Green microalga (Coelastrella sp.) cultivated in wastewater is an effective phosphorus biofertilizer

Abstract Phosphorus (P) is an essential macronutrient for plants, yet its reserves are finite and non-renewable. Microalgae present a promising solution for recovering P from wastewater, providing a sustainable source of biofertilizer. This study assessed the potential of the green microalga Coelastrella sp. cultivated in nutrient-rich synthetic centrate, as a source of slow-release P biofertilizer for crops. A P starvation–repletion approach was applied to enhance P assimilation during outdoor cultivation in photobioreactors, and the resulting biomass was analyzed for nutrient composition and P-release dynamics in calcareous soil in comparison with triple superphosphate (TSP). The algal cells rapidly accumulated more than 2% P (dry weight) within six hours of repletion and showed sustained P release over 84 days, maintaining higher levels of Olsen-extractable P than TSP after four weeks. In pot trials with tomato ( Solanum lycopersicum ), Coelastrella -based fertilization at both 30 and 60 mg P kg⁻ 1 achieved biomass and fruit yields comparable to TSP, while significantly increasing fruit P uptake and micronutrient (Fe, Zn, Ni) concentrations. Moreover, algal fertilization enhanced arbuscular mycorrhizal colonization compared with mineral P. These results demonstrate that Coelastrella sp. can efficiently recover P from wastewater and provide a slow-release biofertilizer that sustains P availability, promotes symbiotic root interactions, and improves plant nutrient acquisition. The use of this algal biomass thus represents a sustainable strategy for recycling wastewater-derived nutrients and reducing dependence on conventional P fertilizers.