scientific attention, in consideration of the imminent increase in water temperatures, continuous expansion of aquaculture operations, resources utilisation in aquafeed and much needed seasonal/adaptive nutritional strategies.

Eicosapentaenoic Acid, Arachidonic Acid and Eicosanoid Metabolism in Juvenile Barramundi Lates calcarifer

“…In agreement, several recent studies have further demonstrated that balanced ARA and EPA are more critical than either individual FA in terms of growth and other metabolic processes further highlighting the need for understanding the optimal LC-PUFA balance (Norambuena et al, 2015;Norambuena et al, 2016).…”

“…Glencross and Rutherford (2011) reported disproportionate EPA and ARA retention in barramundi however this was likely an effect of the increasing DHA level in combination with either EPA or ARA. Atlantic salmon were also shown to conserve ARA (Norambuena et al, 2015) while Senegalese sole attempt to synthesise ARA when dietary supply is limited (Norambuena et al, 2013).…”

The impact of dietary protein: lipid ratio on growth performance, fatty acid metabolism, product quality and waste output in Atlantic salmon (Salmo salar)

“…Additionally, evidence suggests that the synthesis of 20:4n-6 is correlated with immune responses at sub-optimal water temperatures (Norambuena et al 2015). Therefore, the near optimal water temperature experienced during the current trial (Handeland et al 2008;Stehfest et al 2017) were not expected to elicit endogenous 20:4n-6 synthesis in response to temperature stress.…”

Uncoupling EPA and DHA in Fish Nutrition: Dietary Demand is Limited in Atlantic Salmon and Effectively Met by DHA Alone

“…That said, dietary treatments had some variability, albeit small, in their 20:4n-6 content, varying from 1.1 to 6.3 g/kg of lipid, and diets containing lower levels of 20:4n-6 appear to induce in vivo 20:4n-6 biosynthesis (as indicated by increased apparent enzyme activity of Δ-5 desaturase bioconverting 18:2n-6-20:4n-6 in fish fed the NC and DHA-50 diets) to satisfy physiological demand not met through direct consumption. Given that dietary levels of 20:4n-6 were somewhat lower than those typically recommended or observed in feeds for Atlantic salmon and other species (~10 g/kg) [23,[50][51][52] this is perhaps an expected compensatory response. Nonetheless, it would be interesting to directly test whether the apparent sparing effect of SFA and MUFA on n-3 LC-PUFA also extends to n-6 LC-PUFA like 20:4n-6.…”