International Academy of Ecology and Environmental Sciences Proceedings of the International Academy of Ecology and Environmental Sciences 2220-8860 15 4 2025 12 1 Marine fungi as biofactories for chitosan nanoparticles: Green synthesis, characterization and antimicrobial applications 114-131 EN N Gandhi A Samuel Sushma Ch Vijaya http://www.iaees.org/publications/journals/piaees/articles/2025-15(4)/marine-fungi-as-biofactories.pdf> The present study explores an eco-friendly and sustainable approach for synthesizing chitosan nanoparticles (CNPs) using marine fungi as a chitin source and Bacillus sp.-derived chitin deacetylase (CDA) for enzymatic deacetylation. Marine fungal isolates from coastal sediments of Nellore, Andhra Pradesh, were screened and optimized for biomass production and chitin extraction. Enzymatic conversion of chitosan from chitin was achieved using CDA-producing Bacillus sp., yielding chitosan with a high degree of deacetylation (78.5%). chitosan nanoparticles were synthesized via ionic gelation with tripolyphosphate (TPP) and characterized using UV-Vis spectroscopy, DLS, SEM-EDX, XRD, FTIR, and zeta potential analysis. The resulting CNPs were spherical, well-dispersed, and exhibited a particle size of ~138.6 nm with a zeta potential of +32.5 mV, indicating good stability. The nanoparticles demonstrated strong antibacterial activity against Staphylococcus aureus and Escherichia coli, and antifungal activity against Candida albicans and Aspergillus niger in a concentration-dependent manner. MIC, MBC, and MFC values confirmed the potent antimicrobial nature of the nanoparticles, with S. aureus being the most susceptible. The antimicrobial mechanism involved membrane disruption, reactive oxygen species generation, and interference with microbial DNA and metabolism. This study highlights the potential of marine fungal-derived chitosan nanoparticles as efficient biogenic agents for antimicrobial applications and wastewater remediation, particularly in aquaculture settings.