Elisabeth Desbruyeres

Comparison of nitrogenous losses in five teleost fish species

Abstract Three species of marine fish, sea bass, sea bream and turbot, and two salmonids, rainbow trout and brown trout, were raised under similar feeding and environmental conditions in order to compare their nitrogen utilisation. Apparent digestibility of protein, ammonia and urea excretion patterns, plasma ammonia and urea concentrations and liver arginase activities were measured. No differences in protein digestibility were noticed among the five species. Ammonia and urea losses were quantitatively similar in all species, except in turbot, in which ammonia production was significantly lower. Ammonia excretion patterns were linked to ingested nitrogen and showed no inter-species differences. Conversely, urea excretion patterns were specific in turbot and sea bream. Plasma urea levels were higher in marine fish than in salmonids, with the highest values being reached in turbot. Some specificity with regard to liver arginase was also detected in the turbot. Turbot demonstrated some metabolic characteristics that could be connected with its elevated position in the phylogeny of fish.

Excess dietary arginine affects urea excretion but does not improve N utilisation in rainbow trout Oncorhynchus mykiss and turbot Psetta maxima

Abstract The aim of the study was to evaluate the effects of dietary arginine (Arg) supplementation to either a plant protein-based diet or a fish meal-based diet on nitrogen (N) utilisation and ureogenesis in turbot and rainbow trout. Juvenile turbot (7.4 g) and rainbow trout (9.3 g) were fed for 12 weeks two types of diets (plant protein-based (PM) and fish meal-based (FM)) containing different levels of Arg (1.6%, 3% and 4% diet for PM diets and, 3% and 4% for FM diets). Besides, measurement of growth parameters and protein utilisation, ammonia and urea excretion rates were monitored. Plasma ammonia and urea concentrations and activities of selected enzymes of ornithine urea cycle (carbamoyl phosphate synthetase III, ornithine carbamoyltransferase and arginase) were measured. Both species fed PM-based diets had reduced growth performance compared to fish fed FM-based diets. Amino acid imbalance of the PM-based diets had a significant effect on plasma ammonia levels and ammonia excretion. Dietary Arg supplementation did not lead to any improvement in N utilisation or on ammoniogenesis. In both species, there was a linear relationship between dietary Arg and urea excretion. Our data confirm that ornithine urea cycle is incomplete in the liver and that argininolysis plays a major role in ureogenesis in both species.

Imbalanced dietary ascorbic acid alters molecular pathways involved in skeletogenesis of developing European sea bass (Dicentrarchus labrax).

The influence of dietary ascorbic acid (AA) on growth and morphogenesis during the larval development of European sea bass (Dicentrarchus labrax) was evaluated until 45days post hatching. Diets incorporated 0, 5, 15, 30, 50 or 400mg AA per kg diet to give AA-0, AA-5, AA-15, AA-30, AA-50 and AA-400 dietary treatments, respectively. Dietary AA levels lower than 15mg/kg reduced larval growth and survival was affected in specimens fed diets devoid of AA. Globally, disruption of the expression of genes involved in AA and calcium absorption in the intestine (SVCT-1, TRPV-6), skeletogenesis (BMP-4, IGF-1, RARγ) and bone mineralization (VDRβ, osteocalcin) were observed in groups fed doses lower and higher than 50mg AA/kg diet. Such disturbances detected at molecular level were associated with disruptions of the ossification process and the appearance of skeletal abnormalities.

Identification of Hypoxia-Regulated Genes in the Liver of Common Sole (Solea solea ) Fed Different Dietary Lipid Contents

Coastal systems could be affected by hypoxic events brought about by global change. These areas are essential nursery habitats for several fish species including the common sole (Solea solea L.). Tolerance of fish to hypoxia depends on species and also on their physiological condition and nutritional status. Indeed, high dietary lipid content has been recently shown to negatively impact the resistance of sole to a severe hypoxic challenge. In order to study the molecular mechanisms involved in the early response to hypoxic stress, the present work examined the hepatic transcriptome in common sole fed diets with low and high lipid content, exposed to severe hypoxia. The activity of AMP-activated protein kinase (AMPK) was also investigated through the quantification of threonine-172 phosphorylation in the alpha subunit. The results show that hypoxia consistently regulates several actors involved in energy metabolism pathways and particularly AMPKα, as well as some involved in cell growth and maintenance or unfolded protein response. Our findings reveal that (1) the expression of genes involved in biological processes with high energy cost or implicated in aerobic ATP synthesis was down-regulated by hypoxia, contrary to genes involved in neoglucogenesis or in angiogenesis, (2) the consumption of high lipid induced regulation of metabolic pathways going against this energy saving, and (3) this control was fine-tuned by the regulation of several transcriptomic factors. These results provide insight into the biological processes involved in the hepatic response to hypoxic stress and underline the negative impact of high lipid consumption on the tolerance of common sole to hypoxia.

Comparison of brown trout (Salmo trutta) reared in fresh water and sea water to freshwater rainbow trout (Oncorhynchus mykiss): II. Phosphorus balance

Brown trout and rainbow trout (average weight 100 g) were reared in fresh water at 12 °C under the same conditions before transferring brown trout to sea water, in order to compare phosphorus utilisation in both species. Apparent phosphorus availability, orthophosphate excretion and phosphorus accretion in the fish were directly determined. Thus, actual phosphorus mass balance was built. Rainbow trout raised in fresh water had a higher phosphorus retention coefficient (maximum 50 %) than brown trout reared in fresh water (maximum 45 %). Transferring brown trout to sea water induced a reduction in phosphorus retention (maximum 39 %). Orthophosphate excretion, ranging 7–20 mg phosphorus per kg wet weight per day, represented 10–20 % of ingested phosphorus. Phosphorus availability was lower in brown trout raised in sea water (65 %) than brown trout raised in fresh water (76 %). Phosphorus balance measurements showed that 90 to 98 % of phosphorus flow could be accounted for.