## 4 Discussion
corneus_ (formerly _Gracilaria cornea_) were, respectively, of 0.76 and 0.77 mg mL\({}^{-1}\) (BHT, 0.48 mg mL\({}^{-1}\)) [80]. Our results demonstrate that the maximum TPC is found in aqueous extracts obtained at 40 \({}^{\circ}\)C (16.95 mg GAE g\({}^{-1}\) extract), in line with previous studies in red algae, although different species and solvents may give rise to quite different results. Thus, these values make this alga an interesting source of polyphenolic compounds. In addition, the choice of solvent must also consider that these extracts are intended for the feed industry and that less toxic, sustainable, low cost, and high availability solvents should be chosen; that is, effective industrial use should be accessible and able to arouse interest. Regarding the DPPH free-radical activity, this has been widely used to determine the antioxidant potential of seaweed extracts [80]. Using a sequential extraction method with ethanol as solvent, the free radical scavenging activity by the DPPH assay was 59.2% and 39.2% in _Gracilaria folifera_ and _Anthophyllcus longifolius_, respectively (formerly _Sargassum longifolium_) [61]. The authors found greater activity in the ethanol extract when compared to other solvents. In _C. birdiae_ and _C. corneus_, the IC50 values for the ethanol extracts (0.77 and 0.76 mg mL\({}^{-1}\), respectively) were quite low considering that the same value for BHT is 0.48 mg mL\({}^{-1}\). Other authors found greater free radical scavenging activity in the aqueous extracts of _A. vermiculophyllum_, _Devaleratea mollis_ (formerly _Palmaria palmata_), _Chondrus crispus_, _Perplyra purpurea_, and _Asparagopsis taxiformis_ when compared to ethanolic extracts [70,71], similar to the results we found for _G. gracilis_. Aqueous extracts also gave a higher extraction yield, thus the most promising extract in this regard as found in previous studies [70]. Extracts with high levels of TPC had greater activity as DPPH radical scavengers as demonstrated by the coefficient of determination found (Figure 5, R\({}^{2}\) = 0.7327) and according to other authors [17,36,74]. This suggests that seaweed polyphenols may be responsible for the antioxidant properties noticed particularly in aqueous extracts. Bacterial diseases are one of the main causes of mortality and losses in the aquaculture sector with recurrent problems due to the emergence of antibiotic resistance and the presence of the residues of these compounds and their derivatives in animal products. Seaweeds have drawn attention as natural sources of metabolites with antimicrobial activity. Simultaneously, vibriosis is a common disease in aquaculture systems caused by bacteria of the genus _Vibrio_ that can result in high mortality rates. Other bacteria such as _Aeromonas hydrophila_, _Aeromonas salmonicida_, _Edwardsiella pisciida_, _Photobacterium damselae subsp. Damselae_, or _Yersinia ruckeri_ can cause an outbreak of disease with serious consequences. Studies report that the use of lyophilized _Gracilaria chilensis_ (10%) as a feed additive resulted in a significant increase in the growth rate of _Salmo salar_[55,81] but also exhibited a marked anti-viral activity (68.02 \(\pm\) 18.34) against salmon anemia virus. Lipid extracts of _Gracilariopsis longissima_ (formerly _Gracilaria longissima_) exhibited a broad spectrum of activity against _Vibrio_ sp. [33]. Additionally, studies on the antimicrobial activity of chloroform/methanol extracts from marine algae of the _Gracilaria_ genus (_Gracilaria dura_, _G. gracilis_, and _G. longissima_) and their effects against six species of fish pathogenic _Vibrio_ sp. indicated that _G. longissimi_ exhibits high activity against _Vibrio ordali_, _Vibrio salmonicida_, _Vibrio alginolyticus_, and _Vibrio vulinficus_[33]. In general, the authors emphasize the antibacterial potential of the extracts of this species of seaweed as a supplement for health promotion in aquaculture, although _G. gracilis_ extracts were active only against _V. salmonicida_.