## 4 Discussion  
Edible and medicinal mushrooms are good Se accumulators with high nutritional value and biological activity, and Se biofortification with mushrooms is an efficient strategy to enhance human Se intake. It is a promising application to obtain large mycelia yields or bioactive metabolites, via liquid culture technology, and then to develop them into various functional foods. Many studies have shown that Se can significantly affect mushroom growth and mycelium development [12; 22]. This study found that an appropriate amount of Se was favorable for _G. lucidum_ growth. During the liquid culture process, 50 ppm Na\({}_{2}\)SeO\({}_{3}\) was tested as the optimal concentration for growth, and it did not cause morphological changes in _G. lucidum_, but Na\({}_{2}\)SeO\({}_{3}\) at concentrations >100 ppm negatively affected mycelial growth. To satisfy human dietary Se requirements, the _G. lucidum_ mycelia cultured with 50 ppm selenite could be used as dietary supplements. According to the recommended daily intake of Se 60 \(\upmu\)g for adults [23], consuming 140 mg _G. lucidum_ mycelium per day will maintain human health and prevent diseases related to Se deficiency, there including 62.42 \(\upmu\)g SeMet and 12.88 \(\upmu\)g SeCys\({}_{2}\). Mycelium morphology and productivity are highly interlinked, and these associations have been investigated [24]. The _G. lucidum_ mycelia cell ultrastructures were damaged with 200 ppm Na\({}_{2}\)SeO\({}_{3}\) since the 5 th day, including the cell wall, cell membrane, and organelles. Carbon and nitrogen metabolism were seriously disturbed, further influencing the growth and bioactive metabolites, whereas 50 ppm Na\({}_{2}\)SeO\({}_{3}\) did not cause obvious morphological changes. Mushrooms can grow over a wide concentration range of Se, and their tolerance capacity to Se alters with mushroom species, culture conditions, Se sources, and doses. _Hericium erinaceum_ mycelia were also found to be degraded when cultivated with 100 ppm Na\({}_{2}\)SeO\({}_{3}\), disturbing the normal respiratory physiological process in cells. Further, 20 g/kg Selol was determined to contribute to the thickening of the cell wall, which implied an influence on polysaccharide production [25]. Thus, changes in mycelium morphology can be a direct indicator of Se stress to edible and medicinal mushrooms. Volatile substances are essential for the flavor of edible and medicinal mushrooms [26]. Investigations into the odor compositions are crucial for mushrooms, and volatile Se compounds are often neglected because they can easily be lost during sample analyses. Biomethylation is a bioprocess that plays an important role in mushroom Se metabolism and produces volatile Se compounds such as dimethyl selenide (DMSe) and dimethylSI selenide (DMDSe), which will lead to odor change. With the increase in Na\({}_{2}\)SeO\({}_{3}\) concentration, the types and contents of volatile Se compounds released by _G. lucidum_ in the liquid culture increased distinctly, ultimately resulting in a strong stimulating odor. As many as ten volatile Se compounds have been detected in _G. lucidum_, including dimethyl selenide and methyl diselenide, which were the main volatile Se metabolites produced by plants as a result of the phyto-volatilization or biomethylation of Se [27]. Volatile organic sulfur and Se compounds detected in bottled drinking water were reported to have swampy, rotten egg, sulfidic, cooked vegetable, and/or cabbage odors [28]. The production of volatile Se compounds is a self-detoxification mechanism that occurs under high Se conditions, and the concentration and toxicity of inorganic Se compounds can be decreased by the production of volatile Se compounds [29]. Understanding the profile of volatile Se compounds would contribute to disclosing the physiological status of mushrooms during the Se enrichment process, which will help to shed light on the mechanisms of Se toxicity and have implications for the Se biofortification.