Microalgae-Based Transformation of Carbon and Nitrogen: A Pathway for Sustainable Wastewater Treatment

Excessive carbon emissions and nitrogen pollution in aquatic environments drive climate change and eutrophication and therefore present major challenges to global sustainability. Microalgae have been investigated as a bioremediation tool because of their capacity to assimilate inorganic carbon and to transform nitrogenous compounds present in wastewater. Their application enables concurrent nutrient recovery and remediation and is considered in this review with attention to associated energy and cost implications. Accordingly, the review examines carbon transformation by microalgae covering photosynthetic CO₂ fixation, conversion of algal biomass to renewable energy carriers, and integration with wastewater treatment and evaluates cultivation approaches including open raceway ponds, closed photobioreactors, and other relevant systems. Nitrogen transformation processes are addressed in detail, including ammonia removal and the conversion of nitrite and nitrate, strategies for nitrogen capture to control eutrophication, and the physicochemical and biological factors that influence these transformations. The cellular and biochemical pathways underlying carbon sequestration and nitrogen metabolism in microalgae are highlighted. Finally, the review proposes targeted strategies to optimize microalgal applications for carbon capture and nitrogen conversion to improve environmental management and circular nutrient handling.