Metabolic responses and antioxidant mechanisms of Aspergillus cristatus to NaCl-induced osmotic stress in small-leaved Kuding tea media

During the fermentation of small-leaved Kuding tea by Aspergillus cristatus, significant fluctuations in environmental osmotic pressure can affect the growth and metabolism of A. cristatus, thereby influencing the quality of the fermented tea. To investigate osmotic pressure effects on its metabolism, this study used NaCl (0%, 4%, 8% and 12%) in small-leaved Kuding tea medium to establish osmotic gradients, followed by morphological observation, in vitro antioxidant activity assays, and liquid chromatography‒mass spectrometry (LC‒MS) nontargeted metabolomics analysis. The results demonstrated that the NaCl concentration had significant effects on colony morphology and metabolites. The colony diameters and antioxidant activities were significantly greater at 4% and 8% NaCl than at 0% or 12% NaCl. Nontargeted metabolomics identified 556 differential metabolites, 93 of which common differential metabolites covered seven categories, such as amino acids, organic acids and flavonoids. Under 4% NaCl, key antioxidants (melatonin, erythritol, and luteolin) significantly accumulated, which correlated with the activation of the core metabolic pathways, including tryptophan metabolism and flavonoid biosynthesis. Hypertonic stress (12% NaCl) suppressed these core pathways, leading to the induction of repair metabolites (e.g., methyl jasmonate). Low salt (0% NaCl) also inhibited core pathways, leading to a reduction in broad-spectrum antioxidant capacity and an accumulation of specific metabolites (e.g., xanthoxylin). Aspergillus cristatus dynamically regulates metabolism to adapt to different salt concentrations, with 4%-8% NaCl optimal for achieving maximal growth-antioxidant defense equilibrium in small-leaved Kuding tea. These findings provide a foundation for optimizing fermented tea processes and high-antioxidant products.