Numerous studies have reported the disadvantages of using plant protein in aquafeeds owing to the presence of antinutritional factors. Particularly, phytate can be overcome by dietary supplementation with exogenous phytase. On the other hand, the use of microalgae has been proven to be a valuable strategy for enhancing the digestive functionality in fish fed diets with high plant protein content. It was hypothesized that the simultaneous inclusion of phytase with microalgae can be useful to improve growth performance and digestive functionality in fish fed diets with high content of plant protein. In this study, the effects of feeds supplemented with different levels of exogenous phytase (FTU) and 2.5% of a blend of Arthrospira platensis and Nannochloropsis gaditana were evaluated on growth performance, metabolism, and gut functionality in European seabass (Dicentrarchus labrax) juveniles. Fish were fed ad libitum for 83 days with five diets containing different enzyme ratios (FTU), in triplicate: (i) control (0 FTU); (ii) 500 FTU; (iii) 1,000 FTU; (iv) 2,000 FTU; and (v) 10,000 FTU. At the end of the feeding period, zootechnical indexes were estimated, and biological samples were withdrawn for physiological determinations. The results obtained showed an overall improvement in the productive efficiency, general metabolism, and intestinal functionality in fish fed the highest phytase inclusion (2,000 and 10,000 FTU). Final weight (75.51 ± 1.01 g vs. 66.76 ± 1.26 g, p = 0.005), SGR (1.15 ± 0.02 vs. 1.00 ± 0.02, p = 0.006). and feed efficiency (0.80 ± 0.02% day⁻¹ vs. 0.67 ± 0.01% day⁻¹, p = 0.012) were higher in fish fed 10,000 FTU. The chemical composition of muscle and plasma metabolites did not vary among dietary treatments, except for the high glucose and low glycogen content in the liver with 2,000 FTU group. Focusing on gut functionality, enzyme activities tended to be higher in fish fed diets supplemented with phytase and microalgae, and significant differences were found for trypsin, chymotrypsin, and alkaline phosphatase with 1,000, 2,000, and 10,000 FTU, and leucine aminopeptidase with 2,000 and 10,000 FTU (p < 0.0001). Analysis of intestinal morphology revealed that 2,000 and 10,000 FTU combined with microalgae increased villi height and decreased villi diameter and enterocyte height. The presence of microalgae at low phytase supplementation level seems to lack of influence on the different parameters evaluated whereas only increased dietary phytase levels impacted on growth, nutrient utilization, and intestinal functionality. In addition, faecal samples were obtained by stripping the final portion of the intestines, from which the phosphorus and nitrogen were quantified. Apparent digestibility coefficient of phosphorus with 2,000 FTU and 10,000 FTU were higher, 74.3% and 77.8%, respectively, compared to control (52.4%). The results revealed a decrease in the amount of P shed with the excreta from the animals supplemented with phytase, a fact that evidenced an increased intestinal absorption of this element. Overall, the results obtained pointed out that phytase can be used at 10,000, and even 2,000, FTU in combination with microalgae as an approach to reducing feed costs and boost fish growth and digestive functionality of fish, while minimizing the environmental impacts of juvenile European seabass farms.

Tenebrio molitor (TM) meal is a potential alternative ingredient to plant and fishmeal in aquafeeds due to its high protein content, digestibility, and low environmental impact. However, its low n-3 PUFA (EPA and DHA) content represents one of its main limitations. This study evaluated the effects of replacing fishmeal and plant-derived ingredients (plant proteins and soybean oil) with full-fat TM meal on the growth performance and lipid composition of tissues in juvenile European seabass (Dicentrarchus labrax). Four experimental diets, including 5–10% TM for replacing fishmeal and soybean oil (FM5 and FM10) and 10–20% TM for substituting plant protein ingredients and soybean oil (PI10 and PI20), were tested over 49 days against a control diet (CT). Fish fed on PI20 exhibited the highest growth performance (final weight: 49.73 g and SGR: 1.78% day−¹) compared to fish fed on FM (final weight: 47.0 g and SGR: 1.62% day−¹) and CT (final weight: 48.45 g and SGR: 1.71% day−¹). In general, the diets containing TM modulated the lipid profile in fish tissues by reducing the liver lipid content and muscle triglycerides and increasing the n-3/n-6 ratio compared to the CT diet. However, the most effective approach for modulating those parameters seems to be using the TM meal for replacing the plant ingredients instead of substituting fishmeal. Moreover, this feeding strategy, demonstrated to be useful for promoting the growth of fish owing to the use of the TM meal for replacing fishmeal, resulted in a slight reduction in the final weight of the fish.

The present study evaluated the effects of replacing fish oil with microalgal oil on growth performance, nutrient utilization, chemical composition, metabolism and gut functionality in Sparus aurata . Three experimental diets were formulated with increasing levels of oils derived from the microalgae Microchloropsis gaditana and Isochrysis galbana for replacing fish oil: 50% (MO50), 75% (MO75) to 100% (MO100) of total oil. All diets were also supplemented with a protein hydrolysate obtained from Arthrospira platensis to partially replace fishmeal, as well as with an algal carotenoid extract. A total of 198 fish (initial body weight 10.5 ± 0.4 g) were randomly distributed into 12,400 L tanks and fed the experimental diets ad libitum for 83 days under controlled conditions. Growth performance, feed efficiency and somatic indices were assessed, while muscle proximate composition, fatty acid and carotenoid profiles, plasma metabolites, and tissue glycogen were analyzed using standard biochemical and chromatographic techniques. Digestive enzyme activities (trypsin, chymotrypsin, leucine aminopeptidase, alkaline phosphatase) and well as intestinal histology were also evaluated. The results evidenced that use of microalgal oil did not affect fish growth. Fish fed diets supplemented with microalgal products tended to reduce muscle lipid content. Fish fed diets devoid fish oil showed an increase in eicosapentaenoic acid (EPA) content when microalgal oil was included. Plasma cortisol did not vary among experimental groups. The activity of foregut trypsin and alkaline phosphatase increased with the use of microalgal oil, while in the hindgut trypsin and leucine aminopeptidase activities were enhanced. The use of algal products promoted longer intestinal villi and reduced lamina propria thickness in the distal intestine. Overall, the results indicate that total replacement of fish oil with microalgal oil has no detrimental effects on growth performance, and exerts positive effect on specific fatty acids and intestinal functionality.