## 4 Discussion  
Of the four cations tested, barium, which belongs to the group of alkaline earths, was found to have the most toxic effect on the cells of _R. mucilaginous_ MK1. Unfortunately, the literature lacks data concerning its influence on the growth and biosynthesis of carotenoids by yeasts. Among the few studies focusing on the effect of barium on yeast growth, the study by [PERSON] et al. [18] showed that barium added in the form of barium ferrite nanoparticles (BaFeNP) at doses ranging from 15 to 500 ug/mL did not cause any significant effect on the growth of _Candida albicans_ ATCC 10,231 yeast strain. Zinc, which is a chemical element from the zinc group, is a component of many enzymes, transcription factors, and regulatory proteins. It ensures optimal metabolism of nucleic acids and proteins and plays an important role in the growth, division, and functioning of cells. In yeast cells, zinc is present in both free and bound forms, with the latter being dominant [19]. In the present study, the addition of this element did not influence the growth and biosynthesis of carotenoids by _R. mucilaginous_ MK1 yeast at the studied concentrations. In a study by [PERSON] et al. [20], the addition of ZnSO\({}_{4}\) salt at a dose of 0.1 g/L stimulated carotenoid synthesis by _Rhodotorula glutinis_ yeast which produced 638 ug/g\({}_{\text{d.m. }}\) of carotenoids (volume yield: 2.81 mg/L), while the content and volume yield determined in the medium lacking this salt after cultivation were, respectively, 292 ug/g\({}_{\text{d.m. }}\) and 1.13 mg/L. A study by [PERSON] et al. also confirmed that carotenoid biosynthesis was stimulated in a medium supplemented with zinc ions in the form of ZnSO\({}_{4}\)[21]. The yeast _Sporobolomyces salmonicolor_ AL\({}_{1}\) was found to produce 16-fold more \(\beta\)-carotene in the medium containing 112 ppm of Zn\({}^{2+}\) ions (from 11.2 to 189.2 ug/g\({}_{\text{d.m. }}\)) compared to the medium lacking these ions. Interestingly, in the presence of Zn\({}^{2+}\), the biosynthesis of coenzyme Q\({}_{10}\) was completely inhibited. It was assumed that zinc ions activated the enzyme catalytic centers responsible for isoprenoid synthesis and thus contributed to the biosynthesis of \(\beta\)-carotene. Some authors analyzed whether supplementation of zinc in the form of nanoparticles, rather than inorganic salt, caused an increase in the efficiency of biosynthesis of these compounds. [PERSON] and [PERSON] [22] reported that the addition of zinc in the form of ZnO nanoparticles to the culture medium resulted in a high content of carotenoids in the yeast cell biomass. After 72 h of cultivation, the volumetric yield of biosynthesis by _Rhodotorula toruloides_ MH023518 yeast strain increased from 124 mg/L (control medium) to 264 mg/L (medium containing 50 ppm ZnO). It is worth noting, however, that cultivation was carried out in the YPD model medium, which contains optimal nutrients. Nanoparticles generate cellular stress leading to the formationof reactive oxygen species (ROS). This, in turn, increases the biosynthesis of carotenoids to neutralize ROS [22]. The above-presented results prove that it is necessary to select the appropriate doses of zinc, as well as the ideal composition of the propagating medium, for each strain. If higher doses of zinc ions were used in this study, they would have stimulated carotenoid biosynthesis by the _R. mucilaginousa_ MK1 yeast strain. However, a high concentration of this element in the medium may significantly reduce the yeast growth rate and inhibit carotenoid biosynthesis. This was demonstrated in the study by [PERSON] et al. [23], who showed that after 120 h of cultivation of the yeast _R. glutinis_ CCY 020-002-033 in the medium supplemented with 500 mg/L zinc, the volumetric efficiency of \(\beta\)-carotene biosynthesis and the cellular biomass yield were 0.92 mg/L and 0.16 gd.m./L, respectively, while after cultivation in the control medium the values were 2.4 mg/L and 1.94 gd.m./L, respectively.