## 3 Results and Discussion
### Growth Curves of Yeast Strains  
The yeast _R. mucilaginous_ has been described as one of the species present in municipal wastewater treatment plants polluted with Cr, Cd, and nitrates [51]; in olive mill wastewater, where is able to assimilate and degrade phenolic compounds [52]; or in textile wastewater, having the ability to remove Cu\({}^{2+}\), Ni\({}^{2+}\), and Cr\({}^{6+}\) ions with an efficiency of 30.6%, 38%, and 94.5%, respectively [53]. [PERSON] et al. [54] reported that _R. mucilaginous_ planktonic cells were resistant to cadmium concentrations up to 10 mM and nickel concentrations up to 3.200 mg/L. Also, two _R. mucilaginous_ strains isolated from wetlands from Ventanilla (Callao, Peru) showed intensive growth in the presence of 10 mg/L of Pb\({}^{2+}\) at pH 5.0 and pH 6.0, and an ability to tolerate up to 1 g/L Pb\({}^{2+}\) at pH 6.0 [55].  
In our study, the growth of the strains _R. mucilaginous_ CMGB-G1 (P1) and _R. mucilaginous_ CMGB188 (P2) was monitored and recorded through optical density measurements, both in controlled (24 h, Figure 2) and experimental conditions (120 h, Figure 3). Under controlled conditions, the development of yeasts was observed for a shorter duration of 24 h in order to understand their normal growth pattern without any external variables (i.e., temperature, day/night alternation).  
The data from Figure 2a,b reveal that all the tested samples showed the same development pattern over the 24 h test period, with only the lag and exponential development phases being visible. Both yeast strains showed a lag period between 9 and 14 h for most tested synthetic wastewaters, with the order of growth being generally in the direction of \(Pb<Sw<Bio~{}Y+Pb\leq Bio~{}Y\). This trend demonstrates the positive influence of a biosurfactant on the faster growth of \(R\). _mucilaginousa_ cultures. However, a particular case was observed regarding the water contaminated with cadmium ions, where the lag period was 22 h, which was probably due to the more pronounced toxic effect of Cd\({}^{2+}\) ions on the tested strains [31].  
On the other hand, under experimental conditions (Figure 3a,b), the yeast strains were subjected to a more extended observation period of 120 h. This allowed the examination of the long-term growth behavior of the yeast strains and their responses to specific experimental variables.  
The data reflect a steady increase in OD values throughout the experiment. In the case of the biosurfactant-supplemented sample (\(Bio~{}Y\)), a maximum OD was observed for both yeast strains within the 48-96 h period of the experiment. At the end of the 120 h experiment, it becoma evident that the water samples enriched with the biosurfactant yielded the highest optical density (OD) values for both types of yeast strains. Notably, there existed a discernible decreasing trend in OD values, with the samples treated with \(BioY+Pb\) exhibiting the highest values, followed by \(BioY\) and then by \(BioY+Cd\) samples. In contrast, samples with Cd\({}^{2+}\), Pb\({}^{2+}\), and initial synthetic wastewater, as well as those without  
Figure 3: Growth curves of P1 (**a**) and P2 (**b**) recorded in experimental conditions.  
Figure 2: Growth curves of P1 (**a**) and P2 (**b**) recorded in controlled conditions.  
the biosurfactant, showed lower OD values. These findings underline the effectiveness of a biosurfactant in stimulating yeast cell growth, highlighting its importance in the context of the experiment.