• • Optimal Mg2+ dosage of 750 mg/L increased lactic acid yield to (37.4±0.5) g COD/L (1.2× Blank) and L-lactic acid optical activity to (96.3±0.9)%, directly improving product purity for polymer-grade applications.
• • Mg2+ addition significantly enhanced key hydrolytic enzyme activities (α-glucosidase, amylase, protease) and L-lactic acid producing enzymes, while reducing lactate-consuming enzyme activity, accelerating hydrolysis and production rates while slowing lactate degradation—critical for process efficiency.
• • At 750 mg/L Mg2+, the relative abundance of Enterococcus and Streptococcus reached 65.0% and 18.7% respectively (total 83.7%), reshaping the microbial community to favor L-lactic acid producers, which is essential for high optical purity.
• • Metabolic pathway prediction and functional gene analysis confirmed that Mg2+ upregulated carbohydrate metabolism pathways and lactate dehydrogenase encoding genes, providing a mechanistic basis for enhanced fermentation performance and guiding reactor design.