## 3 Results and Discussion
### Stuck Fermentation Altered the Microbial Diversity in Summer Black Tea  
The changes in the microbial community of summer black tea following stuck fermentation were analyzed by 16S rRNA and ITS sequencing. The sequence quality of 16S rRNA and ITS sequencing was excellent, with Q20 greater than 99% and Q30 greater than 97% for all samples (Tables S6 and S7). Shannon and Simpson indices of bacterial (Shannon index: 5.36 to 7.86; Simpson index: 0.85 to 0.97) and fungal (Shannon index: 2.86 to 7.00; Simpson index: 0.53 to 0.97) communities were higher in FHYH compared to YH (Figure S6). The separation of the two groups of samples was evident in the principal coordination analysis (PCoA), especially that of the fungal community, indicating that the microbial community was altered by stuck fermentation (Figure 5A,F). At the phylum level, the abundance of Firmicutes, Proteobacteria, Acidobacteriota, Basidiomycota, and Mucoromycota was significantly higher in FHYH compared to YH samples (Figure 5B,G). In addition, the genera _Pullulanibacillus, Bacillus, Paneibacillus, Colonella, Massilia, Aspergillus, and Penicillinwere more abundant in the FHYH samples (Figure 5C,H). [PERSON] et al. found an increase in the fungal biodiversity of Tibetan tea after stuck fermentation [40], while [PERSON] et al. also concluded that there was a significant increase in the bacterial diversity of Fu brick tea at the later stages of fermentation [41], which is consistent with our research. However, both studies also reported that the microbial community diversity was not always in an increasing state throughout the microbial fermentation process, but was a dynamic change. It should also be noted that the fungal diversity of Fu brick tea decreased in the final stage of fermentation, which may be due to the fact that Fu brick tea needs to be inoculated with _Aspergillus cristatus_ before fermentation, which gradually became the dominant microorganisms in the later stage, while we adopted the method of natural microbial fermentation and did not artificially add any strains of microorganisms, relying on microorganisms in the environment, which resulted in the growth and multiplication of microorganisms, and the diversity of the microorganisms increased. Of course, it is also this increase in microbial abundance that facilitates the isolation and sieving of microorganisms that potentially contribute to tea quality in our subsequent studies. In addition, we identified key microorganisms in YH and FHYH by linear discriminant analysis of effect size (LefSe), with LDA > 4 as the threshold. 16 bacterial and 10 fungal genera were screened as biomarkers in YH, including _Bortysphagia_, _Setophoma_, _Bacteroides_, _Alloprerotella_ and _Paraperoetlla_, and 17 bacterial and 29 fungal genera were the key microbes in FHYH, including _Aspergillus_, _Penicillium_, _Cladophialophora_, _Cumella_, _Paenibacillus_, _Bacillus_ and _Pullululanibacillus_ among others (Figure 5D,I). The evolutionary relationships between these microorganisms were assessed through a cladogram, which showed that the key microbial communities in YH were predominantly of the orders Bacteroidales, Pleosporales and Botryosphaeriales, while those in FHYH were Bacillus, Venturiales, Chateothyriales, Eurotiales and Trechisporales (Figure 5E,J). Taken together, stuck-fermutation significantly increased the microbial richness and diversity of summer black tea.