## 4 Discussion
### Intestinal Microbiota  
Regarding the dietary effects on the intestinal microbiota, it can be argued that no clear effects were detected from the use of HFM or PBM. The gut Bacterial Community Composition (BCC) of _S. aurata_ fed either FM, PBM or HFM was characterized by groups that are commonly found in the fish gut microbiome. The core genera _Pseudomonas_, _Cutiobacterium_, _Staphylococcus_ and _Delflia_ have been previously reported as core microbiomeof _S. aurata_ farmed in several geographical sites [29; 52], suggesting that certain bacterial genera are capable of colonizing the seabream gut independently of the diet and location. Similar findings have been reported for _Salmo salar_[41] with authors stating that the role of fish-hosts in selecting or promoting core microbes is unclear. _Actinobacteria_ species were dominant and unique in the HFM fed fish and their dominance could be related to their antimicrobial functions (i.e., antibiotics) that protect the host [53; 54]. Additionally, the prevalence of terpenoid biosynthesis genes that were prevalent in the HFM fed fish is mostly detected in _Actinobacteria_[55] and was in accordance with the taxonomic BCC (Figure 7A,B). The _Sphingomonas_ species in the gut have been found to be negatively correlated with weight gain [56] which also agrees with our CCA data (Figure 6).  
The predicted pathways that were enriched in the HFM and PBM fed fish compared to FM fed fish were mostly related to functions that imply gut dysbiosis. For instance, increased renin-angiotensin system (RAS) genes have been related to RAS system activation which implies malnutrition [57; 58]. Gut dysbiosis that promotes RAS activation is mostly related to decreased abundances of fermenting bacteria, which was the case in the HFM and PBM groups of fish. Similarly, bacterial retinol metabolism predicted genes indicate a potential need for vitamin A production that enhances mucosal immunity. Thus, bacteria able to participate in retinol metabolism also assist in avoiding pathogen invasion [59].  
Knowledge of the effects of fishmeal replacement by land animal proteins on the intestinal microbiota is extremely limited. In the present study, as stated above, there was no specific pattern in the bacterial communities among fish fed either fishmeal or the tested land animal proteins. [PERSON] et al. [41], working with _Salmo salar_, reported a significant effect of the dietary PBM on the distal intestine digesta and mucosa. This study found significantly higher and lower abundances of specific genera in fish fed PBM than in fish fed on a FM-based diet. [PERSON] et al. [14], also working with _Salmo salar_, reported that the use of HFM and PBM as fishmeal replacements increased the total allochthonous and total autochthonous bacteria in the distal intestine, but the total autochthonous bacteria in the proximal intestine remained unaffected. The authors also reported that the supplementation of HFM caused significant increases in specific genera (_Corynebacteriaceae_, _Lactobacillusace_, _Streptococcaceae_, _Pseudomonadaceae_, _Xanthomonadaceae_) and decreases in another (_Vibrionaceae_). Furthermore, PBM caused increases in _Corynebacteriaceae_ and decreases _in b-Proteobacteria_, _Bacillus-like_, _Peptostreptococcaceae_ and _Vibrionaceae_. Certainly, a better understanding of the functional roles of the intestinal microbiota communities of fish and to what extent these are affected by the use of land animal proteins is needed.