Mangrove Soil as a Natural Catalyst for Green Synthesis of Silver Nanoparticles

Andrea Chacón-CalderónReinaldo Pereira-PérezJuan Miguel Zuñiga-UmañaYendry Corrales Ureña^*JOSE ROBERTO VEGA BAUDRIT

• National Center for High Technology, San José, Costa Rica

The diversity of biogeochemical pathways within mangroves is believed to contribute to the resilience of this ecosystem, enabling it to cope with a wide range of pollutants, from heavy metals to hormones.The extreme physicochemical conditions of mangrove soils, combined with the anaerobic metabolism of their microbiota, create a unique environment for nanometric phenomena. This study compares soil extracts from two mangroves in Costa Rica: Cahuita, located on the Caribbean coast, and Punta Morales, situated on the Pacific coast, including extracts from both surface and deeper layers. Our findings show that nanoparticles are present in mangrove sediments, primarily as silicates and organic fibers, which are suggested to be carbon-based, such as nanocellulose. Moreover, we demonstrate that mangrove soil extracts act as catalysts for forming metallic nanoparticles, such as silver nanoparticles (AgNPs), in high-salinity environments under sunlight irradiation at temperatures of 28-31°C within 15 minutes. Extracts from the Pacific region exhibited a higher affinity for metal NPs than those from the Caribbean, according to isothermal titration studies (ITC). Still, AgNP synthesis was faster using the Caribbean extracts due to the higher number of binding sites of the organic material to the NPs.The formed particles are mainly Ag and AgCl. The mechanism associated with the nanoparticles can be correlated with photolysis, complex formation, reduction of organic matter, and the presence of other ions, such as iron, that catalyze the reaction. Minimum inhibitory concentration (MIC) assays were conducted to monitor optical density over time for Escherichia Coli ( E.Coli), Staphylococcus Aureus (S. Aureus), and Bacillus Subtilis (B. Subtilis). Growth dynamics were evaluated using generalized additive models (GAM) and complemented by non-parametric statistical analysis at defined time points. The results revealed species-specific and concentration-dependent responses to AgNP exposure, with MIC values ranging from 2.5 µg/mL to 20 µg/mL, varying across bacterial strains and nanoparticle synthesis origins. The antimicrobial properties of the NPs synthesized using Caribbean extracts were higher than those synthesized with citric acid, suggesting that coating with humic and fulvic acids plays a role in their properties, as the extracts do not present bacteria growth inhibition.