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Background Free fatty acids (FFAs) are the main cause of fatty liver disease, which can be alleviated by modulation of intestinal microbiota. Lactiplantibacillus plantarum plays a key role in maintaining liver health, but the underlying mechanism remains unclear. Results Here, a strain affiliated to Lactiplantibacillus plantarum was isolated from the intestine of Nile tilapia (Oreochromis niloticus). We used a gossypol-induced fatty liver disease model, which only increased the FFAs level in liver, to investigate the effectiveness of L. plantarum (YC17) in alleviating FFAs-induced lipotoxicity liver injury. We found that dietary gossypol (GOS) induced a significant increase of FFAs in liver, resulting in lipotoxicity in Nile tilapia compared to control. L. plantarum YC17 supplementation reduced FFAs content by restoring esterification process, and then relieved liver injury. Addition of L. plantarum YC17 effectively increased the abundances of Lactobacillus, Clostridium and Cetobacterium in fish intestine, as well as serum levels of the microbial tryptophan metabolites, notably indole-3-propionic acid (IPA) and indole-3-acetic acid (IAA). The addition of L. plantarum YC17 significantly inhibited P53 signaling pathway and up-regulated the expression of FFAs esterification genes. In vitro experiments demonstrated that IPA inhibited P53 through ubiquitination and enhanced FFAs esterification in an aryl hydrocarbon receptor (Ahr) dependent manner. Conclusion The gut microbiota-derived tryptophan metabolites (IPA and IAA) alleviated FFAs induced lipotoxic liver injury by activating Ahr, which promoted P53 ubiquitination, leading to the enhanced FFAs esterification. Our findings demonstrated that gut microbial metabolites alleviated lipotoxicity by promoting the esterification of FFAs in the liver, offering new insights into the study of probiotics and microbial tryptophan metabolites in fatty liver disease.

The roles of dietary cholesterol in fish physiology are currently contradictory. The issue reflects the limited studies on the metabolic consequences of cholesterol intake in fish. The present study investigated the metabolic responses to high cholesterol intake in Nile tilapia ( Oreochromis niloticus ), which were fed with four cholesterol-contained diets (0.8, 1.6, 2.4 and 3.2%) and a control diet for eight weeks. All fish-fed cholesterol diets showed increased body weight, but accumulated cholesterol (the peak level was in the 1.6% cholesterol group). Then, we selected 1.6% cholesterol and control diets for further analysis. The high cholesterol diet impaired liver function and reduced mitochondria number in fish. Furthermore, high cholesterol intake triggered protective adaptation via (1) inhibiting endogenous cholesterol synthesis, (2) elevating the expression of genes related to cholesterol esterification and efflux, and (3) promoting chenodeoxycholic acid synthesis and efflux. Accordingly, high cholesterol intake reshaped the fish gut microbiome by increasing the abundance of Lactobacillus spp. and Mycobacterium spp., both of which are involved in cholesterol and/or bile acids catabolism. Moreover, high cholesterol intake inhibited lipid catabolic activities through mitochondrial β-oxidation, and lysosome-mediated lipophagy, and depressed insulin signaling sensitivity. Protein catabolism was elevated as a compulsory response to maintain energy homeostasis. Therefore, although high cholesterol intake promoted growth, it led to metabolic disorders in fish. For the first time, this study provides evidence for the systemic metabolic response to high cholesterol intake in fish. This knowledge contributes to an understanding of the metabolic syndromes caused by high cholesterol intake or deposition in fish.

The knowledge of socio-economics and livelihoods of milkfish ( Chanos chanos ) pond farming to coastal communities is limited in Tanzania. This study assessed the milkfish pond farming contribution to the socio-economics and livelihoods along the Tanzanian coastline. Sixty-three milkfish farms were interviewed through a semi-structured questionnaire. The results indicated that milkfish pond farming was characterized by low production. The milkfish pond production was positively influenced by farmers’ experience, the number of fingerlings stocked, and pond fertilization. However, milkfish pond farming was significantly negatively affected by days of culture and age of the farmers. The results further showed that milkfish pond farming is a source of income to the owners after selling their fish through retail, wholesale, and on-farm sales via mobile phone communications. However, milkfish pond farms in the studied areas had a negative margin considering all financial factors. Interestingly, milkfish pond farming is a source of food, income, and employment to the community. Nevertheless, milkfish pond farming is constrained by inadequate feeds, seeds, funds, technical support, and existence of theft and predators. Therefore, addressing these challenges is necessary for enhancing milkfish pond farming contribution to the socio-economics well-being and livelihoods support for the coastal communities.

