Optimized melanin production from marine-associated Bacillus sp. EGY7 as a sustainable precursor of carbon dots for biomedical applications

Melanin, a multifunctional biopigment with photoprotective, antioxidant, and therapeutic properties, is limited by scarce sustainable sources and poor aqueous solubility. In this study, melanin production from a newly isolated marine-associated Bacillus sp. EGY7 was optimized using a Box–Behnken design, achieving a yield of 0.208 g/L. The microbial melanin was subsequently hydrothermally converted into water-dispersible melanin-derived carbon dots (MCDs) with quasi-spherical morphology, particle size of 9–15 nm, and a quantum yield of 5.05 ± 0.31%. The MCDs exhibit strong UV absorption, tunable and often excitation-dependent photoluminescence (PL), high photostability, and environment-sensitive fluorescence governed by their core–shell structure and surface functional groups. MCDs exhibited moderate antioxidant activity and high cytocompatibility, maintaining > 70% viability in human skin fibroblasts. Compared with native melanin, MCDs demonstrated enhanced selective cytotoxicity against breast cancer cells (IC50: 0.25 mg/mL for MCF-7 and 0.68 mg/mL for MDA-MB-231), with selectivity indices reaching 19.6 and superior inhibition of long-term colony formation. Mechanistic investigations suggested that their anticancer activity is mediated by reactive oxygen species (ROS)-induced apoptosis coupled with efficient nanoscale cellular uptake. Complementary in silico analyses predicted interactions with multiple cancer-related targets involved in apoptosis, oxidative stress regulation, and cell cycle control, supporting their theragnostic potential. Overall, these findings establish Bacillus sp. EGY7 as a renewable melanin source and highlight MCDs as a sustainable multifunctional platform for selective anticancer and bioimaging applications. Graphical abstract