Lipid digestion in turbot (Scophthalmus maximus). I: Lipid class and fatty acid composition of digesta from different segments of the digestive tract

Gallbladder microbiota in early vertebrates provides evolutionary insights into mucosal homeostasis

The gallbladder (GB) microbiota plays critical roles in mammalian metabolism and immune homeostasis, and its relationship with human disease has been extensively studied over the past decade. However, very little is known about the interplay between GB microbiota and the immune functions of teleost fish, the earliest bony vertebrate with a GB. Therefore, this study sought to investigate the composition of the teleost GB microbiota and the potential mechanisms through which it affects mucosal immunity. In our results, we found that the GB mucosa (GM) and bile bacterial community shared a similar microbiological composition with that of the gut mucosa in naïve individuals. IHNV infection induced a profound GB inflammation and disrupted their microbial homeostasis followed by a strong anti-bacterial response. Interestingly, beneficial bacteria from the Lactobacillales order showed a significant increase in the abundance of the bile microbial community, whereas the structure of the Mycoplasmatales order in the gut microbial community was markedly changed. All in all, our study characterized the structure of the GB microbial ecosystem in teleost fish, and the fish GB microbiome shared a high similarity with the gut microbiota. More importantly, our findings offer solid evidence that the teleost GB evolved immune functions to preserve its mucosal microbial homeostasis, suggesting that both the microbiota and mucosal immunity of the GB might have co-evolved in early vertebrates.

Fine scale transitions of the microbiota and metabolome along the gastrointestinal tract of herbivorous fishes

Background

Gut microorganisms aid in the digestion of food by providing exogenous metabolic pathways to break down organic compounds. An integration of longitudinal microbial and chemical data is necessary to illuminate how gut microorganisms supplement the energetic and nutritional requirements of animals. Although mammalian gut systems are well-studied in this capacity, the role of microbes in the breakdown and utilization of recalcitrant marine macroalgae in herbivorous fish is relatively understudied and an emerging priority for bioproduct extraction. Here we use a comprehensive survey of the marine herbivorous fish gut microbial ecosystem via parallel 16S rRNA gene amplicon profiling (microbiota) and untargeted tandem mass spectrometry (metabolomes) to demonstrate consistent transitions among 8 gut subsections across five fish of the genus of Kyphosus.

Results

Integration of microbial phylogenetic and chemical diversity data reveals that microbial communities and metabolomes covaried and differentiated continuously from stomach to hindgut, with the midgut containing multiple distinct and previously uncharacterized microenvironments and a distinct hindgut community dominated by obligate anaerobes. This differentiation was driven primarily by anaerobic gut endosymbionts of the classes Bacteroidia and Clostridia changing in concert with bile acids, small peptides, and phospholipids: bile acid deconjugation associated with early midgut microbiota, small peptide production associated with midgut microbiota, and phospholipid production associated with hindgut microbiota.

Conclusions

The combination of microbial and untargeted metabolomic data at high spatial resolution provides a new view of the diverse fish gut microenvironment and serves as a foundation to understand functional partitioning of microbial activities that contribute to the digestion of complex macroalgae in herbivorous marine fish.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-022-00182-z.

Vegetable oil and carbohydrate-rich diets marginally affected intestine histomorphology, digestive enzymes activities, and gut microbiota of gilthead sea bream juveniles

For an increased incorporation of plant ingredients in aquafeeds at the expense of fish meal (FM) and fish oil (FO), more knowledge is needed on the effects at the intestine level of dietary vegetable oils (VO) and carbohydrates (CH), and of possible interactions. For that purpose, in this study, the activities of digestive pancreatic enzymes (amylase, lipase, total alkaline proteases), gut microbiota, and histomorphology were assessed in gilthead sea bream (IBW 71.0 ± 1.5 g) fed four diets differing in lipid source (FO or a blend of VO) and carbohydrate content (0% or 20% gelatinized starch) for 81 days. No major changes in digestive enzyme activities were noticed in fish fed the experimental diets. Dietary VO, but not CH content, modified intestinal microbial profile, by increasing the similarity of bacterial communities. Especially when combined with CH, dietary VO promoted abnormal enterocyte architecture. Liver histology was also accessed, and an increased cytoplasmic vacuolization of hepatocytes was related with dietary CH inclusion, being only significantly different in fish fed FO-based diets. Overall, nutritional interactions between dietary lipid source and carbohydrate content were not observed on digestive enzyme activities and microbial profile. However, the intestine histological modifications observed in fish fed the VOCH+ diet suggest a negative interaction between dietary VO and CH. This requires a more in depth assessment in future studies as it can have negative consequences at a functional level.

