## 3 Results and Discussion
### Spectroscopy Analysis
#### 3.4.2 FTIR-ATR  
The algae samples were also analysed using FTIR-ATR spectroscopy as a rapid method for chemical characterisation. The spectra obtained (Figure 6) are similar to those obtained by other authors who have analysed algae and plant material [82; 83]. Figure 5: Principal component analysis to discriminate between the different samples analysed using FT-NIR spectral information (**left** side) and the corresponding loadings of spectral information (**right** side) using the Savitzky–Golay derivate transform with 15 smoothing points. GMB—_Gracilaria multipartita,_ UB—_Ulva_ sp., SMB—_Sargassum muticum_ from Buarcos beach, SMSM—_S. muticum_ from San Marcos beach. The most characteristic bands were seen in the spectral range of 1700-800 cm\({}^{-1}\) and 3500-2800 cm\({}^{-1}\) due to the absorbance of the peptide backbone and the methyl and methylene groups [31]. The 1200-950 cm\({}^{-1}\) regions of the spectra were associated with the stretching (C-O-C) vibrations of the polysaccharides [84], with a most intense band at 1024 cm\({}^{-1}\). This band exhibited varying intensities from _G. multipartita_, followed by _S. muticum_ and a smaller peak in _Ulva_ sp. These features were attributed to different polysaccharide components: agar in red algae, guiluronic acid, which is part of alginate acid in brown algae, and glycosidic C-O-C vibrations form (1,4)-\(\beta\)-D-mannans, which is the primary component of the skeletal cell wall polysaccharides in green algae [85]. The more pronounced peak in _G. multipartita_ indicates a high concentration of phycocolloids, which in this species corresponds to high agar content. In the brown seaweeds, the presence of alginate was also confirmed by a peak at 1320 cm\({}^{-1}\), corresponding to the C-O stretching vibrations in this linear copolymer of \(\beta\)-D-mannuronic acid and \(\alpha\)-L-guiluronic acid [86]. The medium intensity absorbance peaks around 1517 cm\({}^{-1}\) (associated with N-H and C-H stretching) and 1649-1606 cm\({}^{-1}\) (linked to C=O) can be attributed to the amide II and amide I groups, respectively, which are characteristic of protein groups [84]. The Amide I band (\(\sim\)1649 cm\({}^{-1}\)) provided a robust marker for protein quantification. Its higher intensity in _G. multipartita_ compared to _Ulva_ sp. suggests higher protein content in the red seaweed [77, 87], which is consistent with our biochemical analysis. The peaks in the 1600-1500 cm\({}^{-1}\) range corresponded to aromatic compounds (such as flavonoids, phenolic acids, and polyphenols) and are linked to antioxidant properties and secondary metabolites [84]. These peaks were more pronounced in _G. multipartita_, suggesting that it is a potential source of bioactive compounds in response to oxidative stress and UV damage. The band at 1415 cm\({}^{-1}\) was associated with hydroxyl deformation, carbon-oxygen stretching of phenolic hydroxyl groups, and symmetrical C-H bending vibrations in CH\({}_{2}\) or CH\({}_{3}\) groups [88], with greater intensity in _G. multipartita_. The peaks in the 1320-1200 cm\({}^{-1}\) region indicate polysaccharide structures in _S. muticum_ attributed to C-H bending vibrations in sugars [37], indicating the presence of mannuronic acid, another key component of the alginate commonly found in brown  
Figure 6: FTIR-ATR spectra of species of macroalgae samples selected: _Gracilaria multipartita_, _Ulva_ sp., and _Sargassum muticum_ (from Buarcos and San Marcos). seaweeds [89]. The presence of alginate in _S. muticum_ was also confirmed by a peak at 1320 cm\({}^{-1}\), corresponding to the C-O stretching vibrations. The anomeric region (950-700 cm\({}^{-1}\)) includes a significant peak at 930 cm\({}^{-1}\) in red seaweeds, which is attributed to 3,6-anhydro-L-galactose, a key component of agar in _Gracilaria_ species [37]. In biological samples, the peak at 2910 cm\({}^{-1}\) is often related to CH\({}_{2}\) stretching vibrations in lipids or long-chain fatty acids [90].