Synthetic Nanoparticles in Aquatic Systems: Hidden Ecotoxicity and the Sustainability Challenge

Synthetic nanoparticles (NPs) have emerged as versatile materials in a range of industrial and biomedical applications. However, their release into aquatic systems raises growing concern due to their unique physicochemical transformations and ecotoxicological impacts. This review critically explores how nanoparticle properties—such as crystallinity, size, surface reactivity, and light sensitivity—influence their environmental behavior, bioavailability, and toxicity, particularly toward primary producers like microalgae. We highlight recent findings on the mechanisms of nanoparticle-induced photosynthetic inhibition and oxidative stress at the cellular and molecular levels. A key contribution of this review is the introduction of the “Green Synthesis Paradox”, a concept that challenges the assumed environmental safety of green-synthesized nanomaterials by demonstrating their potential hidden toxicity in real ecosystems. We argue that sustainability in nanotechnology must be assessed across the full life cycle of nanoproducts, from synthesis to environmental fate. By integrating ecotoxicological perspectives into the design and regulation of nanomaterials, this review calls for a paradigm shift toward responsible innovation in nanoscience.