## 1 Introduction  
_Baijiu_, a traditional fermented alcoholic beverage, is very popular in China. As is the case with all naturally fermented food products, changes in the microbiome composition during fermentation are essential for the development and final quality of _Baijiu_[1; 2]. Therefore, it is important to elucidate the structure and metabolic function of the microbiome so as to better understand the mechanisms underlying fermentation. Moreover, a comprehensive understanding of food microbiota may help elucidate microbial ecology and evolution in more natural, complex ecosystems [3; 4; 5]. Thus, there is increasing interest in uncovering the characteristics of _Baijiu_-making microbiota and revealing the important effects of changes in microbiota on _Baijiu_ fermentation [1; 6].  
Chinese _Baijiu_ can be classified into different categories based on their flavor profiles, in which sauce-flavor _Baijiu_, strong-flavor _Baijiu_, and light-flavor _Baijiu_ comprise the three dominating categories [6], Light-flavor _Baijiu_ is famous for its pure, pleasantly fruity, and mild taste, and for its refreshing aftertaste [7], which have made it the predominant type of _Baijiu_ in the Chinese liquor market [1; 8]. Light-flavor _Baijiu_ is produced in an open solid-state fermentation process. Typically, sorghum is used as raw material to which low-temperature _Daqu_, the fermentation starter for Chinese _Baijiu_, is added as culture starter, and fermentation is allowed to occur in large earthquake containers. Fermented grain is subsequently distilled to produce _Baijiu_[6]. During light-flavor _Baijiu_ fermentation, microorganisms originating from _Daqu_ and from the environment, such as _Bacillus_, _Lactobacillus_, _Pediococcus_, _Saccharomyces_, _Saccharomyces_, _Pichia_, _Wickerhamomyces_, and _Apergillus_, were found to be predominant [9; 10].  
In previous studies, culture-dependent and -independent methods were used to describe the microbial community in _Daqu_ and during the fermentation process of light-flavor _Baijiu_[9; 10; 11; 12]. The functions of particular species characterized in a simulated environment differed substantially from those of species in situ [13; 14]. In the past few decades, high-throughput amplicon sequencing has helped to uncover and analyze additional microorganisms that participate in _Baijiu_ fermentation [6; 8; 9; 15]. However, amplicon sequencing only provides limited information on microbial metabolic functions [16], whereas shotgun metagenomic sequencing approaches offer a higher resolution regarding taxonomic annotation, circumvent amplification biases, and provide a higher taxonomic accuracy at the species level [17; 18]. Moreover, this method facilitates elucidating the functional distribution of microbiota through non-selective genomic assembly followed by gene annotation [19; 20]. This approach has thus been applied in a few studies to reveal potential metabolic functions in fermented food products, including sausage [21], milk Kefirs [18], pu-erh tea [22], and various cheeses [23; 24].  
In the current study, shotgun metagenomic sequencing, metabolomic analyses, and gene annotation were used to investigate the microbial and metabolic characteristics during light-flavor _Baijiu_ fermentation and further to assess the mechanistic correlations between microbial community composition and fermentation functions. The key microorganisms of _Baijiu_ fermentation and potential metabolic functions of microorganisms were identified. Our results may help increase the production efficiency and improve the quality and flavor of the final product.