Biochemical responses of juvenile shrimp Litopenaeus vannamei (Boone, 1931) exposed to ammonia: The role of açaí (Euterpe oleracea Mart. 1824) as a chemoprotective agent
Exposure to ammonia may influence the excessive increase in the production of reactive oxygen species (ROS) in aquatic organisms. However, the provision of diets with supplementation or inclusion of antioxidants may act as an alternative to chemoprotection, alleviating deleterious effects caused by ROS. The Amazon fruit Euterpe oleracea, popularly known as "açaí", has in its composition large amounts of phenolic compounds that give this fruit an exceptional antioxidant capacity. Because of this, the inclusion of E. olaracea in the diet of L. vannamei could increase the resilience of these animals to deal with ammonia toxicity. In this context, the objective of this study was to evaluate the possible oxidative damages caused by exposure to ammonia in juveniles of L. vannamei, and whether these damages could be attenuated due to the presence of lyophilized açaí in the diet. For 35 days, shrimp with initial weight (mean ± 1 standard error) of 0.97 ± 0.03 g were fed two experimental diets in triplicate: control diet (without the inclusion of açaí) and diet with inclusion of 10% açaí (P/P). After this period, shrimp with an average weight of 3.87 ± 0.04 g were distributed in 24 tanks with 40 L, containing 10 shrimps per tank, and exposed for 96 h to four sublethal concentrations of ammonia (0.01-control; 0.26, 0.48 and 0.91 mg NH3-N L-1), representing a factorial system of 2 (diet with and without açaí) x 4 (total ammonia concentrations tested) in triplicate. The data were submitted to analysis of variance mixed model (ANOVA), being the random factor the different tanks used and the fixed factors the evaluated diets and ammonia concentrations. To detect possible differences between treatments, the Newman-Keuls test with a significance level of 0.05 was applied. Samples of gills, hepatopancreas and muscle were collected for analyses of the content of polyphenols and flavonoids, of total antioxidant capacity (ACAP), reduced glutathione (GSH), glutathione-S-transferase (GST), protein-associated sulfhydryl groups (P-SH) and lipid peroxidation (TBARS). The presence of açaí in the diet did not affect the zootechnical parameters of L. vannamei. The hepatopancreas was the only tissue that presented an increase in the content of polyphenols and flavonoids in organisms fed with açaí (p <0.05). The ACAP of the gills decreased significantly in both diets after exposure to ammonia (p <0.05), whereas in the muscle of açaí-fed prawns there was an increase in antioxidant capacity in the same situation (p <0.05). The loss of antioxidant competence in the gills favored increased levels of TBARS in the açaí diet in this tissue (p <0.05). In contrast to the gills, the decrease in lipid damage observed in treatments of 0.48 and 0.91 mg NH3-N L-1 in the muscle of shrimp that consumed açaí was influenced by the increase in ACAP (p <0.05). There were no significant differences in GSH concentrations in the organs for both diets (p> 0.05). In the gills, açaí promoted a significant drop in GST activity in all concentrations of ammonia in relation to the control diet, and in the muscle, unlike the control diet, the açaí acted to maintain the GST, which reinforces the results found in the content of TBARS in these tissues, since lipid damage increased in the gills and decreased in the muscle. The concentration of sulfhydryl groups (P-SH) decreased significantly in the gills and muscle of the control diet shrimp when exposed to ammonia, inducing protein damage, a response that was not observed in the açaí inclusion diet. In conclusion, ammonia induced oxidative stress in the gills and muscle, however, the inclusion of açaí in the diet of L. vannamei presented antioxidant effects in the different organs, not aggravating the protein damage in the gills and muscle and attenuating the lipidic damage in the muscle.