Process optimization of a halotolerant yeast Candida tropicalis ZQ for efficient azo dye biodegradation under high salinity

A novel halotolerant yeast, Candida tropicalis ZQ, isolated from marine sediment, was evaluated for its potential in azo dye biodegradation under high-salt conditions. The optimization of culture parameters was performed using the single-factor experiment. The following optimal settings were established: sucrose 2.0 g/L, (NH4)2SO4 0.1 g/L, yeast extract 0.06 g/L, pH 6.0, 35 °C, ≥160 rpm. In the aforementioned conditions, the strain exhibited >96% decolorization of 1200.0 µmol/L Acid Red 3 R within 12 h. It is noteworthy that it maintained high activity (>97.0% within 32 h) even at 100.0 g/L NaCl. Intracellular enzymes, including NADH‑DCIP reductase, lignin peroxidase, manganese peroxidase and laccase, were identified as key catalysts in this process. Acute toxicity assays confirmed substantial reduction in acute toxicity of the metabolites. Integrative genomic and transcriptomic analyses suggested a proposed degradation pathway (azo bond cleavage, hydroxylation, desulfurization, ring-opening and mineralization) and multiple osmoregulatory strategies (“high-salt-in”, “compatible-solute-in”, ergosterol synthesis and casein kinase II). The combination of outstanding halotolerance, high dye-loading capacity, and efficient reduction in acute toxicity positions C. tropicalis ZQ as a promising biocatalyst for bioremediation of azo-dye-contaminated high-salinity wastewater.