Insights of the precious "starter": indigo dyeing vat inoculated with muddy sediment from an older vat
The industrialization of traditional indigo dyeing is constrained by a time-consuming fermentation cycle and unstable initiation. Among the Li people of Hainan, a rapid fermentation technique (completed within 2 days) markedly shortens this process compared to the conventional method (7 days), crucially through the addition of muddy sediment from a mature dye vat as a starter culture. To elucidate the microbial mechanisms underlying this acceleration, we integrated amplicon and metagenomic sequencing to systematically compare the microbial communities, environmental parameters, and functional genes of the two fermentation processes. Our analysis revealed that the quick fermentation process immediately established a more favorable environment for reduction, characterized by a lower oxidation-reduction potential (ORP) and suitable pH. Its bacterial community displayed reduced diversity and greater stability, dominated by the Firmicutes, with core genera including Alkalibacterium, Corynebacterium, Amphibacillus, and Bacillus. These key functional taxa, along with genes associated with carbohydrate metabolism and electron transfer, were enriched in the muddy sediment inoculum. Furthermore, fungal abundance remained low during quick fermentation and was negatively correlated with the core bacterial genera. In conclusion, the muddy sediment functions as a natural inoculant that accelerates indigo reduction by delivering a functionally redundant microbial consortium, optimizing the initial fermentation conditions, and suppressing fungal proliferation. This study elucidates the ecological mechanism behind this traditional practice and provides a theoretical foundation for developing efficient and stable biological indigo reduction technologies.