## 3 Results and Discussion
### Electrical Conductivity and pH Measurements  
EC monitoring during the cultivation of _A. platensis_ is an important indicator of metabolic activity that affects culture health. Conductivity measures the medium's ability to conduct an electric current, which is influenced by ion concentration. During growth, the microalga absorbs the nutrient components such as nitrates (NO3\({}^{-}\)) and phosphates (PO4\({}^{3-}\)) from the cultivation medium, which are then integrated into the cells and thus reduce the ion concentration, reducing conductivity. However, in response to stressors such as metal exposure or nutrient limitation, metabolic processes of cells are disrupted  
Figure 3: NIR reflectance response of cultures at 1062.6 nm: (**a**) Cd-MT; (**b**) Zn-MT; (**c**) Pb-MT; (**d**) Cu-MT; (**e**) Ni-MT; (**f**) Multi-MT. Values were normalized to Day 0 of cultivation to account for slight variations in initial biomass. Each value is average of two measurements, with uncertainties less than 3% over each value. and the production of metabolic byproducts like hydrogen ions (H\({}^{+}\)) or ammonium ions (NH\({}_{4}\)\({}^{+}\)) could increase, thus increasing EC. Increased conductivity may therefore become an indication of metabolic disturbances or culture stress [55]. Besides this, changes in conductivity can serve to monitor the general health and growth of the culture. In stressed _A. platensis_ cells, conductivity remains stable or slightly increases due to reduced metabolic activity and limited nutrient absorption. On the other hand, in healthy cells, conductivity decreases due to the absorption of nutrients by the cells, reducing ion concentration in the medium. In all cases, EC started with a value of 0.91 mS/cm for all cultures, while on Day 7, a minor variation is observed (<5% of the initial value). In Figure 4, the pH measurements of all mono- and multi-MT cultures are presented, along with those of the Control culture, for all days of cultivation. In general, the measurements are summarized as follows:  
* Zn, Pb, and Cd mono-MT cultures follow the Control one closely, at least up to Day 6. * Ni and Cu mono-MT cultures present similar behavior. * Regarding the multi-MT cultures, Mix 1 ppm seems to be unaffected up to Day 2 of cultivation. Afterwards, it starts declining. Mix 5 and 10 ppm cultures present reduced values even from Day 1. The increase in pH during _A. platensis_ cultivation is a common phenomenon associated with the organism's photosynthetic activity and metabolic processes [19]. Optimal pH values for the microalga cultivation are between 9.5 and 11 [19]. _A. platensis_ requires CO\({}_{2}\) in photosynthesis, and this diminishes the concentration of CO\({}_{2}\) in the medium. The carbonate equilibrium is moved, which increases alkalinity and pH. Besides this, the consumption of protons due to NO\({}_{3}\)\({}^{-}\) taken up as a nitrogen source increases the pH. These processes are balanced by the excretion of metabolic end products, such as CaCO\({}_{3}\), especially in hard water systems. Such pH increases are typical for healthy _A. platensis_ growth and provide an alkaline environment that suppresses the growth of competitive  
Figure 4: pH measurements of cultures during all cultivation days: (**a**) Cu-MT; (**b**) Cd-MT; (**c**) Ni-MT; (**d**) Pb-MT; (**e**) Zn-MT; (**f**) Multi-MT. Each value is average of two measurements, with uncertainties less than 4% over each value. microorganisms. According to [PERSON] [55], this interrelationship between photosynthetic CO\({}_{2}\) uptake and medium pH in _A. platensis_ cultivation has a dual importance for the optimization of growth and prevention of contamination. Indeed, in our experiments, a clear difference in pH behavior was noticeable between the unstressed Control and the stressed MT cultures of _A. platensis_.