## 4 Discussion
Antioxidant defense systems are activated in stressful situations, whether due to changes in water quality or an increase in cultivation density; therefore, diet can increase resistance [26]. In this study, gill data showed that increasing the levels of macroalgae inclusion in the diet led to increased levels of reduced glutathione (GSH) and increased antioxidant capacity (ACAP), as well as increases in the P-SH groups. GSH is one of the first defense mechanisms against the action of free radicals [65]. [PERSON] et al. [66] indicated that a curcumin-inclusive diet may modulate GSH biosynthesis by regulating gene expression. In situations of stress, the gill is the most affected tissue because it is in direct contact with possible contaminants dissolved in water, and it is important to recruit antioxidant molecules to maintain homeostasis [61]. Therefore, the use of macroalgae in diets represents an alternative to using the biomass produced and exogenous input of antioxidant enzymes. According to [PERSON] et al. [67], because macroalgae are able to develop in stressful environments, they have defense mechanisms such as an increase in antioxidant enzymes, such as polyphenols and carotenoids, which are important characteristics in the application of the biomass produced. Muscle, which is the most common part of fish, is interesting for observing the absence of damage and improvement in antioxidant capacity under stressful conditions. In this study, even when the animals were subjected to salt stress, there was an improvement in the detoxification capacity and absence of lipid damage in animals with 5 and 10% macroalgae inclusion. According to [PERSON] et al. [68], antioxidants, such as glutathione S-transferase (GST), glutathione (GSH), and superoxide dismutase (SOD), have been reported in the body. This antioxidant effect of algae may be due to the fact that macroalgae of the _Ulva_ genus have several important metabolites in their composition, such as phenolic compounds, carotenoids, chlorophylls, and minerals [67]. According to [PERSON] et al. [69], carotenoids and chlorophylls can be characterized as antioxidants that seek to prevent/intercept ROS. Therefore, the inclusion of macroalgae in the diet can increase the preparation of animals to prevent oxidative damage. However, the inclusion of 10 and 15% macroalgae in the diet caused lipid peroxidation compared with the control treatments. According to [PERSON] et al. [62], an inappropriate amount of antioxidants can convert bioactive compounds into pro-oxidants, caused by the neutralization of physiologically beneficial radicals, which may have occurred at higher levels of macroalgae inclusion. Some anti-nutritional characteristics, such as high ash and fiber content, can also affect animals physiologically, given that the nitrogen digestibility coefficient of macroalgae is lower than that of soybean meal [70]. GST activity also increased in the liver at a 10% macroalgae inclusion concentration compared with that in the unstressed control. GST plays a role in the biotransformation and elimination of toxic compounds in the cell [71], and in this process, GSH is used as a co-substrate in detoxification to maintain homeostasis, so its levels can be altered by GST activity [72]. However, in our study, unlike the accumulation of GSH and GST activity that occurred in the gills, GSH levels in the liver did not increase, which probably indicates that they were used for the detoxification of some compounds accumulated in the body at inclusions of 10 and 15%, which may be related to salinity stress or possible anti-nutritional characteristics of the macroalgae [48]. In addition to the presence of a high fiber and carbohydrate content, which according to the requirements of the species can negatively interfere in the growth of organisms, some factors such as phenolic compounds, which are bioactive compounds present in macroalgae, can be anti-nutritional factors and can cause oxidative stress in the organism [73].