Improved biosynthesis and characteristics of silver nanoparticles using marine endophytic fungi exposed to hypo-osmotic stress

Silver nanoparticles (AgNPs) are valued for their unique properties and can be synthesized biologically, offering a greener alternative to traditional methods. This study assessed the potential of cell-free filtrates (CFFs) of seaweed-associated marine fungi to produce AgNPs. More specifically, Talaromyces pinophilus, Aspergillus fructus, Stemphylium gracilariae, Purpureocillium lilacinum, Microascus croci, Penicillium toxicarium, Exophiala mesophila, and Emericellopsis maritima, when cultured in media prepared with artificial seawater or distilled water (hypo-osmotic conditions) as a possible stress inducer. Optimization of the reaction conditions including changes in pH, temperature, reaction time, and the concentration of AgNO₃ and CFF were evaluated for obtaining high relative concentrations and theoretically small sized AgNPs. High pH (10), elevated temperature (100 ºC), and the highest AgNO₃ concentration tested (1.5 mM) were found to be key. Moreover, CFFs from marine endophytic fungi grown under hypo-osmotic conditions, in particular T. pinophilus, A. fructus, P. lilacinum, P. toxicarium, and E. maritima, biosynthesized higher relative concentrations of generally monodispersed AgNPs with moderate stability (zeta-potential between - 20 and - 30 mV) and varying nanoparticle average sizes (between 40.64 and 191.60 nm) with high percentages of very small particles (bellow 50 nm). This study demonstrates the potential of marine endophytic fungi for AgNPs biosynthesis, showing that modifying fungal growth conditions enhances green AgNPs production. Future research on diverse stress conditions may further optimize this eco-friendly approach.