Red drum Sciaenops ocellatus growth and expression of bile salt-dependent lipase in response to increasing dietary lipid supplementation

Sciaenops ocellatus has a long history in aquaculture and many difficulties associated with its commercial culture have been addressed and successfully resolved; nevertheless, further research in lipid nutrition could address more comprehensive questions on the way these nutrients are utilized. The purpose of this study was to evaluate S. ocellatus growth and lipase gene expression in response to increasing dietary lipid supplementation. Four experimental diets were formulated to provide 3, 10, 16, or 23% lipid using menhaden fish oil. Twenty juveniles (mean initial weight 2.3 ± 0.1 g) were stocked per aquaria in a recirculating system; each diet was assigned to three aquaria and fed to fish for 6 weeks. At the end of the study, fish fed 3% of dietary lipid were significantly (P < 0.0001) smaller and showed significantly lower feed efficiency, condition factor, hepatosomatic index, and intraperitoneal fat than fish fed the other diets, but no differences were observed among fish fed 10, 16, or 23% lipid. A straight broken-line regression model for thermal growth coefficient provided an estimated value of 9.4% of dietary lipid as the optimal inclusion level. The bile salt-dependent lipase (BSDL) of red drum was 80.3 kDa. Relative gene expression of BSDL was significantly higher (P = 0.0007) in fish fed 10% lipid, with no differences among the other dietary treatments. Results provided could help monitor the metabolic status of farmed fish and contribute to optimize diet formulations based on maximum gene expression of BSDL for supplementation of dietary lipid.

Effects of fish oil replacement by vegetable oil blend on digestive enzymes and tissue histomorphology of European sea bass (Dicentrarchus labrax) juveniles

The impact of replacing circa 70% fish oil (FO) by a vegetable oil (VO) blend (rapeseed, linseed, palm oils; 20:50:30) in diets for European sea bass juveniles (IBW 96 ± 0.8 g) was evaluated in terms of activities of digestive enzymes (amylase, lipase, alkaline phosphatase, trypsin and total alkaline proteases) in the anterior (AI) and posterior (PI) intestine and tissue morphology (pyloric caeca-PC, AI, PI, distal intestine-DI and liver). For that purpose, fish were fed the experimental diets for 36 days and then liver and intestine were sampled at 2, 6 and 24 h after the last meal. Alkaline protease characterization was also done in AI and PI at 6 h post-feeding. Dietary VO promoted higher alkaline phosphatase activity at 2 h post-feeding in the AI and at all sampling points in the PI. Total alkaline protease activity was higher at 6 h post-feeding in the PI of fish fed the FO diet. Identical number of bands was observed in zymograms of alkaline proteases of fish fed both diets. No alterations in the histomorphology of PC, AI, PI or DI were noticed in fish fed the VO diets, while in the liver a tendency towards increased hepatocyte vacuolization due to lipid accumulation was observed. Overall, and with the exception of a higher intestine alkaline phosphatase activity, 70% FO replacement by a VO blend in diets for European sea bass resulted in no distinctive alterations on the postprandial pattern of digestive enzyme activities and intestine histomorphology.

Genomic characterization and expression analysis of four apolipoprotein A-IV paralogs in Senegalese sole (Solea senegalensis Kaup)

