Adaptative cellular and metabolomic responses to salinity of black yeasts isolated from deep-sea sediments of the Gulf of Mexico

Black yeasts are extremotolerant fungi that thrive in high-salinity environments, supported by melanin production and morphological and molecular adaptations. In this study, we isolated three black yeast species, Salinomyces thailandicus, Neophaeotheca triangularis, and Neophaeotheca salicorniae, from deep sediments of the Gulf of Mexico. We examined their morphology, cell division, growth, and metabolomic profiles under varying salinities (3.5% sea salt; 10% and 20% NaCl), and in the presence of the melanin inhibitor phthalide (PHT). Species-specific responses emerged: S. thailandicus shifted from filamentous to yeast-like growth with increasing salinity; N. triangularis showed the reverse; N. salicorniae remained dimorphic but exhibited reduced cell division at higher salinities. S. thailandicus divided by budding, while both Neophaeotheca species employed endoconidiogenesis. Phthalide exposure suppressed hyphal development in all three. Pores observed on Neophaeotheca cell surfaces, may mediate extracellular melanin granules transport. Untargeted metabolomics revealed salinity-dependent shifts in metabolite profiles, with higher metabolite abundance under PHT treatment. Fatty acids represented the most prominent metabolic pathway and may contribute to melanin biosynthesis. Additional metabolites included amino acids, peptides, alkaloids, terpenoids, and polyketides, some with potential biotechnological relevance. These findings advance our understanding of black yeasts adaptation to osmotic stress and highlight their ecological significance in marine environments.