Short- and medium-term effects of raw and hydrolyzed microalgae inclusion on fatty acid profile, intestinal functionality, and gene expression in gilthead seabream (Sparus aurata)

Introduction: Microalgae are promising functional ingredients for sustainable aquafeeds, but the resistant cell wall of some species may limit nutrient availability during larval development. This study evaluated the short- and medium-term effects of low dietary inclusion levels of raw or enzymatically hydrolyzed microalgal biomass in gilthead seabream (Sparus aurata).

Methods: Larvae (initial weight: 0.034 g) were fed for 45 days with a control diet or diets containing 5% or 10% raw (R5, R10) or hydrolyzed (H5, H10) microalgae and were evaluated in triplicate (3 tanks per dietary treatment; n =400 fish/tank; initial density 80 g m⁻³). Fish were sampled after 22 and 45 days and then reared for an additional 90 days on a common control diet to assess prolonged effects. Muscle fatty acid profiles, intestinal morphology, and intestinal and hepatic gene expression were analyzed. The significant differences between fish fed different dietary treatments were detected by ANOVA and calculated with Fisher’s least significant difference (LSD) post hoc test (p < 0.05) using SPSS version 25 (IBM Corporation Inc).

Results: Microalgae supplementation altered muscle fatty acid composition. In larvae, all supplemented diets increased MUFAs and reduced PUFAs, n-3 PUFAs, EPA, DHA, and the n3/n6 ratio compared with the control (29.24 ± 1.12 % PUFAs), with the lowest PUFA levels observed in H5 (22.01 ± 0.19 %). After the prolonged phase, previously supplemented juveniles showed higher n-3 PUFA and DHA contents than controls (18.50 ± 0.28 % and 8.68 ± 0.12 %, respectively), especially R10 (20.47 ± 0.97 % and 10.68 ± 0.02 %, respectively). Intestinal morphology remained normal in all treatments, with no signs of damage or inflammation. Intestinal gene expression was mostly unchanged, except for significant CD209d downregulation in H10. Conversely, imuc was upregulated in R5 larvae, while muc2 increased in R5 at 45 days and R10 juveniles. Hepatic gpx was significantly downregulated in R10 and H5 compared with CT.

Conclusions: Dietary microalgae during the larval stage modified lipid metabolism and induced persistent transcriptional responses without compromising intestinal integrity. Raw and hydrolyzed biomass showed distinct physiological effects, supporting their use as functional ingredients in sustainable larval feeds.