The apolipoprotein A-IV (ApoA-IV) plays a key role in lipid transport and feed intake regulation. In this work, four cDNA sequences encoding ApoA-IV paralogs were identified. Sequence analysis revealed conserved structural features including the common 33-codon block and nine repeated motifs. Gene structure analysis identified four exons and three introns except for apoA-IVAa1 (with only 3 exons). Synteny analysis showed that the four paralogs were structured into two clusters (cluster A containing apoA-IVAa1 and apoA-IVAa2 and cluster B with apoA-IVBa3 and apoA-IVBa4) linked to an apolipoprotein E. Phylogenetic analysis clearly separated the paralogs according to their cluster organization as well as revealed four subclades highly conserved in Acanthopterygii. Whole-mount analyses (WISH) in early larvae (0 and 1day post-hatch (dph)) showed that the four paralogs were mainly expressed in yolk syncytial layer surrounding the oil globules. Later, at 3 and 5dph, the four paralogs were mainly expressed in liver and intestine although with differences in their relative abundance and temporal expression patterns. Diet supply triggered the intensity of WISH signals in the intestine of the four paralogs. Quantification of mRNA abundance by qPCR using whole larvae only detected the induction by diet at 5dph. Moreover, transcript levels increased progressively with age except for apoA-IVAa2, which appeared as a low-expressed isoform. Expression analysis in juvenile tissues confirmed that the four paralogs were mainly expressed in liver and intestine and secondary in other tissues. The role of these ApoA-IV genes in lipid transport and the possible role of apoA-IVAa2 as a regulatory form are discussed.

Dietary probiotic supplementation (Shewanella putrefaciens Pdp11) modulates gut microbiota and promotes growth and condition in Senegalese sole larviculture

Probiotic supplementation in fish aquaculture has significantly increased in the last decade due to its beneficial effect on fish performance. Probiotic use at early stages of fish development may contribute to better face metamorphosis and weaning stress. In the present work, we studied the influence of Shewanella putrefaciens Pdp11 supplementation on growth, body composition and gut microbiota in Senegalese sole (Solea senegalensis) during larval and weaning development. S. putrefaciens Pdp11 was incorporated using Artemia as live vector (2.5 × 10⁷ cfu mL⁻¹) and supplied to sole specimens in a co-feeding regime (10-86 DAH) by triplicate. Probiotic addition promoted early metamorphosis and a significantly higher growth in length at 24 DAH larvae. S. putrefaciens Pdp11 also modulated gut microbiota and significantly increased protein content and DHA/EPA ratios in sole fry (90 DAH). This nutritional enhancement is considered especially important after weaning, where significantly higher growth in length and weight was observed in probiotic fish. Moreover, a less heterogeneous fish size in length was detected since metamorphosis till the end of weaning, being of interest for sole aquaculture production. After weaning, fish showed significantly higher growth (length and weight) and less variable lengths in fish when supplemented with probiotics. Both the enhancement of nutritional condition and the decrease in size variability associated with probiotic addition are highly interesting for sole aquaculture production.

Pre-digestion of dietary lipids has only minor effects on absorption, retention and metabolism in larval stages of Atlantic cod (Gadus morhua)

The hypothesis of the present study was that cod larvae have a limitation in lipid digestion, and that absorption of lipids would increase by pre-hydrolysation. The diets used were designed to contain 15 % lipid, of which 40 % was phosphatidylcholine (PC) and 60 % was TAG. Cod larvae (40 d post hatch (dph)) were fed a single meal where either PC or TAG was radioactively labelled, and the labelled PC or TAG was either intact or hydrolysed (pre-digested). The larvae were then incubated individually in chambers with collection of CO2 for 10 h. The following fractions were analysed for radioactivity: the incubation water (evacuated feed); the intestine; the body; the CO2 trap. The larvae ate a 16–29 μg diet, equivalent to 3·4–5·2 % of dry body weight. In the whole population, 0–16 % of the lipid was evacuated. The larvae that had eaten less than 1·9–2·7 μg lipid absorbed close to 100 % of the lipid, absorption being defined conservatively as the amount contained in the carcass and CO2, excluding the intestinal tissue. In these larvae, approximately 100 % of the absorbed lipid was also catabolised. In the larvae that ingested more than 1·9–2·7 μg lipid, there was a linear reduction in lipid absorption to a minimum of 55 % at the highest lipid intakes parallel to an increasing retention of lipids in the carcass. There were only minor differences in digestion, absorption, retention and metabolism of lipids between the larvae fed the different diets, and the larvae tended to retain lipid classes as they were present in the feed. The study shows that 40-dph Atlantic cod larvae have an efficient utilisation of dietary lipids supplied as intact PC and TAG.

