## 3 Results and Discussion
### Correlation Analysis of Core Microorganisms and Principal Metabolites  
Spearman correlation analysis was performed on 40 metabolites (including 20 non-volatile metabolites and 20 volatile metabolites, \(p\leq 0.05\), \(\mid\) Log\({}_{2}\)F\(\mid\) top 20) and 5 core genera  
Figure 5: Diversity and composition of the microbial community in YH and FHYH. (**A**) Principal coordination analysis (PCoA) for bacteria. (**B**) Relative abundance of bacteria at the phylum level. (**C**) Relative abundance of bacteria at the genus level. (**D**) The linear discriminant analysis effect size (LefSe) of LDA score distribution histogram for bacteria. (**E**) LefSe analysis of an evolutionary branching diagram for bacteria. (**F**) PCoA for fungi. (**G**) Relative abundance of fungi at the phylum level. (**H**) Relative abundance of fungi at the genus level. (**I**) LefSe of LDA score distribution histogram for fungi. (**J**) LefSe analysis of an evolutionary branching diagram for fungi. (_Aspergillus_, _Pullululululanibacillus_, _Bacillus_, _Pneibacillus_and _Basidiumycota_gen_Incertae_sedis_, \(p\leq 0.05\), Richness \(\geq 3\%\)) to understand the relationship between microbial communities, metabolites, and tea quality. As shown in Figure 6A, _Aspergillus_, _Pullululanibacillus_, and _Bacillus_ were negatively correlated with L-histidine, L-lysine, L-norleucine, L-phenylalanine, and L-valine. Since these amino acids are associated with a bitter taste, these microbes likely play a key role in improving the taste of black tea in summer. _Aspergillus_ reduces free amino acids through Maillard reaction and enzymatic breakdown [42]. [PERSON] et al. also revealed a negative correlation between L-phenylalanine and Aspergillus in green tea during stuck fermentation, and the period of stuck fermentation in this study (37 days) was similar to ours, which suggests that stuck fermentation is not only effective for black tea base in flavor improvement but may also be applicable to other tea types [18]. In fact, _Aspergillus_ is known to secrete multiple enzymes, including \(\alpha\)-amylase, glucoamylase, various cellulases, pectinase, xylanase and hemicellulose, proteases, and polyphenol oxidase [42]. _A. niger_ reduces acidity, bitterness, and astringency in unfermented tea through extracellular enzyme action, and promotes the sweet and mellow quality of fermented tea [42]. _Pneibacillus_also secretes a variety of enzymes, including amylase, cellulase, hemicellulose, lipase, pectinase, and lignin-modifying enzymes [43]. [PERSON] et al. found that _Bacillus_ can oxidize tea catechchins into theaflavins by producing pectinase and polyphenol oxidase, which reduces bitterness and brightens the color of tea soup [44]. The breakdown of cellulose and pectin increases the sugar content of tea soup, which is consistent with the significant positive correlation of _Penicillium_, _Basidiumycota_gen_Incertae_sedis_, _Pullululanibacillus_ and _Bacillus_ with various sugars (D-mannitol (CAS:69-65-8), D-sorbitol (CAS:50-70-4), dulticid (CAS:608-66-2)). [PERSON] et al. showed that the surface of tea leaves is covered with microorganisms during fermentation, which eventually disrupts the leaf cell structure [33]. _Penicillium_ also degrades cellulose to produce a small amount of free sugar, which lends sweetness to the tea soup. In addition, its secondary metabolite penicillin inhibits the growth of harmful bacteria [42]. These results suggest that microorganisms contribute significantly to the degradation of tea leaf cell walls. _Basidiumycota_gen_Incertae_sedis_, a genus of phylum Basidiumycota, may be involved in the production of various metabolites (D-mannitol, D-sorbitol, dulticid, guanine (CAS:73-40-5), 5-hydroxymethylfurfural (CAS:67-47-0), 3-methyl-2-oxobutanoic acid (CAS:759-05-7) in tea, although only a few studies have reported on this species. Overall, our findings suggest that microbial activity may play a significant role in metabolite conversion, reduced bitterness, and increased sweetness in stuck-fermented summer black tea.