Green-synthesized silver nanoparticles from fresh and waste Posidonia oceanica extracts: characterization and evaluation of antibacterial, cytotoxic activity against HCT-116 cells, and ecotoxicological properties.

Posidonia oceanica, an endemic Mediterranean seagrass, represents a valuable source of bioactive compounds and an abundant marine biomass waste. In this study, aqueous extracts from fresh (POV) and waste (POD) leaves of P. oceanica were employed as sustainable reducing and capping agents for the green synthesis of silver nanoparticles (AgNPs). The resulting nanoparticles were characterized using UV-vis spectroscopy, FTIR, XRD, SEM, TEM, AFM, and zeta potential measurements, and subsequently evaluated for their antimicrobial, cytotoxic, antidiabetic, anti-inflammatory, anti-hemolytic, and ecotoxicological activities. Phytochemical profiling revealed higher levels of phenolic acids and flavonoids in the POV extract, particularly ellagic acid (511.29 µg g-1) and rutin (470.24 µg g-1), whereas estragole was identified as the predominant volatile constituent in both POD (57.86%) and POV (23.04%) extracts. The biosynthesized AgNPs were spherical, polydisperse, and exhibited stable colloidal properties with average particle sizes of 13.31 ± 5.2 nm and 8.61 ± 2.6 nm, and zeta potentials of -37.42 mV and -34.85 mV for POV and POD AgNPs, respectively. POV AgNPs exhibited superior antioxidant performance while POD AgNPs showed greater antibacterial efficacy against both Gram-positive and Gram-negative bacteria, with a ZOI shown even at low concentrations of 0.0625 mg mL-1. Both formulations displayed dose-dependent cytotoxicity against HCT-116 human colorectal carcinoma, with IC50 values of 5.03 ± 0.25 µg mL-1 and 2.22 ± 0.11 µg mL-1 for POV AgNPs and POD AgNPs, respectively. In contrast, POV and POD AgNPs showed no detectable antidiabetic, anti-inflammatory, or anti-hemolytic activities under the tested conditions. Acute cytotoxicity testing via Artemia salina showed moderate toxicity after 24 hours ranging from 63.3 ± 5.8% to 86.6 ± 5.8% for POV AgNPs and from 73.3 ± 5.8% to 86.6 ± 5.8% for POD AgNPs, indicating a lower toxicity profile for POD AgNPs. Importantly, these toxic effects were observed at concentrations substantially higher than those required for antibacterial and cytotoxic activities. Overall, P. oceanica-derived AgNPs represent sustainable marine-derived nanomaterials with promising antioxidant, antibacterial, and cytotoxic properties, supporting their further investigation for biomedical and biotechnological applications.