In previous text main groups of biotechnologically important metabolites used for enrichment of red yeast biomass were described. Mainly carotenoids, ergosterol, lipids and metal accumulation in red yeast cells makes them attractive for industrial applications.

Ergosterol is provitamin D, part of was followed partly as the additional parameter of biomass quality and also to monitor the competition of two specialized branches of isoprenoid pathway, which is used for the biosynthesis of both carotenoids and sterols. The production of ergosterol was very similar to the production of P-carotene, even if these metabolites were formed in competitive branches of isoprenoid metabolic pathway (Marova et al., 2010). Practically simultaneous oscillation in carotenoid and ergosterol production under optimal conditions could be caused by the role of both metabolites in R. glutinis stress response. Carotenoids act as antioxidants and may prevent cells or cell membranes against negative effects of increased oxidative stress. Ergosterol is an integral component of yeast cell membranes, which are very sensitive to external stress. Recently it has been found that the major changes in intact cells of red yeast Rhodotorula minuta irradiated by UV-B were interpreted as combination of changes observed in the cell wall and membrane, the changes observed in the membrane preparations were attributed to ergosterol (Tan et al., 2003). Ergosterol is a precursor of Vitamin D2 and it is also used for the production of cortisone (Metzler 2003). Now ergosterol as single product is commercially produced by yeast fermentation using Saccharomyces cerevisiae strains. The popular means to improve the ergosterol fermentation are optimization of the culture medium, screening of the high ergosterol producing strains. Different carbon sources, nitrogen sources and other nutrient materials had different influences on cell growth and accumulation of ergosterol in yeast biomass. A new yeast strain, obtained by way of protoplast fusion, increased the biomass to 2.45 g/100 ml (dry cell weight) and the ergosterol content to 3.07% (Frengova a Beshkova, 2009). It was reported that the synthesis of ergosterol was not determined by cell growth but by the oxygen consumption rate. Ethanol was formed in yeast fermentation and it had an obvious influence on the growth of yeast. In yeast culture process, glucose is preferred and when the glucose concentration reaches a low value, the cell growth is confined. Then after a short period of adaption, cells continue to grow by consuming the ethanol produced in the first phase as the carbon source. The whole process appeared to be a two-phase process. The ergosterol content increased when the specific growth rate decreased. The environmental and physiological parameters such as the dissolved oxygen, oxygen uptake rate of yeast cells culture had direct or indirect influences on the accumulation of ergosterol and the growth of yeast cells. The interaction relation might help to optimize the ergosterol fermentation. But until now little work has been reported on this relation (Tan et al., 2003). Carotenoids are important natural pigments that play an essential role as accessory light­harvesting pigments and, especially, in protection against damage by photosensitized oxidation. Several yeast genera—Rhodotorula, Sporobolomyces, Rhodosporidium, and

Cryptococcus — produce also coenzyme Q10 (CoQ10; Dimitrova et al., 2010). CoQ10 has a similar isoprenoid chain in its structure. It is also an interesting product for biotechnology. CoQ10 is present in all cells and membranes, and in addition to being a member of the mitochondrial respiratory chain, it also has several other functions of great importance for the cellular metabolism, such as participation in the extra-mitochondrial electron transport (plasma membranes and lysosomes), regulation of the mitochondrial permeability of transition pores, and regulation of the physicochemical properties of membranes. CoQ10, especially, is widely used as an essential component of ATP generation in the oxidative phosphorylation process and as an antioxidant preventing lipid peroxidation and scavenging superoxide. It has been proved that yeast CoQ10 is much better absorbed by the skin than the synthetic CoQ10. Peroxide reduction in the stratus corneum is considerably more pronounced after yeast CoQ10 application. Therefore, research efforts on the production of CoQ10 by microorganisms focus on the development of potent strains by conventional mutagenesis and metabolic engineering, analysis and modification of the key metabolic pathways, and optimization of fermentation strategies. Various microorganisms, including bacteria (e. g., Agrobacterium, Rhodobacter) and yeasts (e. g., Candida, Rhodotorula, and Saitoella), are reported as CoQ10 producers in patented laid-open applications purposely applied in pharmaceutical and cosmetic industry (Dimitrova et al., 2010, Yurkov et al., 2008).

Strains of basidiomycetous yeasts isolated from different sources were studied in order to determine the content of carotenoid pigments and ubiquinone Q10 for subsequent selection work to obtain producers of these substances. The high specific productivity of carotenoids (600-700 mg/ g) was revealed in the representatives of the following species: Cystofilobasidium capitatum, Rhodosporidium diobovatum, R. sphaerocarpum, Rhodotorula glutinis, Rhodotorula minuta, and Sporobolomyces roseus. The ratio of the major pigments (torulene, torularhodine and P-carotene) in the representatives of different species was studied. Certain specific features of pigment formation in relation to the taxonomic position of the yeasts were determined. Eurybiont species with substantial ecological lability are the most active producers of carotenoids and ubiquinone Q10 among the epiphytes. It is the first time a comparative analysis of the coenzyme Q10 content in different taxa has been performed using several strains of the same species. The maximal coenzyme Q10 production (1.84 mg/g of dry biomass) was found in the yeast species R. sphaerocarpum (Yurkov et al., 2008).