Dietary neutral lipid level and source in Senegalese sole (Solea senegalensis) larvae: effect on growth, lipid metabolism and digestive capacity

Contrary to larval essential fatty acid (EFA) requirements, the effect of dietary neutral lipid supply has been little investigated in marine fish larvae. The present work investigates the effect of feeding Senegalese sole larvae on Artemia enriched with higher or lower doses of lipid emulsion. Two lipid sources - soybean oil and fish oil - were compared. From 16 days after hatching (DAH) onwards, larvae were fed one of four experimental treatments: Artemia enriched on a high or low dose of soybean oil emulsion (HS and LS) or Artemia enriched on a high or low dose of fish oil emulsion (HF and LF). In terms of growth, the dietary lipid level did not have a significant effect while the soybean oil treatments induced a lower growth than the fish oil-enriched Artemia. The fatty acid (FA) composition of the larvae closely reflected the dietary quantitative and qualitative FA profile. Only slight dietary effects were noted in the activity of trypsin, lipase and alkaline phosphatase. A higher amount of lipid droplets was noticeable in the posterior intestine epithelia and in the hepatocytes of larvae fed Artemia enriched with higher lipid doses, while LS-Artemia induced the lower lipid accumulation on the basal zone of the enterocytes, in accordance with the lowest total lipid level measured in this treatment. These results suggest an important effect of dietary total lipid level on lipid accumulation in the enterocytes and on FA absorption. At 33 DAH a tube feeding trial was conducted with 14C-labelled oleic acid (OA) or triolein (TRI), showing that the lower accumulation of lipid droplets in the larvae fed LS was associated with a significantly higher absorption and retention in the gut and body tissues of the TRI label. For OA no significant differences between treatments were found. TRI label was considerably more evacuated than OA, indicating that sole larvae may have a lower capacity to incorporate a triacylglycerol, which needs to be digested. Finally, OA appears to be preferentially utilized for energy production, accumulating more in larval tissues when absorbed in higher amounts.

Dietary protein:lipid ratio and lipid nature affects fatty acid absorption and metabolism in a teleost larva

Studies with teleost larvae have reported poor performance associated with quantitative lipid imbalances in the diet. The present study examined the effect of dietary protein:neutral lipid ratio on fatty acid (FA) absorption efficiency and metabolism in larval Senegalese sole. In addition, the effect of lipid class (triolein (TRI) and l-3-phosphatidylcholine-1,2-di-oleoyl (PC)), carbon number and degree of saturation of the labelled NEFA, stearic acid (SA), oleic acid (OA) and DHA) was tested. FA absorption was determined by tube feeding [1-14C]-labelled lipids and NEFA after a single meal of either non-enriched Artemia (NEA) or Artemia enriched on a soyabean oil emulsion (EA), or after feeding these diets over an extended period of time (18 d). The tested dietary protein:lipid ratios had no short-term influence but long-term feeding of a diet higher in neutral lipid (EA) increased lipid accumulation within the gut epithelium and resulted in lower FA absorption (higher label evacuation and lower retention of dietary FA), which may partially explain the trend for lower growth observed with this diet. The lipids and NEFA, showed different digestive and metabolic properties, independent of feeding regime. FA absorption increased with unsaturation, being lowest for SA, followed by OA, and highest for DHA. In addition, sole larvae had a lower capacity to digest and absorb FA esterified to TRI, compared with PC, with the order of decreasing absorption being NEFA>PC>>TRI. Moreover, larvae appeared to discriminate between the source of OA, as this FA in the free form or esterified to PC was catabolised less than TRI.

