## 3 Results and Discussion
### FTIR Spectra
\begin{table}
\begin{tabular}{c c c} \hline
**Frequency (cm\({}^{-1}\))** & **Assignments** & **Reference** \\ \hline
3630–3040 (3280) a  & N-H Stretch/protein (overtone of amide II band) & [45,48] \\ C–H symmetric stretch/CH2, CH3 lipid acyl chains, CH & [45,48] \\
3040–2800 (2860,2930, num b) & stretch/aminoacids or carbohydrates & [45,48,47] \\
1775–1720 (1745) & C=O symmetric stretch/lipids, fatty acids & [45,48] \\
1590–1715 (1650) & CO stretch, CN stretch, NH bend/amide I band (proteins), C=C & [45,47] \\ stretch/o/delta and aromatic compounds & & \\
1490–1590 (1540) & NH bend, CN stretch, CH stretch/amide II band (proteins) & [45,47,48] \\
1490–1430 (1450) & CH3 and CH2 asymmetric bend/proteins or lipids & [47,48,49] \\
1430–1350 (1400) & CH3 and CH2 Symmetric bend/proteins or lipids, COO- symmetric & [47,49] \\  & & stretch/acids & \\
1280–1185 (1240) & P = O asymmetric stretch/nucleic acids (DNA and RNA) or & [45,47,49] \\  & phosphorylated proteins, C-O-S/sulpholipids & [45,47,49] \\
1185–1135 (1150) & C-O-C stretch and O-H bend/carbohydrates or lipids & [45,47,49] \\
1135–1095 (1105) & C-O-H deformation, C-O-C stretch/polysaccharides, carbohydrates & [45,49] \\
1095–1070 (1015, 10058 n c) & or lipids, P=O symmetric stretch/nucleic acids or phospholipids & [45,49] \\
950–880 (940) & CH deformation/carbohydrates, P-O-P/polphosphates & [45,48] \\
880–825 (836-control samples only) & P-O asymmetric stretch/lipids, CH deformation/carbohydrates, & [45] \\
825–740 (760) & CH deformation/carbohydrates & [45] \\  & N-H bend/amide V band, CH2 rocking/lipids, CH2 rocking—NH & \\
740–690 (690 sh) & deformation/polylyclines, O-CO/CO2/CO/C=O & [45] \\  & deformation/carboxylic acids or esters & \\ \hline \end{tabular}
\end{table} 
Table 4: FT-IR frequencies observed in spectra collected from _A. platensis_ samples and possible assignments of the vibration modes. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & \multicolumn{6}{c}{**Performance Indices**} \\ \cline{3-8}
**Parameter** & **LV a** & \multicolumn{3}{c}{**Internal Validation (LOOCV)**} & \multicolumn{3}{c}{**External Validation**} \\ \cline{3-8}  & & **R2 b** & **RMSE c** & **RPD d** & **R2** & **RMSE** & **RPD** \\ \hline Proteins & 4 & 0.968 & 2.581 & 5.662 & 0.847 & 4.981 & 2.645 \\ Carbohydrates & 3 & 0.978 & 2.576 & 6.825 & 0.895 & 5.287 & 3.194 \\ Lipids & 2 & 0.440 & 1.054 & 1.360 & \(-\)5.449 & 2.255 & 0.408 \\
1.3:1.6-\(\beta\)-glucans & 3 & 0.979 & 1.066 & 7.031 & 0.930 & 1.920 & 3.913 \\ \(\alpha\)-glucans & 3 & 0.970 & 0.562 & 5.920 & 0.777 & 1.226 & 2.194 \\ Dietary fibers & 4 & 0.951 & 2.352 & 4.617 & 0.937 & 2.721 & 4.121 \\ Glucose & 4 & 0.921 & 3.630 & 3.623 & 0.973 & 2.053 & 6.286 \\ \hline \end{tabular}
\end{table} 
Table 5: Performance indices for the internal validation (LOO-CV) and the external validation (test set) estimates for each PLS-R model. Figure 6: Comparison of the PLS-R–FTIR model estimates for the different _A. platensis_ parameters against the experimentally observed values. **a**: calibration (training) data; **b**: internal validation (LOO-CV) data; **c**: external validation (test set) data. Previous studies have also included PLS-R modelling for the prediction of protein, carbohydrate and lipid in microalgae and cyanobacteria [51; 52]. The study of [PERSON] et al. [51] showed good correlations for all the parameters studied in _Nammochloropsis_ sp. between the predicted values using the FTIR spectra and the observed ones [51], while the study of [PERSON] and [PERSON] [52] also reported sufficient prediction models for the protein and carbohydrate content of Spirulina but insufficient models for lipids, a finding that was observed in this study as well. However, the aforementioned works applied one data set (one biological replicate/batch) to model the data and the models were not validated with an external independent data set (test set from a different batch), so the results cannot be fully compared with the current study, which included two batches for training and one for external validation.