Fu brick tea (FBT) is one of the major brands of dark tea. Microbial fermentation is considered the key step in the development of the special characteristics of FBT. The systemic corelationship of the microbiome and metabolomics during manufacture of Fu brick tea is not fully understood. In this study, we comprehensively explored the microbiome and metabolite dynamic evolution during the FBT manufacturing processes, and revealed decisive factors for the quality and safety of FBT based on the grouped methods of metabolomics combined with biochemical measurements, microbiome sequencing combined with quantitative polymerase chain reaction (PCR), and multiplex analysis. Both the microbiome and quantitative PCR showed that fungi displayed concentrated distribution characteristics in the primary dark tea samples, while bacterial richness increased during the flowering processes and ripening period. All microorganism species, as well as dominant fungi and bacteria, were identified in the distinct processes periods. A total of 178 metabolites were identified, and 34 of them were characterized as critical metabolites responsible for metabolic changes caused by the corresponding processes. Metabolic analysis showed that most metabolites were decreased during the FBT manufacturing processes, with the exception of gallic acid. Multivariate analysis verified that the critical metabolites were correlated with specific dominant microbial species. All the top fungal species except unclassified_g_ Aspergillus showed positive correlations with six critical metabolites (L-The, epigallocatechin (EGC), Gln, tea polyphenol (TP), tea polysaccharides (TPs) and caffeine). Five of the top bacteria species (Cronobacter, Klebsiella, Pantoea, Pluralibacter, and unclassified_ f_Entero-bacteriaceae) showed positive correlations with epigallocatechins and tea polyphenols, while the other 11 top bacterial species correlated negatively with all the critical metabolites. The content of amino acids, tea polyphenols, tea polysaccharides, and flavonoids was reduced during microbial fermentation. In conclusion, our results reveal that microbial composition is the critical factor in changing the metabolic profile of FBT. This discovery provides a theoretical basis for improving the quality of FBT and enhancing its safety.

The sensory quality and health benefits of Pu-erh tea are mainly determined by microbial fermentation processing. The directed exogenous inoculation of specific microorganisms is an effective method to improve the quality and flavor of Pu-erh tea. In this study, Lactobacillus plantarum and Saccharomyces cerevisiae were introduced into the fermentation processes of Pu-erh tea, as they are the main contributors to enzyme secretion, to change the tea’s functional components. The raw tea materials, spontaneous fermentation tea and microbiological fermentation tea were analyzed by microbiomics and metabolomics. A total of 248 metabolites were characterized, 71 of which were identified as essential metabolites involved in the metabolic changes. These essential metabolites were produced by specific dominant microbial species with multivariate analysis methods. Metabolites essential to the sensory quality and health benefits of Pu-erh tea, such as flavonoids and free amino acids, were increased in tea samples inoculated with Lactobacillus plantarum and Saccharomyces cerevisiae following fermentation. Fungal diversity decreased after fermentation, and both the diversity and richness of bacteria were significantly decreased. In conclusion, our results demonstrate the advantages of Lactobacillus plantarum and Saccharomyces cerevisiae in forming the unique sensory characteristics of Pu-erh tea, and they indicate that the microbial composition is a key factor in altering the tea’s metabolic profile. Our work establishes a theoretical foundation for the promotion of the safety and quality of Pu-erh tea through exogenous inoculation with Lactobacillus plantarum and Saccharomyces cerevisiae.

Nickel is a widespread environmental pollutant originating from industrial activities, and its pollution problem persists globally. Currently, research on nickel ion metabolism and detoxification mechanisms primarily focuses on prokaryotes, while the molecular mechanisms of nickel detoxification in eukaryotes remain underexplored. In this study, Saccharomyces cerevisiae was used as a eukaryotic model to demonstrate that the plasma membrane-localized chloride transporter Gef1p participates in nickel ion metabolism. The GEF1 knockout strain (gef1Δ) showed strong resistance to excess nickel ions, and the content of nickel ions in gef1Δ cells was significantly elevated. The results of transcriptomics analysis showed significant upregulation of MMT2 and CUP1 in gef1Δ cells supplemented with nickel. Both MMT2 and CUP1 overexpressed in the gef1Δ strain showed a growth advantage on nickel media. Nickel ion content in the mitochondria of cells overexpressing MMT2 was significantly elevated, and the levels of reactive oxygen species were significantly decreased in strains overexpressing either the MMT2 or CUP1 genes. This study reveals that the GEF1 gene plays an important role in nickel resistance, and that upregulation of MMT2 and CUP1 is critical for gef1Δ strains to counteract ROS formation and growth defect by nickel ions. The high intracellular accumulation of nickel ions caused by gef1Δ during the process of resisting nickel toxicity makes it a strong candidate for ecological remediation of nickel ion pollution.

Nickel is a trace element essential for human life. Currently, studies on the metabolism and function of nickel ions mainly focus on prokaryotes, while the role of nickel ions in eukaryotes is still being explored. In this study, we found for the first time that GEF1 which encodes Gef1p, a chloride transport protein located in the Golgi membrane, is involved in nickel ion metabolism in yeast. The GEF1 knockout strain (gef1∆) showed strong resistance to excess nickel ions, and the content of nickel ions in gef1∆ cells was significantly elevated. The results of transcriptomics analysis showed significant upregulation of MMT2 and CUP1 in gef1∆ cells supplemented with nickel. Both MMT2 and CUP1 overexpressed in the gef1∆ strain showed a growth advantage on nickel media. Nickel ion content in the mitochondria of cells overexpressing MMT2 was significantly elevated, and the levels of reactive oxygen species were significantly decreased in strains overexpressing either the MMT2 or CUP1 genes. This study reveals that the GEF1 gene plays an important role in nickel homeostasis, and that upregulation of MMT2 and CUP1 is critical for gef1∆ strains to counteract ROS formation and growth defect by nickel ions.