It is well known that the gut microbiome and its interaction with the host influence several important factors for fish health such as nutrition and metabolism. Diet is one of the main factors influencing the composition of the gut microbiome in reared fish. Microalgae, due to their high fatty acid content, appear to be a promising alternative for replacing fish oil in aquafeed. Thus, the aim of this work was to evaluate the effects of dietary microalgae blends as fish oil replacers οn the midgut bacterial microbiota of the gilthead sea bream ( Sparus aurata ). The control diet (FO) contained only fish oil as a source of lipids, EPA and DHA fatty acids, while three experimental diets were used where fish oil was replaced at 67% by one of the following microalgae biomass blends: Microchloropsis gaditana and   Isochrysis sp. ( Tisochrysis lutea ) (MI), Phaeodactylum tricornutum and   Isochrysis sp. (PI) and Schizochytrium sp.   and P. tricornutum (SP). The midgut bacterial community composition of the experimental diets was altered compared to the control diet. There were 11 operational taxonomic units (OTUs) that were highly abundant in FO compared to the three experimental diets (FO, MI, SP) and two OTUs that were found in high abundance in both FO and the experimental diets in all comparisons (FO-MI, FO-PI, FO-SP). Most of the highly abundant OTUs in the experimental diets were unique to each experimental diet, with two OTUs being found in common between FO-MI and FO-PI. Additional evidence from the presumptive bacterial functional metabolic pathways suggested that the microalgae-based diets resulted in one over-expressed and one under-expressed pathway. The overexpressed pathway was related to the metabolism of fucose, a major constituent of the polysaccharide content of several microalgal species. Peptidoglycan biosynthesis was the under-expressed metabolic pathway. This suggests that a new gut microbiota profile was selected due to the microalgae inclusion in the provided diet. This study showed that, with the absence of mortality in fish, the gilthead sea bream gut microbiome can smoothly adapt its function according to the metabolic capacity of the dietary microalgae combinations that were used. The MI feed seems to promote several beneficial bacteria with potential probiotic abilities in the fish gut, belonging to the Pseudoalteromonas, Pseudomonas, Bacillus and Rhodopseudomonas genera.

Sea cucumbers have become a highly valuable fishery product, and therefore the determination of their proximate compositions and fatty acid profiles are useful indicators of their physiological status and nutritional value. Thus, various raw and processed products, such as raw, dried, frozen, boiled, and salted Holothuria polii and Holothuria tubulosa that were collected from the Aegean Sea were analyzed. Although there were some notable differences in the proximate and fatty acid profiles between the two species and among the various processing forms, both H. polii and H. tubulosa had a high nutritional value. They were rich in proteins (up to 68.5% in the dried form), and although their lipid fraction was low (0.3–3.9%), it was characterized by very high levels of arachidonic acid (19.1–30.9% of total fatty acids) and eicosapentaenoic acid (8.5–15.7) that play vital roles in human growth, development, and health. In addition, their n-3/n-6 fatty acid ratios were within the recommended ranges, while their low Atherogenic and Thrombogenic indices and high Hypocholesterolemic index denote their high protective role against coronary artery disease. The provided information sheds light on the high nutritional value of this important marine fishery resource and provides valuable information for its preservation and processing. This information could also help policy makers, stakeholders, and the public to recognize the importance of this valuable fishery resource for human nutrition and to adopt preventative measures toward its sustainable exploitation.

This study evaluated the dietary fishmeal substitution by full‐fat (FF) and defatted (DF) Zophobas morio meals regarding growth, feed efficiency, proximate and fatty acid compositions, digestive enzymes activities, histology and midgut microbiota in gilthead seabream ( Sparus aurata ). Juveniles initially weighing 3.4 g were distributed to triplicate groups and fed at satiation six isonitrogenous (8.41%) and isocaloric (21 Mj/kg) diets for 100 days. An insect meal‐free diet was the control (CTRL), two diets contained a FF Z. morio meal at 49 g/kg (FF‐49) and 97 g/kg (FF‐97), and three diets contained a DF Z. morio meal at 58 g/kg (DF‐58), 116 g/kg (DF‐116) and 174 g/kg (DF‐174) at the expense of fishmeal. Neither the form nor the inclusion level of Z. morio meals affected the feed intake of fish denoting a similar acceptability to that of fishmeal. Fish survival, growth and feed efficiency were not impaired by all dietary inclusion levels of Z. morio meals. Proximate composition of fish was altered but without indicating a clear correlation with the form or inclusion level of Z. morio . Increasing inclusions of both forms of Z. morio meals tended to decrease 22:6n‐3, 20:5n‐3, 18:3n‐3 and 18:2n‐6 levels in fish tissues. All fish exhibited similar proteolytic enzyme activities, but the increasing inclusions of both insect meal forms led to gradual increases in the lipase and α‐amylase activities indicating a compensatory mechanism for lipid and carbohydrate digestion. The use of Z. morio meals led to some mild histomorphological changes in the intestine and liver that were more pronounced in fish fed the FF form at the highest inclusion level. Midgut bacterial communities of the groups were similar and dominated by potentially beneficial members of Saccharimonadales and Rhodobacteraceae , except FF‐97 fish that had high abundances of Legionella ‐ and Pandoraea ‐like bacteria. To conclude, Z. morio meal, either FF or DF, is a suitable insect protein for fishmeal substitution towards more sustainable aquafeeds for S. aurata .

It is well known that the gut microbiome and its interaction with the host influence several important factors for fish health such as nutrition and metabolism. Diet is one of the main factors influencing the composition of the gut microbiome in reared fish. Microalgae, due to their high fatty acid content, appear to be a promising alternative for replacing fish oil in aquafeed. Thus, the aim of this work was to evaluate the effects of dietary microalgae blends as fish oil replacers on the midgut bacterial microbiota of the gilthead sea bream ( Sparus aurata ). The control diet (FO) contained only fish oil as source of lipids, EPA and DHA fatty acids, while three experimental diets were used where fish oil was replaced at 67% by one of the following microalgae biomass blends: Microchloropsis gaditana and Isochrysis sp. ( Tisochrysis lutea ) (MI), Phaeodactylum tricornutum and Isochrysis sp. (PI) and Schizochytrium sp. and P. tricornutum (SP). The midgut bacterial community composition of the experimental diets was altered compared to the control diet. There were 11 operational taxonomic units (OTUs) which were highly abundant in FO compared to the three experimental diets (FO, MI, SP) and two OTUs that were found in high abundance in both FO and the experimental diets in all comparisons (FO-MI, FO-PI, FO-SP). Most of the highly abundant OTUs in the experimental diets were unique to each experimental diet, with two OTUs being found in common between FO-MI and FO-PI. Additional evidence from the presumptive bacterial functional metabolic pathways suggested that the microalgae-based diets resulted in one over-expressed and one under-expressed pathway. The overexpressed pathway was related to the metabolism of fucose, a major constituent of the polysaccharide content of several microalgal species. Peptidoglycan biosynthesis was the under-expressed metabolic pathway. This suggests that a new gut microbiota profile was selected due to the microalgae inclusion in the provided diet. This study showed that, with the absence of mortality in fish, the gilthead sea bream gut microbiome can smoothly adapt its function according to the metabolic capacity of the dietary microalgae combinations that were used. The MI feed seems to promote several beneficial bacteria with potential probiotic abilities in the fish gut, belonging to the Pseudoalteromonas, Pseudomonas, Bacillus and Rhodopseudomonas genera.

Biomass productivity of Microchloropsis gaditana cultivated in a variety of modes and effect of cobalt and molybdenum on its lipid distribution