Abstract: In this study, amplicon sequencing was used to characterize the community structure of rare microbial groups in Nongxiangxing Daqu, null model analysis was applied to elucidate the assembly mechanisms of the microbial community, and co-occurrence network analysis was employed to evaluate the influence of rare microbial groups on the overall microbial community structure of Daqu. The results showed that 83 rare bacterial taxa and 65 rare fungal taxa were detected. Null model analysis indicated that the succession of rare microbial groups was overall dominated by stochastic processes, whereas rare bacterial communities were significantly influenced by deterministic processes during the early and late stages of fermentation. Meanwhile, rare microbial groups exhibited greater phylogenetic distances and broader niche breadths than abundant microbial groups, suggesting that they possessed stronger adaptability to environmental changes. Co-occurrence network analysis further revealed that some rare taxa occupied key positions in the network and might contribute to the maintenance of community structure through synergistic interactions with abundant taxa. After the removal of 11 key rare genera, the complexity and connectivity of the microbial co-occurrence network decreased, whereas module separation increased, indicating that key rare microorganisms play an important role in maintaining the structural integrity of the network. These findings provide fundamental information for understanding the roles of rare microbial groups during Daqu fermentation and may contribute to the regulation of microbial growth and metabolism in Nongxiangxing Daqu fermentation.

Key words: rare microbial groups; Nongxiangxing Daqu; community composition; null model; succession mechanism; co-occurrence network