## 3 Results  
### Identification and Cultivation of Yeast Strains  
Based on the molecular genetic analysis of ITS1-5.8S-ITS4 regions of the 18S rRNA gene, the first strain was found to belong to _Vishniacozyma victoriae_, while the second one was identified only at the genus level as _Tremellomycetes_ sp. The most likely explanation could be the lack of available data in NCBI GenBank, which is used for comparison during the identification process. It is highly probable that this strain is a new one. Its taxonomy will be further revealed in full detail.  
Furthermore, the yeast growth, accumulation of EPSs, and the change of pH during the fermentation process of both strains in flasks and bioreactor were monitored and assessed. The results are illustrated in Figures 1 and 2, respectively. The relatively high temperature of cultivation (22 \({}^{\circ}\)C) was chosen to induce heat stress in the psychrophilic yeast cultures, aiming for an extensive accumulation of EPSs as a protective response mechanism. The kinetic of the fermentation process of _V. victoriae_ showed preservation of the standard course of the exponential curve and entering the stationary phase after 72 h (Figure 1A). The amount of accumulated biomass at the 120 th h of cultivation was about 6 g/L. At this stage, a carbon source was utilized, and EPS was formed. The quantity of accumulated EPS was over 4 g/L. Considering _Tremellomycetes_ sp., a similar trend was observed, as the synthesized biopolymer at the end of the fermentation was 4.5 g/L. The accumulated biomass was about 7 g/L at the 96 th h; after that, it entered the stationary phase of growth (Figure 1B). Interestingly, we found a considerable decrease in pH from the initial 5.3 to 2 for both producers during the first 24 h of cultivation.  
Figure 1: Time course of cell growth, EPS accumulation, and pH during the cultivation of (**A**) _Vishniacozyma victoriae_ and (**B**) _Tremellomycetes_ sp. in flasks.  
Figure 2 shows the growth kinetics of the yeast strains in a 4 L bioreactor. Trends in cell growth and EPS accumulation were similar to that of flask cultivation. Interestingly, the _Trenellomycetes_ sp. (Figure 2C) cell growth had a longer exponential phase by comparison with that of _V. victoriae_ strain (Figure 2A). It seemed that both species were similar in regard to EPS production, although _Trenellomycetes_ sp. produced a slightly higher amount of EPS (4.8 g/L), while the amount of EPS in the culture medium of _V. victoriae_ reached not more than 4.5 g/L at the 120 th h. The biomass productivity of _V. victoriae_ was 0.055 g/L/h, and that of _Trenellomycetes_ sp. was 0.058 g/L/h. The accumulation of EPSs was 0.038 g/L/h and 0.040 g/L/h, respectively. Interestingly, a large part of EPSs (>50%) was synthesized during the first 24 h of the cultivation, which was also accompanied by high oxygen consumption and effective microbial growth. The DO\({}_{2}\) level was plotted in Figure 2B,D. It is shown that after the initial decrease in DO\({}_{2}\), the level of DO\({}_{2}\) gradually increased, which was closely associated with a smooth transition between the exponential and stationary phase. It is worth noting that the DO\({}_{2}\) is a key parameter for the support and activation of cell growth and other anabolic processes of the aerobic metabolism, such as accumulation of the secondary metabolites (EPSs). It may be that a better stirring and aeration of the growth medium contributed to a slightly higher EPS synthesis by _Tremellomycetes_ sp. during the cultivation in the bioreactor.