The yeasts submitted to Treatment T2 (60 µg mL− 1 of sodium selenite) grew within the estimated time of 24 to 48 hours, without the need for cell adaptation of the yeasts to the Se; therefore, the maximum concentration of yeast (S. cerevisiae), strain Y904, supports without the adaptive process. Similar results were found in the works of Assunção (2011) and Rajasheree et al. (2013), using the same species of yeast, but from another strain.
According to Assunção (2011), the objective was to evaluate the inhibitory effect of 0.0 concentrations; 5.6; 34.8; 49.7 and 94.0 µg mL− 1 of sodium selenite in YEPDA in the growth of the yeast S. cerevisiae EVN 166, in 24 h. As a result, the colony-forming unit (CFU mL− 1) was the same for all Se concentrations after 24 h of growth; however, a slightly lower value of CFU mL− 1 was observed for the 94.0 µg mL− 1 of sodium selenite.
The study by Rajashree et al. (2013), aimed to analyze the toxicity of Se (0, 10, 20, 30, 40, 50, 75, 100, 125, 150 µg mL− 1 of sodium selenite) in yeast cells S. cerevisiae NCYC 1026 and the effects on biomass production, in sterile Sabouraud Dextrose, under aseptic conditions and incubation for 72 hours at 30˚C. As a result, it obtained the highest concentration of 75 µg mL− 1 with no change in biomass production, and 50 µg mL− 1 taking into account the bioaccumulation of Se by the cell.
According to the studies by Nagodawithana et al., Kaur et al. (2006), Stabnikova et al. (2008) and Marinescu et al. (2011), the decrease in the number of yeast cells is directly related to the increase in the concentration of sodium selenite in the culture medium, because the higher amounts of sodium selenite in the culture medium has a strong inhibitory effect on growth of yeast. Explained by Kieliszek et al. (2019a), the slowdown in yeast growth may be a result of the occurrence of oxidative stress caused by the presence of high concentrations of Se in the culture medium, which can lead to another phenomenon called the level of lipid peroxidation.
With the increase in the concentration of sodium selenite, the yeasts showed an intense reddish brown color. Changes in colony odor were also observed. It was similar to the smell of hydrogen sulfide. According to Suhajda et al. (2000), this may occur due to the substitution of Sulfur by Se in the enzymes. The Se compounds follow the same metabolic pathways and the metabolites are analogous to that of S. These changes in cell staining can be explained by the biotransformation that happens inside the cell, when the selenite (transparent coloring) is reduced to Se amorphous (reddish coloring) (KONETZKA 1977).
In the work of Bierla et al. (2013), the substitution and degree of substitution of sulfur for Se in methionine and cysteine, in Se-rich yeasts, using plasma mass spectrometry inductively coupled by capillary HPLC (ICP-MS), used in parallel to capHPLC- ESI-MS was investigated. As a result, substitution of cysteine sulfur is three times less frequent than that of methionine sulfur. Taking into account the amounts of methionine and cysteine available in yeast cells, they concluded that the estimate of selenocysteine concentration in Se-rich yeasts was 15 to 30% of selenomethionine.
Birringer et al. (2002) demonstrated that the small amounts of organic Se compounds in plants, yeasts, bacteria or animals are isologists of sulfur compounds and the enzymes involved in the metabolism and trans-sulfurization pathway do not normally discriminate between sulfur and Se compounds.
Also according to Birringer et al. (2002), Se causes morphological changes in yeast, possibly altering the structure of the cell wall and membrane complex. In the work by Kieliszek et al. (2019b), Se supplementation increased the participation of unsaturated acids, such as linoleic acid and linolenic acid, in the biomass of Candida utilis ATCC 9950 and S. cerevisiae MYA-2200. As the biosynthesis of these acids may be associated with increased desaturase activity and lipid peroxidation, these processes may be directly linked to changes in yeast cell morphology. Some changes already mentioned in the literature are: increase in the size of cells, shrinkage of yeasts, thickening of the cytoplasm or changes in the structure of the vacuole (Kieliszek 2016).
The morphological change in yeasts according to the increase in the concentration of sodium selenite was also found in the work of Rajashree et al. (2013), where the control group without the addition of sodium selenite had yeast cells with a smooth edge surface, while the treatments with sodium selenite acquired roughness on the surface. Damage to the cell wall was observed in yeast cells, which was caused by high concentrations of sodium selenite, resulting in a reduction in the number of yeast cells. According to Kieliszek et al. (2016), the addition of sodium selenite (salt) to the medium causes osmotic stress in the yeast cells, and as a response, the formation of grooves in the cell wall and consequently the wrinkling occurs, thus being the identified roughness.
The results of the optical microscopy images (Fig. 7) corroborate with those observed in the study conducted by Kieliszek et al. (2016), where the analysis of microscopic images of yeasts of the species C. utilis TCC 9950, demonstrated that the concentrations of 20, 30 and 40 µg mL− 1 of sodium selenite in the substrate caused a significant increase in size and cross-sectional area of cells due to agglomerations and vacuoles among them, when compared to cells without the addition of Se. Comparing with other similar results, Rajashree et al. (2013) identified the change in the surface of the yeasts by means of SEM, observing that the smooth surface of S. cerevisiae without the addition of sodium selenite, contrasted yeast cells with rough surfaces when grown in a medium enriched with 50 µg mL− 1 of sodium selenite, while those grown in 100 µg mL− 1 were partially damaged (with small cracks).
Despite being of the same species, S. cerevisiae, and having the ability to bioaccumulate several elements and, as a result, tolerate higher concentrations in the medium, the different strains show different behaviors to stress and specific variations. There are studies with yeasts of the same species that tolerate different concentrations of Se (WHITE 1987). As in the results obtained by Wang et al. (2010), the addition of 90 µg mL− 1 in the late exponential development phase of yeast of lineage GS2, was the highest tolerated concentration taking into account the decrease in biomass production.
In the present work, the adaptive process allowed the yeast to tolerate up to 246 µg mL− 1 of sodium selenite in the culture medium. Due to the gradual increase of Se in the medium and the metabolic interactions caused by Se, the yeasts presented different characteristics, such as, the reduction of the cell multiplication speed, roughness, intense hydrogen sulfide odor and increase in the intensity of the reddish brown color. Such results were similar to those cited in the literature. Suhajda et al. (2000), enriched S. cerevisiae also using sodium selenite, obtained a reddish color in the yeast cells.
The adaptive ability of yeast can be acquired when exposed to the appropriate selection pressure (WHITE 1987). According to Bronzetti et al. (2001), the addition of high concentrations of sodium selenite to the culture medium demonstrated to have a mutagenic effect in the yeast cells of the species S. cerevisiae D7, generating a 70% decrease in cell survival when compared to the control group.
The change in all these characteristics (change in color, roughness, odor, size and number of cells) with the increase in the concentration of sodium selenite, demonstrates how the yeasts have undergone modifications and / or adaptations in order to survive the stress caused by the enrichment of the culture medium with Se, which may induce to believe that adaptive evolution of yeast cells has occurred.