Algae as Nature’s Filters: Micro- and Macroalgal Solutions for Nutrient and Contaminant Removal in Aquatic Systems

Algae are emerging not merely as passive nature-based engineered bio-platforms, but as dynamic bio-circular platforms capable of simultaneously removing complex contaminant mixtures while generating vaporizable biomass within integrated water-resource systems. Freshwater and marine micro- and macroalgae possess diverse physiological and biochemical traits that enable them to assimilate excess nitrogen and phosphorus, adsorb and accumulate metals, bind organic pollutants, and immobilize emerging contaminants such as pharmaceuticals, hydrocarbons, and microplastics. This review synthesizes current knowledge on algal remediation across ecosystems, highlighting species‑specific mechanisms including nutrient assimilation, biosorption, bioaccumulation, enzymatic biotransformation, extracellular polymeric substance production, and physical interception of suspended particles. Freshwater species such as Chlorella, Scenedesmus, Oedogonium, and Cladophora demonstrate high nutrient uptake rates and strong capacity to remove metals and organic contaminants in wastewater treatment and agricultural runoff systems. Marine taxa including Nannochloropsis, Tetraselmis, Ulva, Gracilaria, and Sargassum exhibit exceptional tolerance to salinity and complex pollutant mixtures, in context-dependent performance variability, making them effective in coastal remediation, aquaculture effluent treatment, and biosorption-based applications. Case studies from municipal wastewater facilities, constructed wetlands, integrated multi-trophic aquaculture, and coastal bioremediation platforms demonstrate high removal efficiencies and the potential for biomass valorization within circular bioeconomy frameworks. Despite these advances, challenges remain related to environmental variability, biological contamination, scalability, regulatory constraints, and the safe management of pollutant‑laden biomass. Future progress will depend on advances in strain selection, system engineering, microbial consortia design, digital monitoring, and supportive policy frameworks. Overall, algae represent a robust, adaptable, and ecologically compatible approach to restoring water quality in freshwater and marine ecosystems increasingly affected by anthropogenic pressures.