Patrick Quazuguel

Highly unsaturated fatty acids of the omega-3 series (HUFA) are major constituents of cell membranes, yet poorly synthesised de novo by consumers. Their production, mainly supported by aquatic microalgae, has been decreasing with global change. Understanding the consequences of such reductions is essential for ectotherm consumers, since temperature tightly regulates the HUFA content in cell membranes, maintaining their functionality. Integrating individual, tissue and molecular approaches, we examined the consequences of the combined effects of temperature and HUFA depletion on the key cardio-respiratory functions of the golden grey mullet, an ectotherm grazer of high ecological importance. For four months, fish were exposed to two contrasting HUFA diets (4.8% ecosapentaenoic acid (EPA)+docosahexaenoic acid (DHA) on dry matter (DM) vs. 0.2% EPA+DHA on DM) at 12°C and 20°C. Ventricular force development coupled with gene expression profiles measured on cardiac muscle suggest that combining HUFA depletion with warmer temperatures leads to (1) a proliferation of sarcolemmal and SR Ca²⁺ channels and (2) a higher force-generating ability by increasing extracellular Ca²⁺ influx via sarcolemmal channels when the heart has to sustain excessive effort due to stress and/or exercise. At the individual scale, these responses were associated with a relatively greater aerobic scope, maximum metabolic rate and net cost of locomotion, suggesting the higher energy cost of this strategy. These impaired cardiac performances could have wider consequences on other physiological performances such as growth, reproduction or migration, all greatly depending on heart function.

European sea bass (Dicentrarchus labrax) inhabits coastal waters and may be exposed to hypoxia at different life stages, requiring physiological and behavioral adaptation. In the present study, we attempted to determine whether regulation of hemoglobin (Hb) gene expression plays a role in the physiological response to chronic moderate hypoxia in whole larvae and hematopoietic tissues (head kidney and spleen) of juveniles. We also tested the hypothesis that hypoxia exposure at the larval stage could induce a long-term effect on the regulation of Hb gene expression. For this purpose, D. labrax were exposed to a non-lethal hypoxic condition (40% air saturation) at the larval stage from 28 to 50 days post hatching (dph) and/or at the juvenile stage from 196 to 296 dph. Data obtained on larvae indicate that hypoxia induced a subtype-specific regulation of Hb gene expression, with significant decrease of MN-Hbα3, MN-Hbβ4 and MN-Hbβ5 and increase of MN-Hbα2, LA-Hbα1 and LA-Hbβ1 transcript levels. Hypoxia did not induce regulation of Hb gene expression in juveniles, except in the head kidney for those that experienced hypoxia at the larval stage. The latter exhibited a significant hypoxia-induced stimulation of MN-Hbα2, LA-Hbα1 and LA-Hbβ1 gene expression, associated with stimulation of the PHD-3 gene involved in the hypoxia-inducible factor oxygen-sensing pathway. We conclude that subtype- and stage-specific regulation of Hb gene expression plays a role in the physiological response of D. labrax to cope with hypoxia and that early exposure to low oxygen concentration has a long-term effect on this response.

The better understanding of how intestinal microbiota interacts with fish health is one of the key to sustainable aquaculture development. The present experiment aimed at correlating active microbiota associated to intestinal mucosa with Specific Growth Rate (SGR) and Hypoxia Resistance Time (HRT) in European sea bass individuals submitted to different nutritional histories: the fish were fed either standard or unbalanced diets at first feeding, and then mixed before repeating the dietary challenge in a common garden approach at the juvenile stage. A diet deficient in essential fatty acids (LH) lowered both SGR and HRT in sea bass, especially when the deficiency was already applied at first feeding. A protein-deficient diet with high starch supply (HG) reduced SGR to a lesser extent than LH, but it did not affect HRT. In overall average, 94 % of pyrosequencing reads corresponded to Proteobacteria, and the differences in Operational Taxonomy Units (OTUs) composition were mildly significant between experimental groups, mainly due to high individual variability. The highest and the lowest Bray-Curtis indices of intra-group similarity were observed in the two groups fed standard starter diet, and then mixed before the final dietary challenge with fish already exposed to the nutritional deficiency at first feeding (0.60 and 0.42 with diets HG and LH, respectively). Most noticeably, the median percentage of Escherichia-Shigella OTU_1 was less in the group LH with standard starter diet. Disregarding the nutritional history of each individual, strong correlation appeared between (1) OTU richness and SGR, and (2) dominance index and HRT. The two physiological traits correlated also with the relative abundance of distinct OTUs (positive correlations: Pseudomonas sp. OTU_3 and Herbaspirillum sp. OTU_10 with SGR, Paracoccus sp. OTU_4 and Vibrio sp. OTU_7 with HRT; negative correlation: Rhizobium sp. OTU_9 with HRT). In sea bass, gut microbiota characteristics and physiological traits of individuals are linked together, interfering with nutritional history, and resulting in high variability among individual microbiota. Many samples and tank replicates seem necessary to further investigate the effect of experimental treatments on gut microbiota composition, and to test the hypothesis whether microbiotypes may be delineated in fish.

The aim of the study was to evaluate the influence of different dietary protein and lipid levels and their ratios on larval growth, survival and the incidence of cannibalism in Pseudoplatystoma punctifer. Larvae were raised in a recirculation system from 3 to 26 days post-fertilization (dpf) (2–25 days post hatching, dph) at an initial density of 40 larvae L−1, 27.8 ± 0.65°C and 0L : 24D photoperiod. Larvae were fed from 4 to 12 dpf with Artemia nauplii and weaned onto four different compound diets from 13 dpf within 3 days, then fed exclusively with these diets until 26 dpf. These diets contained 30 : 15, 30 : 10, 45 : 15 or 45 : 10 protein : lipid (P : L) (in % of dry matter) levels. A control group was fed Artemia nauplii until 17 dpf and weaned thereafter with the 45P : 10L compound diet. The experiment was carried out in triplicate. Results showed higher growth and survival rates and lower incidence of cannibalism in the group fed the 45P : 15L diet than in the other treatments. Differences in larval survival and growth performance were associated with the higher protein and lipid content rather than the protein : lipid ratio of this diet. When comparing diets with the same protein level, the increase in dietary lipid led to an improvement in growth, suggesting that energy from lipids spares protein for growth in P. punctifer fingerlings. An Artemia feeding period longer than 12 dpf did not improve larval growth or survival. (Résumé d'auteur