## 4 Conclusions  
In the present study, five different heavy metals were used to produce mono-MT and multi-MT cultures, to examine the behavior of multi-metal systems, and to compare the results regarding the ability of _A. platensis_ for the bioremediation of water contaminated with heavy metals. Experiments in multi-MT systems demonstrated that the removal of their components remains efficient compared to mono-MT systems, as particularly encouraging rates were achieved, i.e., 87% for Cu, 98% for Cd, 67% for Ni, 96% for Pb, and 97% for Zn, depending on the cultivation period and the concentration of heavy metals. The effect of Cu seems to be antagonistic to Ni. Regarding the simultaneous effect using different metals, their mixture may exhibit antagonistic or synergistic behavior, or it may show no interactive effects [39]. The results support that regarding the quality of the cultures and the cells themselves, Ni and Cu presented the most damaging effects, which is in agreement with the literature [19]. Cd, Pb, and Zn in mono-MT cultures had a moderate influence on cell viability in comparison with the damaging effects caused by Ni and Cu. This is supported by the literature in the case of Cd, Pb [19], and Zn. In the case of Cd, this finding is in contrast with the results of [PERSON] et al. [19]; the authors, experimenting with Cd-, Ni-, and Pb-treated cultures, concluded that Cd had the most damaging result on the microalga. The removal efficiency of _A. platensis_, considering the concentration of heavy metals and the day of cultivation, follows this order, in both mono- and multi-MT cultures, as presented in Table 2. In most cases, removals on Days 3 and 7 are quite close thus a shorter treatment period can be adopted. The removal mechanism that is induced by _A. platensis_ can be considered as antagonistic over Ni 1 and 5 ppm- and Zn 1 ppm-treated cultures, as its removal efficiency decreased in comparison to the mono-MT culture. On the other hand, the removal mechanism is considered synergistic for all Cu, all Cd, Ni 10 ppm, all Pb, and Zn 5 and 10 ppm components of the multi-MT cultures, as the mix removal is elevated or close to the one of the mono-MT cultures. The quality of the cultures was assessed by means of pH and EC measurements. EC measurements showed observed changes that did not exceed 5%; this is consistent with reduced metabolic activity and limited nutrient absorption from the cultivation medium. These small variations highlight that EC has limitations to be used as a standalone analysis to check cellular responses under the present study's conditions. On the other hand, pH measurements of the Cu-, Ni-, and multi-MT cultures of 5 and 10 ppm indicated that they may not be healthy. In particular, the multi-MT cultures (Mix) of 5 and 10 ppm show similar pH trends to those of Cu and Ni mono-MT cultures. The pH values of Mix 1 ppm presented a declining trend, indicating that this culture may also not be healthy, but the effect is delayed in comparison to the 5 and 10 ppm Mix cultures. NIR spectroscopy was applied for biomass growth estimation. The Mix cultures' behaviors show similar results to those of the Cu-treated cultures. Regarding the quality of the microalga cells, optical microscopy observations performed on Day 7 of cultivation show that the 1 ppm multi-MT culture seems to be the most affected, in comparison with the 1 ppm cultures of the mono-MT cultures. In contrast, the 5 and 10 ppm multi-MT cultures contain fragmented filaments, but they are not as affected as the Ni and Cu mono-MT cultures. Following this, the decrease in chlorophyll-a content of seemingly unhealthy cultures shows that the cells are not able to photosynthesize. The collected ATR-FTIR spectra showed that the multi-MT cultures present characteristics that better agree with the Cu and Ni mono-MT cultures. The most affected ones are Cu 10 ppm, Ni 5 and 10 ppm, and Mix 10 ppm, as seen by the decline of the protein bands, while the saccharide- and lipid-associated bands seem to be unaffected.