Dietary supplementation of heat-treated Gracilaria and Ulva seaweeds enhanced acute hypoxia tolerance in gilthead sea bream (Sparus aurata)

Abstract

Intensive aquaculture practices involve rearing fish at high densities. In these conditions, fish may be exposed to suboptimal dissolved O₂ levels with an increased formation of reactive O₂ species (ROS) in tissues. Seaweeds (SW) contain biologically active substances with efficient antioxidant capacities. This study evaluated the effects of dietary supplementation of heat-treated SW (5% Gracilaria vermiculophylla or 5% Ulva lactuca) on stress bioindicators in sea bream subjected to a hypoxic challenge. 168 fish (104.5 g average weight) were distributed in 24 tanks, in which eight tanks were fed one of three experimental diets for 34 days: (i) a control diet without SW supplementation, (ii) a control diet supplemented with Ulva, or (iii) a control diet with Gracilaria Thereafter, fish from 12 tanks (n=4 tanks/dietary treatment) were subjected to 24 h hypoxia (1.3 mg O₂ l^-1) and subsequent recovery normoxia (8.6 mg O₂ l^-1). Hypoxic fish showed an increase in hematocrit values regardless of dietary treatment. Dietary modulation of the O₂-carrying capacity was conspicuous during recovery, as fish fed SW supplemented diets displayed significantly higher haemoglobin concentration than fish fed the control diet. After the challenge, survival rates in both groups of fish fed SW were higher, which was consistent with a decrease in hepatic lipid peroxidation in these groups. Furthermore, the hepatic antioxidant enzyme activities were modulated differently by changes in environmental O₂ condition, particularly in sea bream fed the Gracilaria diet. After being subjected to hypoxia, the gene expression of antioxidant enzymes and molecular chaperones in liver and heart were down regulated in sea bream fed SW diets. This study suggests that the antioxidant properties of heat-treated SW may have a protective role against oxidative stress. The nature of these compounds and possible mechanisms implied are currently being investigated.

Keywords: Hypoxia; Nutritional background; Oxidative stress; Sea bream; Seaweeds.

Fig. 1.

Changes in accumulative mortality of sea bream fed seaweed- supplemented diets and subjected to hypoxia followed by normoxia (recovery). Data are represented as mean±s.e.m. of four tanks per treatment. Bars with different letters indicate significant differences between dietary treatments (P<0.05, one-way ANOVA, followed by Holm-Sidak test).

Fig. 2.

Hepatic and cardiac indexes related to OXPHOS and FA oxidation genes in sea bream fed seaweed supplemented diets in normoxia or subjected to hypoxia followed by normoxia (recovery). (A,C) Hepatic; (B,D) cardiac. Data are represented as mean±s.e.m. (n=5-7). Bars with different letters indicate significant differences between dietary treatments (P<0.05, one-way ANOVA, followed by Holm-Sidak test).

Fig. 3.

Fold-changes of mRNA expression levels of differentially expressed genes in liver of sea bream fed seaweed-supplemented diets and subjected to normoxia or to hypoxia followed by normoxia (recovery). (A) Gracilaria versus control; (B) Ulva versus control. Data are represented as mean±s.e.m. (n=5-7). Asterisks indicate significant differences with respect to the control diet group within each environmental O₂ condition (P<0.05, Student t-test).

Fig. 4.

Fold-changes of mRNA expression levels of differentially expressed genes in heart of sea bream fed seaweed-supplemented diets and subjected to normoxia or to hypoxia followed by normoxia (recovery). (A) Gracilaria versus control; (B) Ulva versus control. Data are represented as mean±s.e.m. (n=5-7). Asterisks indicate significant differences with respect to the control diet group within each environmental O₂ condition (P<0.05, Student t-test).

Fig. 5.

Experimental set-up used to control dissolved oxygen (DO) levels in the experimental tanks to implement hypoxia. The system consisted of a loop of 12 tanks with reduced DO levels, which were separated from the remaining tanks (12) in normoxia as detailed in the Materials and Methods section.

Fig. 6.

Changes in dissolved oxygen (DO) levels during the trial. Sea bream fed the experimental diets for 34 days were subjected to normoxia (8.6 mg O₂ l⁻¹) or to hypoxia (1.3 mg O₂ l⁻¹) followed by normoxia (recovery) as described in the Materials and Methods section. Sampling points for each experimental condition are represented as dots.