Dietary TAG source and level affect performance and lipase expression in larval sea bass (Dicentrarchus labrax)

The influence of dietary TAG source (fish oil, triolein, and coconut oil) and level (7.5 and 15% of the diet) on growth, lipase activity, and mRNA level was studied in sea bass larvae, from mouth opening until day 24 and from day 37 to 52. Fish oil and triolein induced better growth in both experiments, this being significant at a higher dietary level. Coconut oil significantly decreased growth at the higher level, possibly as the result of an excessive supply of medium-chain TAG. Growth was not related to lipase specific activity, suggesting a production in excess to dietary needs. Body lipid content was positively related to dietary lipid level and was affected by lipid quality. In addition, larval FA composition generally reflected that of the diet. The source of dietary lipid, but not the quantity, was shown to affect lipase activity significantly. Coconut oil diets induced the highest lipase activity, whereas the effect of fish oil was age dependent-it was similar to coconut oil at day 24 but induced the lowest lipase activity in 52-d-old larvae. The differential lipase response was probably caused by differences in the FA composition of the diet, related to the specificity of lipase toward FA differing in chain length and degree of saturation. No significant differences were found in lipase/glyceraldehyde-3-phosphate dehydrogenase mRNA, which suggests the existence of a posttranscriptional regulation mechanism.

Interactive effects of dietary palm oil concentration and water temperature on lipid digestibility in rainbow trout, Oncorhynchus mykiss

An experiment was conducted to evaluate the interactive effects of dietary crude palm oil (CPO) concentration and water temperature on lipid and FA digestibility in rainbow trout. Four isolipidic diets with 0, 5, 10, or 20% (w/w) CPO, at the expense of fish oil, were formulated and fed to groups of trout maintained at water temperatures of 7, 10, or 15 degrees C. The apparent digestibility (AD) of the FA, measured using yttrium oxide as an inert marker, decreased with increasing chain length and increased with increasing unsaturation within each temperature regimen irrespective of CPO level fed to the fish. PUFA of the n-3 series were preferentially absorbed compared to n-6 PUFA in all diet and temperature treatments. Except for a few minor FA, a significant (P < 0.05) interaction between diet and temperature effects on FA digestibility was found. Increasing dietary levels of CPO lead to significant reductions in the AD of saturates and, to a lesser extent, also of the other FA. Lowering water temperature reduced total saturated FA digestibility in trout regardless of CPO level. Based on the lipid class composition of trout feces, this reduction in AD of saturates was due in part to the increasing resistance of dietary TAG to digestion. Increasing CPO level and decreasing water temperature significantly increased TAG content in trout fecal lipids, with saturates constituting more than 60% of the FA composition. Total monoene and PUFA digestibilities were not significantly affected by water temperature in fish fed up to 10% CPO in their diet. The potential impact of reduced lipid and FA digestibility in cold-water fish fed diets supplemented with high levels of CPO on fish growth performance requires further research.

Lipid digestion in turbot (Scopthalmus maximus) 11: Lipolysis in vitro of (14)C-labelled triacylglycerol, cholesterol ester and phosphatidylcholine by digesta from different segments of the digestive tract

Preparations of digesta from the stomach, foregut, hindgut and rectum of turbot (Scophthalmus maximus) were assayed for the ability to hydrolyse glycerol tri[1-(14)C]oleate ((14)C-TAG), 1,2-di[1-(14)C]palmitoyl L-3-phosphatidylcholine ((14)C-PC) and cholesterol [1-(14)]oleate ((14)C-CE) over 1,2,3 and 17h. In the assay of foregut digesta with (14)C-TAG substrate, 37.8% of the total radioactivity was found in the FFA class after the first hour of incubation. This value increased to a maximum of 68.5% of the available label after 17h incubation. Over the same time the proportion of radioactivity in diacylglycerols (DAG, 31.6%-7.4%) decreased while that in the monoacylglycerols increased (MAG, 14.0%-22.3%). In assays of digesta from the hindgut and rectum, after 1 h of incubation, the proportion of radioactivity recovered in FFA represented 64.9% and 74.8%, respectively, whereas the proportions in both DAG and MAG decreased with incubation time. Similarly to (14)C-TAG, the highest rate of lipolytic hydrolysis of (14)C-CE occurred in digesta from the posterior digestive tract where the proportions of radioactivity recovered in FFA of the hindgut (50.0%) and rectum (81.9%) preparations were substantially higher than those of the stomach (3.5%) and the foregut (14.4%) after 1h. With (14)C-PC as substrate the levels of radiolabelled FFA in both the foregut and the hindgut (2.4% and 7.6%, respectively) were markedly lower than the 37.5% in the rectum. The results suggest that the posterior digestive tract is very active in non-specific and phospholipid lipolysis and a region where the major part of lipid digestion takes place.