## 1 Introduction  
Selenium (Se), an essential micronutrient for humans, participates in a series of cellular metabolic processes and possesses vital biological functions, specifically with potent antioxidant, anticancer, anti-inflammatory, and antiviral properties, among others [1, 2]. Plants and microorganisms have been developed as major Se biofortification sources and are considered as dominant functional foods or food ingredients of daily Se supplementation. Se-enriched arable crops, vegetable crops, and Se-yeast and Se-algae are presently recommended as the primary sources of dietary Se for humans [3, 4, 5].  
Edible and medicinal mushrooms are widely consumed as favorable food supplements because they are rich in polysaccharides, proteins, and other nutrients, thereby having multiple nutritive values [6]. The production of functional mushrooms by Se biofortification is an effective way to improve the demand for Se in humans due to their strong capacity to uptake and accumulate this element [7]. Mushrooms can transform inorganic Se into organic Se by incorporating it into active macromolecules during metabolism [8]. The bioaccumulation of Se and their bioactive metabolites is associated with elevated nutraceutical properties in many mushroom species, such as antimicrobial and antioxidant activities in _Coriolus versicolor_[9], antioxidant activity and flavor properties in _Pleutotus eryngii_[10], and immunomodulatory effects in _Grifola fondosa_[11].  
Current research on Se-enriched mushrooms has mainly centered on fruiting body cultivation by adding inorganic Se compounds to solid substrates, further investigating Se metabolite structural characterizations and the potential bioactivities. Se enrichment ability varies among mushroom species, culture conditions, Se sources, and doses. _C. versicolor_ biomass decreases much more with Na\({}_{2}\)SeO\({}_{3}\) than with selenourea addition, and selenourea has less impact and lower toxicity on mycelium growth than Na\({}_{2}\)SeO\({}_{3}\)[9]. However, Se concentrations and forms significantly affect the _Hericium erinaceus_ mycelium and fruiting body growth, and both respond better to inorganic forms [12]. The growth rate of _Flammulina veluipes_ is suppressed at lower selenite concentrations in solid and static cultivation as compared to that with shaking cultivation [13].  
In comparison to solid cultivation, liquid culture has a shorter production cycle and higher production benefit [14]. It is a promising application direction to obtain large mycelium yields or bioactive metabolites via liquid culture technology, which could be developed into various functional foods in the food industry. However, few studies have explored the influence of Se on mushroom mycelia during liquid culture.  
_Ganderna lucidum_, a traditional valuable source of nutrients for centuries, has a high amount of polysaccharide and is considered functional due to its anticancer, antioxidant, and anti-inflammatory activities [15; 16]. This study used _G. lucidum_ as the research object and compared the growth and bioactive metabolites alterations in mycelia cultured with different concentrations of selenite, providing evidence in terms of pellet morphology, cell structure components, and odor change. The total Se content, selenoamino acids, and polysaccharide were also analyzed to define variations in the major bioactive metabolites. This study provides comprehensive insights into the influence of Se on G. _lucidum_ mycelia during the liquid culture process, and is useful for the production of Se-enriched mushrooms with high nutritive values.