## 3 Results and Discussion
### Heavy Metal Removal Efficiency of A. platensis in MT Cultures  
Figure 9 shows the removal efficiency of _A. platensis_ regarding mono- and multi-MT cultures. It was observed that as the concentration of Cu and Ni increases in a mono-component system, their toxicity becomes more pronounced at higher levels, potentially leading to lethal effects. However, at lower concentrations, Cu plays a vital role in the metabolic functions of both prokaryotic and eukaryotic organisms [18]. On the other hand, in multi-metal systems, Cu removal appears to increase compared to the mono-component system. When comparing the results between mono- and multi-metal systems, it is evident that Ni exhibits reduced removal in comparison to the other metals in the multi-MT cultures. This behavior can be explained by the competitive Langmuir model, which describes the interactions among various ions or molecules being adsorbed [2,45]. The model provides insights into the adsorption capacity in systems where multiple types of ions coexist, leading to competition for available adsorption sites. Low metal removal efficiency by  
Figure 8: FPA/FTIR imaging analysis in micro-ATR mode of _A. platensis_ filament (Control culture, Day 7 of cultivation). (**a**) Optical image (350 \(\upmu\)m \(\times\) 350 \(\upmu\)m). Micro-ATR measured area is indicated with red rectangle (70 \(\upmu\)m \(\times\) 70 \(\upmu\)m). FPA chemical images showing distribution of FTIR bands at (**b**) 1643, (**c**) 1728, and (**d**) 837 cm\({}^{-1}\), that correspond to amide I, lipids, and saccharides, respectively. _A. platensis_ is directly related to cell damage, as demonstrated in Figures 1, 2, 5, and 7, since cell integrity is crucial for metal binding effectiveness. As for the other metals, such as Cd, Pb, and Zn, their removal remains unaffected in the mono- and multi-component system. Furthermore, for these metals, higher concentrations could be reached without a noticeable decline in removal, suggesting that their uptake is not hindered by increased metal concentrations in the system [1; 6]. The primary mechanisms involved are surface adsorption, ion exchange, and chelation. Functional groups found in the cell walls, such as carboxyl, amino, and phosphate groups, attract and bind metal ions because of their negative charge, which draws in the positively charged heavy metal ions. Ion exchange enables the replacement of certain ions, like H\({}^{+}\) or Na\({}^{+}\), with metal ions, promoting their absorption. Additionally, metal ions can form complexes with organic molecules on the surface of _A. platensis_ cells [1]. It is also observed that in some cases, heavy metal removal reduces on Day 7. This occurs primarily due to the reversible nature of the biosorption process where metal ions adhere to the surface of _A. platensis_ cells but can be released when environmental conditions change. Factors such as pH, temperature, or the presence of competing ions can interfere with the binding sites, leading to the metals being freed [35]. When the microalga experiences stress due to excessive metal accumulation, it may actively expel excess ions  
Figure 9: Removal (%) of heavy metals from culture media by _A. platensis_ after mono- (**a**) and multi-metal (**b**) treatment of cultures. Each value is average of two measurements, with uncertainties less than 5%. as part of its detoxification mechanisms to protect itself [77]. This is often facilitated by efflux pumps that transport metals out of the cell. Furthermore, prolonged metal stress can cause damage to the cell wall structure or lead to the instability of metal-ligand complexes inside the cells, which may result in the release of free metal ions into the environment [78]. Oxidative stress, a byproduct of heavy metal toxicity, can exacerbate cellular instability and disrupt intracellular storage mechanisms. These processes highlight the dynamic nature of metal removal efficiency of _A. platensis_ and other microalgae.