The Fish Microbiota:Research Progress and Potential Applications

The gut microbiota plays an important role in host health and disease.Our understanding of the fish microbiota lags far behind our knowledge of that of humans and other mammals.Nevertheless,research has highlighted the importance of the microbiota in the health,performance,and various physiological functions of fish.The microbiota has been studied in various fish species,including model animals,economic fish,and wild fish species.The composition of the fish microbiota depends on host selection,diet,and environmental factors.The intestinal microbiota affects the nutritional metabolism,immunity,and disease resistance of the fish host,while the host regulates the intestinal microbiota in a reciprocal way through both immune and non-immune factors.Improved and novel gnotobiotic fish models have been developed,which are important for the mechanistic study of host–microbiota interactions in fish.In this review,we discuss recent progress in fish microbiota research.We describe various aspects of this research,including both studies on fish microbiota variations and fundamental research extending our knowledge of host–microbiota interaction in fish.Perspectives on how fish microbiota research may benefit fish health and industrial sustainability are also discussed.

Dietary ribose supplementation improves flesh quality through purine metabolism in gibel carp(Carassius auratus gibelio)

Since the aquaculture industry is currently observing a deterioration in the flesh quality of farmed fish,the use of nutrients as additives to improve the flesh quality of farmed fish species is a viable strategy.The aim of this study was to investigate the effect of dietary D-ribose(RI)on the nutritional value,texture and flavour of gibel carp(Carassius auratus gibelio).Four diets were formulated containing exogenous RI at 4 gradient levels:0(Control),0.15%(0.15RI),0.30%(0.30RI)and 0.45%(0.45RI).A total of 240 fish(150±0.31 g)were randomly distributed into 12 fibreglass tanks(150 L per tank).Triplicate tanks were randomly assigned to each diet.The feeding trial was carried out in an indoor recirculating aquaculture system for 60 d.After the feeding trial,the muscle and liver of gibel carp were analysed.The results showed that RI supplementation did not result in any negative impact on the growth performance and 0.30RI supplementation significantly increased the whole-body protein content compared to the control group.The contents of collagen and glycogen in muscle were enhanced by RI supplementation.The alterations in the flesh indicated that RI supplementation improved the texture of the flesh in terms of its water-holding capacity and hardness,therefore improving the taste.Dietary RI facilitated the deposition of amino acids and fatty acids in the muscle that contributed to the meaty taste and nutritional value.Furthermore,a combination of metabolomics and expression of key genes in liver and muscle revealed that 0.30RI activated the purine metabolism pathways by supplementing the substrate for nucleotide synthesis and thereby promoting the deposition of flavour substance in flesh.This study offers a new approach for providing healthy,nutritious and flavourful aquatic products.

Effects of fish meal replacement with Chlorella meal on growth performance,pigmentation,and liver health of largemouth bass(Micropterus salmoides)

Chlorella meal is a potential protein source for aquafeeds.However,the physiological response of carnivorous fish fed Chlorella meal remains elusive.This study evaluated the effects of replacing dietary fish meal with Chlorella meal on growth performance,pigmentation,and liver health in largemouth bass.Five diets were formulated to replace dietary fish meal of 0%(C0,control),25%(C25),50%(C50),75%(C75),and 100%(C100)with Chlorella meal,respectively.Total 300 fish(17.6±0.03 g)were randomly assigned to 15 tanks(3 tanks/group).Fish were fed the experimental diet twice daily for 8 weeks.The increased dietary Chlorella meal quadratically influenced the final body weight(FBW),weight gain rate(WGR),specific growth rate(SGR),and feed intake(FI),which were significantly lower in the C100 group than in the other groups(P<0.05).The feed conversion ratio(FCR)increased linearly or quadratically with dietary Chlorella meal.Dietary Chlorella meal linearly or quadratically increased the lutein content of plasma,liver,and dorsal muscle of largemouth bass(P<0.05).Compared to the C0 group,all supplemented Chlorella meal groups significantly improved the yellowness(b*)of the dorsal body(1.5 to 2.0 fold),abdominal body(1.5 to 1.8 fold),and dorsal muscle(3.8 to 5.4 fold)of largemouth bass(P<0.05).In addition,compared to the C0 group,the liver vacuolation area of fish was significantly increased in the C75 and C100 groups(P<0.05).Transcriptional levels of apoptosis-related genes of b-cell lymphoma-2(bcl2),caspase-9-like(casp9),and caspase-3a(casp3)were markedly upregulated(0.9 to 1.6 fold)in the C100 group compared to the C0 group(P<0.05).Based on the quadratic regression analysis between FBW,WGR,or SGR and dietary Chlorella meal level,largemouth bass had the best growth when replacing 31.7%to 32.6%of fish meal with 15.03%to 15.43%dietary Chlorella meal.The present results indicated that dietary supplementation with Chlorella meal(11.85%to 47.45%)significantly enhanced the pigmentation;however,total replacement of fish meal(40%)with Chlorella meal(47.45%)caused growth retardation,apoptosis,and liver damage in largemouth bass.

