Marine Algae as Sustainable Platforms for the Green Synthesis of Metal Nanoparticles

The potential of marine algae as sustainable agents for the green synthesis of nanoparticles is examined as an environmentally benign alternative to conventional chemical and physical approaches that are commonly associated with toxic byproducts. This review aims to provide a comprehensive and accessible overview of algae-mediated green synthesis of nanoparticles, with a specific focus on the synthesis mechanisms, nanoparticle types, and application potential of <i>Nannochloropsis oculata</i> and <i>Tetraselmis</i> spp., while highlighting current challenges and future research directions in algae-based nanotechnology. In particular, the review focuses on <i>N. oculata</i> and <i>Tetraselmis</i> spp., which have been widely reported to exhibit strong metal ion reduction and stabilizing capabilities. The biosynthetic mechanisms governing algae-mediated nanoparticle synthesis are systematically examined, with algal biomolecules including proteins, polyphenols, fatty acids, and pigments identified as key natural reducing and capping agents. Two principal synthesis routes are addressed, namely, intracellular synthesis occurring within living algal cells and extracellular synthesis using cell-free extracts, with comparisons drawn in terms of particle controllability, scalability, and recovery. The biosynthesis of silver (AgNPs), gold (AuNPs), and zinc oxide nanoparticles (ZnONPs) is illustrated through representative case studies, together with commonly employed characterization techniques such as TEM, FTIR, XRD, UV-vis spectroscopy, and DLS. The reviewed studies demonstrate that algae-mediated approaches are capable of producing stable metallic and metal oxide nanoparticles with sizes ranging from 2 to 79 nm, exhibiting confirmed antimicrobial, antioxidant, and anticancer activities. Structural and functional analyses have validated the physicochemical properties of the biosynthesized nanoparticles. Despite these demonstrated advantages, significant challenges remain, particularly in relation to nanoparticle uniformity, reproducibility, and large-scale production. The integration of mechanistic insights, synthesis strategies, and application perspectives underscores the potential of marine algae as cost-effective, sustainable, and biocompatible platforms for nanoparticle production in biomedical and environmental applications.