## 3 Results and Discussion
### Determination of Selenium Content in Yeast  
During 24 h of experimental cultivation in media supplemented with the lowest doses of selenium (4 and 8 mg/L), the selenium content permanently bound to yeast cell biomass was estimated at 140 and 326 ug/g, respectively. In the next day of cultivation (after 48 h), the selenium level was higher and accounted for 169 and 770 ug/g, respectively (Figure 2). Extension of the cultivation until the third day did not result in any significant increase in the accumulation of this element in the yeast cell biomass of _C. utilis_. Presumably, defense mechanisms leading to detoxification of the element in yeast cells were the main reason for different selenium content in the cellular biomass. The excess of selenium can be transported in yeast to the extracellular environment by means of membrane vesicles. They carry selenoprotein complexes across the cytoplasmic membrane [30; 31; 32]. Experiments carried out by [PERSON] et al. [33] showed that detoxification of selenium in the presence of thiol compounds is associated with the release of H\({}_{2}\)Se into the cell exterior of yeast. Experiments carried out by [PERSON] et al. [34] showed that baker's yeast (_Saccharomyces cerevisiae_) grown for 72 h in media supplemented with sodium hydroxyselenite (IV) (2-12 mg/L) were able to bind selenium at a concentration range between 15 and 203 ug/g. Results obtained by [PERSON] et al. [26] showed that the addition of selenium to the YM (Yeast Mold) culture medium (24 h) at a concentration between 30 and 125 mg/L led to an increase in the content of selenium in the cell biomass of _Saccharomyces cerevisiae_ 6 M in the range between 1039 and 5003 ug/g. The process of selenium uptake by _Pediococcus acidilactici_ ATCC 8042 was described by [PERSON] et al. [35]. It was found that lactic acid bacteria cultured for 24 h in MRS medium with sodium selenite (IV) at a concentration in the range between 0.5 and 4 mg/L bound large amounts of selenium (0.17-1.89 mg/g). [PERSON] et al. [36] reported that the concentration of accumulated selenium in the biomass of two strains of _Lactobacillus paracasei_ (ML13 and CH135) were approximately 38 and 40 mg/g in a culture exposed to 150 mg Se\({}^{4+}\)/L in the MRS medium. The results obtained indicate the ability of _C. utilis_ ATCC 9950 yeast cells to accumulate selenium not only in the logarithmic growth phase, but also during the stationary phase, as evidenced by the increase in the content of this element in the biomass of the yeast cells. According to the literature [29], dead microbial cells may bind metal ions through different physico-chemical mechanisms. The described results are in accordance with the conclusions made by [PERSON] and [PERSON] [37]. The authors of the cited report showed that dead cells of _S. cerevisiae_ 1452-L6F (baker's yeast) bound more uranium and zinc, approximately 40%, when compared to living cells. In terms of medium supplemented with selenium at a dose of 20 mg/L, one obtained 903 ug/g after the first day of culturing and 1742 ug/g in the second day. Further extension of the experimental cultivation of _C. utilis_ till 72 h resulted in the reduction of selenium content (1650 ug/g) in yeast cell biomass. It was probably related to the formation of volatile methyl derivatives of selenium compounds during the conversion of selenium from yeast biomass [38]. At selenium concentrations above 30 mg/L in experimental media, one observed an increase in the content of this element in yeast biomass after 72 h of cultivation. Evaluation of selenium binding to yeast cells after 72 h of cultivation at a concentration of this element expressed by a dose of 80 mg/L in the experimental medium showed a rapid increase in the content of this element (4658 ug/g) in the yeast cell biomass. At the same time, it was observed that yeast cell biomass was red-brown in color (Figure 3). This also occurred to a lesser extent for lower doses of selenium (30, 40 mg/L) after 72 h of cultivation. The color of yeast biomass in this case was orange.