l-Carnitine is essential for mitochondrial β-oxidation and has been used as a lipid-lowering feed additive in humans and farmed animals. d-Carnitine is an optical isomer of l-carnitine and dl-carnitine has been widely used in animal feeds. However, the functional differences between l- and d-carnitine are difficult to study because of the endogenous l-carnitine background. In the present study, we developed a low-carnitine Nile tilapia model by treating fish with a carnitine synthesis inhibitor, and used this model to investigate the functional differences between l- and d-carnitine in nutrient metabolism in fish. l- or d-carnitine (0·4 g/kg diet) was fed to the low-carnitine tilapia for 6 weeks. l-Carnitine feeding increased the acyl-carnitine concentration from 3522 to 10 822 ng/g and alleviated the lipid deposition from 15·89 to 11·97 % in the liver of low-carnitine tilapia. However, as compared with l-carnitine group, d-carnitine feeding reduced the acyl-carnitine concentration from 10 822 to 5482 ng/g, and increased lipid deposition from 11·97 to 20·21 % and the mRNA expression of the genes involved in β-oxidation and detoxification in the liver. d-Carnitine feeding also induced hepatic inflammation, oxidative stress and apoptosis. A metabolomic investigation further showed that d-carnitine feeding increased glycolysis, protein metabolism and activity of the tricarboxylic acid cycle and oxidative phosphorylation. Thus, l-carnitine can be physiologically utilised in fish, whereas d-carnitine is metabolised as a xenobiotic and induces lipotoxicity. d-Carnitine-fed fish demonstrates increases in peroxisomal β-oxidation, glycolysis and amino acid degradation to maintain energy homeostasis. Therefore, d-carnitine is not recommended for use in farmed animals.

The adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL)–mediated lipolysis play important roles in lipid catabolism. ATGL is considered the central rate-limiting enzyme in the mobilization of fatty acids in mammals. Currently, severe fat accumulation has been commonly detected in farmed fish globally. However, the ATGL-mediated lipolysis and the potential synergy among ATGL, HSL, and autophagy, which is another way for lipid breakdown, have not been intensively understood in fish. In the present study, we added Atglistatin as an ATGL-specific inhibitor into the zebrafish diet and fed to the fish for 5 weeks. The results showed that the Atglistatin-treated fish exhibited severe fat deposition, reduced oxygen consumption, and fatty acid β-oxidation, accompanied with increased oxidative stress and inflammation. Furthermore, the Atglistatin-treated fish elevated total and phosphorylation protein expressions of HSL. However, the free fatty acids and lipase activities in organs were still systemically reduced in the Atglistatin-treated fish, and the autophagy marker LC3 was also decreased in the liver. On the other hand, glycogenolysis was stimulated but blood glucose was higher in the Atglistatin-treated fish. The transcriptomic analysis also provided the hint that the protein turnover efficiency in Atglistatin-treated fish was likely to be accelerated, but the protein content in whole fish was not affected. Taken together, ATGL plays crucial roles in energy homeostasis such that its inhibition causes loss of lipid-sourced energy production, which cannot be compensated by activation of HSL, autophagy, and utilization of other nutrients.

The overall performance of an aquaculture system is partly determined by its water quality parameters. Poor water quality stresses and adversely affects fish growth causing low production, profit and product quality. Diurnal dynamics of water quality parameters were investigated in Nile tilapia ( Oreochromis niloticus ) monoculture and polyculture with Clarias gariepinus in earthen ponds. Dissolved oxygen, temperature and pH were measured and monitored for 24 h. Water samples for nutrient analysis were collected from the middle of ponds in triplicate at about 30–35 cm below the water surface using a plastic bottle. Nitrite–nitrogen (NO 2 –N), nitrate–nitrogen (NO 3 –N), unionised ammonia (NH 3 –N), soluble reactive phosphorus and free carbon dioxide were analysed following standard methods and procedures. The results show that dissolved oxygen concentrations during the past midnight and pre-dawn hours were significantly lower than the levels in the morning and afternoon hours (ANOVA, F  = 45.709, P  < 0.05) which is dangerous to the life of the cultured fishes. The levels of unionised ammonia and temperature were higher and lower, respectively, than the acceptable levels for optimum growth of O. niloticus and C. gariepinus . Nitrite–nitrogen, nitrate–nitrogen, pH, soluble reactive phosphorus and free carbon dioxide were within the recommended limits for fish growth in aquaculture. Growth parameters, feed conversion ratio and survival rate were not significantly different between culture systems ( P  > 0.05). Fish yield was relatively higher in polyculture (45.74 ± 0.44 tons/ha) than monoculture (30.77 ± 0.54 tons/ha). During fish farming, optimum fish growth and hence economic benefits can be accrued by devoting some efforts on monitoring the fish pond water at regular intervals. This quality assurance process will ensure that fish farmers produce fish with maximum growth and yield without polluting pond water and the surrounding environment.

Prolific breeding and production of high percentage of recruits are main problems in mixed-sex Nile tilapia ( Oreochromis niloticus ) culture in earthen ponds. The current study assessed the efficiency of different sizes of African sharptooth catfish ( Clarias gariepinus ) in controlling recruitment in mixed-sex Nile tilapia cultured in tanks and its effect on growth performance, percentage survival and yield. Three treatments consisting of triplicate ponds stocked with mixed-sex Nile tilapia in monoculture and in polyculture with large (62.50 ± 3.26 g) and small (40.00 ± 2.68 g) African sharptooth catfish were fed on a 297.50 g kg −1 crude protein diet for 126 days. Results showed that, the number of recruits was significantly lower in larger African sharptooth catfish predator than smaller ones and monoculture of Nile tilapia. Polyculture with larger African sharptooth catfish resulted in significantly higher growth performance of Nile tilapia. Large African sharptooth catfish in polyculture reduced the amount of small, low-value recruits, while the yield of large and high value Nile tilapia was increased. This study revealed that fish farmers can reduce prolific breeding, obtain higher growth performance and produce larger size of marketable Nile tilapia by predominantly stocking ponds with large African sharptooth catfish predator of at least 60 g.