## 4 Discussion  
The nutritional composition of macroalgae depends on the environmental variables in which they are cultivated. In this study, the protein content of macroalgae cultivated in an integrated system with biofoc was 19%, which was higher than that reported by [PERSON] et al. [42], who obtained 14% protein from macroalgae collected in the environment, and by [PERSON] et al. [43], working with _Ulva olnoi_, who collected from the environment with a protein content of 13.6% in the macroalgae. The protein increase observed in the present study may be associated with the high concentrations of nitrogen available for absorption in the biofoc system, as well as a high load of solid production, causing an increase in chlorophyll-a as a way of maximizing photosynthesis, as reported by [PERSON] et al. [10]. [PERSON] et al. [44], working in an integrated system with shrimp, mullet, and macroalgae, showed an increase in nitrogen in the macroalgae, from 1% nitrogen at the beginning of the experiment to 2.9% at the end of the experiment, associated with an increase in chlorophyll-a and carotenoids, showing an increase in the nutritional value of the macroalgae when cultivated in association with other organisms. This increase in protein and biocompatibility in macroalgal tissues makes it a promising and sustainable activity to be included in the feed processes for aquatic organisms. [PERSON] et al. [21] used a 10, 15, and 20% inclusion of macroalgae in the diet and the replacement of fishmeal for tilapia, with the result that, up to 10% inclusion, the animal's performance was not affected, with digestibility, apparent retention of protein, and energy being essential factors for the success or failure of the substitution [45]. A similar result was found by [PERSON] et al. [46] for tilapia, showing that small percentages of _Ulva_ macroalgae inclusion (5 and 10%) did not negatively affect animal performance compared with the control diet. For carnivorous species such as sea bass fry _Dicentrarchus labrax_, it was found that the inclusion of up to 5% macroalgae meal promoted improvements in growth and survival rates, related to the protein content, minerals, and essential amino acid profile present in the macroalgae [47]. In the present study, the inclusion of macroalgae in the diet resulted in the replacement of ingredients from terrestrial plant sources such as soybean meal and wheat bran. Although macroalgae meal presented lower energy and protein digestibility, as presented by [PERSON] et al. [45], it did not interfere with the growth performance of tilapia at any level of inclusion in this experiment. [PERSON] and [PERSON] [48] evaluated the digestibility indices of _U. lactuca_ for tilapia and obtained values of 82% for protein, 92% for lipids, 63% for ash, and 74% for energy, showing good indices and feasibility of inclusion in the diet. The results of the present study were similar to those found by [PERSON] et al. [49], who tested the inclusion of 10% _Gracilaria_ from an integrated cultivation system in the diet of tilapia, and showed that there were no differences in the performance or body composition of tilapia between the treatment with _U. lactuca_ and the control. Thus, the application of macroalgal biomass produced in the integrated biofoc system in tilapia feed reflects the circular economy, which consists of adopting sustainable practices related to the better use of the nutrients produced and waste management [17]. Several studies have pointed to different situations regarding the effects of including macroalgae on the proximal composition of fish carcasses. [PERSON] et al. [20] tested the replacement of wheat flour with macroalgae flour and showed that higher inclusions(75 and 100 g kg\({}^{-1}\)) resulted in greater growth and changes in the tilapia carcass, with higher protein and lower lipid values as the inclusion of macroalgae in the feed increased. [PERSON] et al. [50] also found a decrease in lipid values with an increase in the inclusion of macroalgae, but protein values were not altered.