## 3 Results
### Intestinal Histology  
All the experimental groups of fish of both feeding trials revealed a normal intestinal histology and none of them showed any signs of inflammation (Figures 3 and 4). Enterocytes were distinct, while goblet cells and apical epithelial vacuoles were normally present (Figure 3A). In addition, abundant eosinophils cells were normally observed within the submuc layer of all fish (Figure 3B). Figure 2: Liver histopathological examination of _S. aurata_ fed on HFM diets. (**A**) fish fed FM diet—normal liver structure; (**B**) fish fed HFM25 diet—normal liver structure; (**C**) fish fed HFM100 diet—hemorrhage signs (*) and large lipid droplets (arrows); (**D**) fish fed HFM100 diet—signs of liver cirrhosis; (**E**) fish fed HFM25+ diet—normal liver structure; (**F**) fish fed HFM50+ diet—hepactocytes with no central nuclei were detected. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline  & **FMa** & **FMb** & **HFMa** & **HFMb** & **HFMc** & **HFMd** & **PBMa** & **PBMb** \\ \hline Richness & 14 & 14 & 48 & 29 & 14 & 28 & 15 & 16 \\ Sequences & 2283 & 132 & 2206 & 325 & 196 & 181 & 234 & 319 \\ Shannon & 1.32 & 1.71 & 2.27 & 2.97 & 2.05 & 2.87 & 2.10 & 2.23 \\ Cumulative & & & & & & & & \\ abundance & 99.21 & 96.21 & 92.84 & 97.54 & 97.45 & 97.24 & 98.72 & 98.12 \\ \(>\)1\% & & & & & & & \\ Coverage & 1.00 & 0.95 & 0.99 & 0.99 & 0.97 & 0.97 & 0.99 & 0.99 \\ \end{tabular}
\end{table} 
Table 2: Sequencing results, diversity indices and coverage values of fish fed the FM, PBM50 and HFM50 diets. Figure 4: Midgut histopathological examination of _S. aurata_ fed on HFM diets. (**A**) fish fed FM diet; (**B**) fish fed HFM50 diet; (**C**) fish fed HFM25 diet; (**D**) fish fed HFM50+ diet. In all images, the gut structure appeared normal with goblet cells (*) present. Figure 3: Midgut histopathological examination of _S. aurata_ fed on PBM diets. (**A**) fish fed FM diet—normal gut structure with goblet cells (*) present; (**B**) fish fed PBM50 diet—eosinophil cells (arrow) accumulation within the muscularis layer. Abundant eosinophils cells were normally observed within the submuc layer (arrowhead); (**C**) fish fed PBM25 diet—normal structure; (**D**) fish fed PBM100 diet—normal structure with goblet cells (*) present. Morisita similarities between samples were very low (<50%) showing no specific pattern according to the different diet fed (Figure 5B). Canonical correspondence analysis also exhibited no pattern among fish of either the same or of a different dietary group but revealed the importance of body weight for the ordination of the samples (\(p<0.05\)) (Figure 6). At the phylum level, all samples were characterized by _Proteobacteria_ and _Actinobacteria_, which were the most abundant in almost all fish, as well as by _Bacteroides_ and _Firmicutes_ (Figure 7A). At the OTU level, a total number of 64 OTUs were detected in relative abundances >1% in at least one fish. Overall, these OTUs accounted for more than 97% of the total diversity in all samples (Table 2). Similarly, OTUs with relative abundance >10% clearly representing persistent members of BCC reached cumulative abundances >50% in all samples (Figure 7B) and represented different species of _Alpha-, Beta-, Gamma-proteobacteria, Actinobacteria, Flavobacteria, Bacilli_ and _Clostridia_ (Figure 7B). In total, 6 OTUs were shared amongst all dietary groups (Figure 5) and belonged to the genera _Staphylococcus_, _Pseudomonas_, _Deflia_, _Cutiobacterium_ and _Hydrogenophaga_ (Figure 7B). Most of them (5) belonged to the abundant species that dominated (>10%) at least in one sample (Figure 7B). The cumulative abundances of this 'core' microbiome that was identified from habitat ranged from 6.1% (HFMa) to 99.2% (FMa) (Figure 7B). The lowest values for core microbiome relative abundances were observed in the HFM fish with an average of 20.15% contrary to 63.34% and 43.81% in the FM and PBM fish, respectively.