## 4 Discussion
It remains to be determined whether these volatile Se compounds are common metabolites in other mushroom species cultured with Se. As one of the most important bioactive metabolites of _G. lucidum_, polysaccharide synthesis was greatly influenced by Na\({}_{2}\)SeO\({}_{3}\) addition, especially under high Se dose conditions. With the increase in Na\({}_{2}\)SeO\({}_{3}\) concentrations, polysaccharide production was increased followed by a decrease. Specifically, 50 ppm Na\({}_{2}\)SeO\({}_{3}\) promoted polysaccharide synthesis, but 200 ppm Na\({}_{2}\)SeO\({}_{3}\) significantly inhibited its synthesis after the 5 th day, which was associated with cell degradation. This result was consistent with findings that10 \(\mu\)g/mL Na\({}_{2}\)SeO\({}_{3}\) promoted the production of _Lentinula edodes_ mycelium exopolysaccharideac-charide (EPS), whereas 20 \(\mu\)g/mL Na\({}_{2}\)SeO\({}_{3}\) reduced EPS [30]. The addition of 7 mg/L Na\({}_{2}\)SeO\({}_{3}\) enhanced polysaccharide biosynthesis in _Cordyceps militaris_, and the key enzymes PGM, PGL, and UGP involved in polysaccharide synthesis were found with elevated gene expression in the Se-enriched group [31]. Presently, there have been numerous studies aiming to improve mushroom polysaccharide yield, with a 62.50% increment in _G. lucidum_ after the addition of L-phenylalanine [17]. Hence, low concentrations of Se addition in the liquid culture is a viable strategy, which not only positively affects the polysaccharide yield but also increases the active substances of organic Se. Organic Se is more bioavailable or bioactive than inorganic Se in humans. SeMet is more bioaccessible than most other organic Se compounds such as SeCys\({}_{2}\)[32]. The Se sources and doses supplied in the media affected the selenoamino acid composition and proportion. In addition, different mushrooms have diverse selenoamino acid types. SeMet and MeSeCys reached their maximum values when irrigated with 20 mg Se/L, whereas 10 mg Se/L was required for SeCys in _Agaricus bisporus_ fruiting bodies [33]. Studies have also shown that mushrooms such as _Pleurotus ostreatus_[34] and _L. edodes_[35], given Se fortification, yield good bioaccessibility in the form of SeMet. Moreover, MeSeCys was identified in _H. erinaceus_[12], _Auriclaria auricular_[7], and _P. ostreatus_[34] fruiting bodies. Here, 50 ppm Na\({}_{2}\)SeO\({}_{3}\) was beneficial for SeCys\({}_{2}\) and SeMet production throughout the whole process, whereas the content of SeCys\({}_{2}\) and SeMet decreased in _G. lucidum_ cultured with 200 ppm Na\({}_{2}\)SeO\({}_{3}\). Tolerance to high Se in Se-hyperaccumulating plants is correlated with the ability to biosynthesize MeSeCys, an amino acid derivative that is not incorporated into proteins [36]. The same phenomenon was observed for _G. lucidum_; MeSeCys content increased with the addition of 200 ppm Na\({}_{2}\)SeO\({}_{3}\), which was produced to alleviate the negative effects. _G. lucidum_ mycelia were bright red, especially under high Se conditions, and it was speculated that they could convert Na\({}_{2}\)SeO\({}_{3}\) into SeNPs. Similarly, _L. edodes_ has also been reported to transform Se into SeNPs, and SeNPs are localized inside the mycelial hyphae [37]. SeNPs have been used in the maintenance of human health [38]. They can not only be absorbed and used by humans but also have unique functions, such as antioxidant, immune-regulation, anti-cancer, and antibacterial activities. SeNPs produced by microorganisms are often characterized by low toxicity and good stability, and they form Se\({}^{0}\) aggregates alone or in combination with exopolysaccharideacharides and proteins [39; 40]. The transition of mycelia color from white to red was the starting point of notable changes in cells and metabolites, which was also the most intuitive embodiment of complex changes in cells. Mycelial growth and morphology can serve as indicators of Se stress in mushrooms. The incorporation of Se into the mycelia caused variations in morphology and growth in a manner dependent on the Se dose.