Ectoine Mitigates Stress and Enhances Lignin to Vanillin Bioconversion in Halalkalibacterium ligniniphilum L1

Engineered microbial systems offer a route for converting lignin into value‐added aromatics, yet lignin‐derived intermediates are cytotoxic and suppress growth and metabolic flux. Here, we show that ectoine, a compatible solute, mitigates lignin‐induced stress in Halalkalibacterium ligniniphilum L1 and improves bioconversion performance. At 0.1 g L −1 , ectoine alleviated surface damage, improved chemical oxygen demand (COD) removal from about 40% to about 55%, and increased decolorization from about 25% to about 48%–50% over 7 days. Assays using crude extracts from recombinant Escherichia coli expressing individual L1‐derived enzymes showed that ectoine was associated with broader apparent pH and temperature operating ranges, up to a 12.3% point increase in fixed time substrate conversion, and improved time‐dependent retention of measurable activity. Transcriptomic analysis identified 24 differentially expressed genes (9 upregulated and 15 downregulated; false discovery rate (FDR) < 0.05, |log 2 fold change (log 2 FC) | ≥ 1), indicating a focused rather than global response. Ectoine‐responsive changes were associated with membrane/envelope‐related functions, transport, and oxidoreductase‐related processes, whereas motility and generalized stress‐associated programs were reduced. In a 5‐L bioreactor, ectoine increased vanillin titers by 13.9%–23.0% at lignin concentrations of 20–40 g L −1 . These results support ectoine as an additive for lignin bioconversion under stress.