Polyphasic characterization, halotolerance, melanin production, and in vitro bioactivities of Neophaeotheca triangularis AUMC 17201 from hypersaline El Hamra Lake, Egypt

Background Hypersaline environments harbor extremotolerant fungi with specialized adaptations to osmotic, oxidative, and radiation stress. Among them, melanized black yeasts are of particular interest because their melanized cell walls, morphological plasticity, and yeast-like, hyphal, or meristematic growth forms support survival in extreme saline habitats and may contribute to their biotechnological potential. Results In this study, a halotolerant black yeast strain, AUMC 17201, recovered from hypersaline water of El Hamra Lake, Wadi El Natrun, Egypt, was identified and evaluated for salinity tolerance, eumelanin-like pigment production, antioxidant activity, and antimicrobial potential. Polyphasic characterization based on cultural, microscopic, physiological, and ITS rDNA analyses identified the strain as Neophaeotheca triangularis , representing, to the best of our knowledge, the first documented record of this species from Egypt. The isolate exhibited broad halotolerance, growing across 0–30% NaCl, with maximum biomass production at 5% NaCl (14.304 ± 1.623 g L⁻¹). Salinity differentially regulated pigment production, with the highest total melanin yield at 5% NaCl (1.273 ± 0.093 g L⁻¹) and the highest mean cellular melanin content at 15% NaCl (109.24 ± 7.63 mg g⁻¹ dry biomass). Physicochemical assays, FT-IR spectroscopy, and qualitative HPLC chromatographic comparison supported the eumelanin-like nature of the extracted pigment. Crude culture filtrates exhibited salinity-dependent antioxidant activity, with maximum hydroxyl radical scavenging activity (87.71 ± 2.53%) and total antioxidant activity (0.109 ± 0.0142 mg AAE mL⁻¹) at 5% NaCl. Maximum ferric reducing power was recorded at 10% NaCl (1.417 ± 0.098 mg AAE mL⁻¹). The filtrates also showed salinity-dependent antibacterial activity, with the largest inhibition zone recorded against Bacillus subtilis at 20% NaCl, while antifungal activity was detected against M. furfur and T. mentagrophytes , with the numerically largest inhibition zone against T. mentagrophytes at 10% NaCl. The distinct salinity optima for biomass production, cellular pigmentation, and bioactivity further demonstrate differential physiological responses of AUMC 17201 across the salinity gradient. Conclusions Neophaeotheca triangularis AUMC 17201 combines broad halotolerance, salinity-regulated eumelanin-like pigment production, and preliminary in vitro antioxidant and antimicrobial activities, supporting further investigation of the strain and its bioactive metabolites for potential biotechnological applications.