Depletion of insulin receptors leads to β-cell hyperplasia in zebrafish

Hyperglycemia in type 2 diabetes results from an inability of insulin to regulate gluconeogenesis.To characterize the role of the insulin/insulin receptor pathway in glycometabolism and type 2 diabetes,we created a zebrafish model in which insulin receptors a and b(insra and insrb) have been ablated.We first observed that insra and insrb were both expressed abundantly during embryonic development and in various adult tissues.Increased expression of insulin and number of b-cells were observed in insra-/-/-insrb-/-fish together with higher glucose in insra-/-,insrb-/-,or insra-/-/-insrb-/-fish,indicating that insra and insrb were knocked out effectively.However,compared to the wild-type fish,insra-/-/-insrb-/-fish died between 5 and 16 days post-fertilization(dpf) with severe pericardial edema and increased level of cell apoptosis,which was not induced by increased total body glucose content.Increased gluconeogenesis and decreased glycolysis were also observed in both single and double knockout fish,but no mortality or malformation was observed in single knockout fish.Given the importance of insulin receptors in glucose homeostasis and embryonic development,transcriptome analysis was used to provide an important model of defective insulin signaling and to study its developmental consequences in zebrafish.The results indicated that both insra and insrb played a pivotal role in glucose metabolism and embryonic development,and insra was more critical than insrb in the insulin signaling pathway.

Genomic polymorphisms at the crhr2 locus improve feed conversion efficiency through alleviation of hypothalamus-pituitary-interrenal axis activity in gibel carp(Carassius gibelio)

Improvement in fish feed conversion efficiency(FCE)is beneficial for sustaining global food fish supplies.Here,we show that a set of polymorphisms at locus of the corticotropin releasing hormone receptor 2(crhr2),which is involved in hypothalamuspituitary-interrenal(HPI)axis signaling,is associated with improved FCE in farmed allogynogenetic gibel carp strain CAS Ⅲ compared with that in the wild gibel carp strain Dongting(DT).This set of polymorphisms downregulates the expression levels of crhr2 mRNA in the brain and pituitary tissues in gibel carp strain CAS Ⅲ compared with those in strain DT.Furthermore,compromised HPI axis signaling is observed in gibel carp strain CAS Ⅲ,such as decreased α-melanocyte stimulating hormone protein levels,plasma cortisol content,and stress responses.Moreover,enhanced activation of protein kinase B/mammalian target of rapamycin complex 1 signaling observed in the muscle tissue of strain CAS Ⅲ in comparison to that in strain DT indicated elevated anabolic metabolism in strain CAS Ⅲ.Thus,these studies demonstrate that the genetic markers associated with compromised HPI axis signaling,such as crhr2,are potentially useful for genetic selection toward improvement in farmed fish growth and FCE,which would reduce fishmeal consumption and thereby indirectly facilitate sustainable fisheries.