Matheus Quintão Braga, Juliana Ferreira Lorentz, Alexia Saleme Aona de Paula Pereira, Iara Barbosa Magalhães, Thiago Abrantes Silva, Nayara Vilela Avelar, Bruno Silva Henriques, Ana Paula Maciel de Castro, Paula Peixoto Assemany, Maria Lúcia Calijuri
The application of nanoparticles (NPs) in microalgae-based systems has emerged as a flexible strategy to enhance wastewater treatment and biomass valorization, while promoting circular bioeconomy pathways. This critical review analyzes over one hundred studies published between 2015 and 2025, which evaluated the use of metallic, metal oxide, and carbon-based NPs in microalgal cultivation, harvesting, green synthesis, and toxicity assessment. Based on these studies, optimized NP concentrations can enhance photosynthetic efficiency, improve nutrient and organic pollutant removal, and modulate biomass composition toward biofuels and value added bioproducts, including pigments, biopolymers, and biofertilizers. Magnetically assisted harvesting and microalgae-mediated NP synthesis can be combined synergistically to reduce energy demands and separation costs. However, key challenges remain, notably the definition of safe and effective dosage ranges, the lack of standardized ecotoxicological protocols, and the scarcity of pilot- and industrial-scale validations. Additional obstacles include limited performance benchmarking against conventional technologies and uncertainties related to life cycle assessments and cost analyses. By integrating nanotechnology, microalgal biotechnology, and sustainability assessment, this review outlines strategic pathways to translate laboratory advances into scalable, economically viable, and environmentally safe applications, reinforcing the role of microalgae–NP systems in environmental sanitation and the bioeconomy. • Nanoparticles boost microalgae growth and wastewater remediation. • Concentration-dependent effects include enhanced uptake or oxidative toxicity. • Magnetic nanoparticles enable rapid and reusable biomass harvesting. • Green synthesis from microalgae reduces nanomaterial environmental impacts. • Gap: few pilot scale studies integrating LCA and real wastewater.