## 3 Results and Discussion
### FTIR Analysis  
ATR-FTIR analysis was performed on the _A. platensis_ biomasses collected from all cultures as a qualitative method to check the changes occurring to microalga because of the heavy metal influence. Figure 6 shows the ATR-FTIR spectra collected from the Control culture the initial (Day 0) and the final (Day 7) day of cultivation. Both spectra are almost identical, and they present FTIR bands that are typical for _A. platensis_[36; 63; 64; 65; 66; 67; 68]. In particular, the wide and intense overlapping bands at 3500-3200 cm\({}^{-1}\) are attributed to OH and NH stretching vibrations, and the bands at 3000-2800 cm\({}^{-1}\) are attributed to CH stretching vibrations [64; 69]. The shoulder at 1728 cm\({}^{-1}\) is attributed to C=O stretching of lipids [65]. The strong bands at 1643, 1541, and 1394 cm\({}^{-1}\) are assigned to amide I (mainly C=O stretching), amide II (mainly N-H bending), and COO\({}^{-}\) stretching, respectively. Some less intense bands and shoulders are also present at 1480-1180 cm\({}^{-1}\). In particular, these bands are located at 1451 (with many attributions to groups, such as OCH and COH [65], CH\({}_{3}\)[63; 66; 67], CH and CH\({}_{2}\)[36], and C-CH\({}_{3}\) and C-CH\({}_{2}\)[64]), 1308 (N-H bending and C-N stretching of amide III), and 1242 cm\({}^{-1}\) (P=O of phospholipids and nucleic acids [63; 68; 69; 70], and ester O-C-O [68]). Finally, the broad and intense complex at 1180-950 cm\({}^{-1}\), with defined bands at 1151, 1060, and 1029 cm\({}^{-1}\), is attributed to the saccharide content of the microalga (pyranose compounds), and especially to C-O and C-C bonds [36; 65; 66; 67; 68; 69; 71], and maybe with a contribution by P-O bonds [68; 70]. Regarding the variation in the Control culture spectra, a band at 837 cm\({}^{-1}\) is present only in the Day 0 spectrum, while the area at 1180-950 cm\({}^{-1}\) is stronger in Day 7, where the band at 1029 cm\({}^{-1}\) is more intense. Both these variations are connected with saccharides [36; 69; 72; 73; 74; 75]. The ATR-FTIR spectra of the biomass collected from all cultures on Day 7 of cultivation are presented in Figure 7. In every case, the main spectral differences between the MT cultures and the Control spectrum are noted for comparative reasons. The collected spectra show that the different metals cause changes in the microalga biomass. These differences can be summarized into two major categories: the decrease in the intensity of the 1643 (amide I) and 1541 (amide II) cm\({}^{-1}\) bands, and the arise of bands attributed to saccharide content (\(\sim\)1400 cm\({}^{-1}\) and 900-800 cm\({}^{-1}\)). Similar results were also found in _A. Figure 6: ATR-FTIR spectra of _A. platensis_ biomass collected from Control culture on initial (Day 0) and final days (Day 7) of cultivation. _platensis_ cultivations with different cultivation media and environmental conditions [22], and in mono- [45] and multi-MT cultivations [46]. Concerning the adsorption of metals by cells, their cell walls and specifically the chemical groups present in them are considered responsible for binding metal ions [45]. Previous studies mention shifting of bands in the area of 1650-1300 cm\({}^{-1}\), which is indicative of metal binding to characteristic groups that present bands in this area, such as COO\({}^{-}\) and OH groups [36; 43; 46]. In the present study, the shifting of bands in this area is not observed. Instead, the bands under question decrease in intensity in all MT cultures. This is indicative of the decline of these bands, i.e., the disruption of structure by the breaking of bonds, or the induction of oxidation stress leading to denaturation and fragmentation, up to the degradation of proteins, and not the coordination of their said functional groups with metals. Regarding Cu, Zn, Cd, and Ni effects on _A. platensis_ mono-MT biomass, similar spectral behavior was reported by [41].