Wastewater nutrients valorization into biostimulant via engineered polyphosphate-accumulating bacterium

Phosphorus (P) and nitrogen (N) are critical nutrients increasingly lost to wastewater streams. Existing methods focus on removal rather than recycling, and are poorly equipped for valorization. Building on our group's prior work engineering Citrobacter freundii (CPP) overexpressing ppk for enhanced P removal, we demonstrate here that CPP simultaneously valorizes both P and N from real municipal wastewater into intracellular polyphosphate (polyP) and spermidine (Spd). Under optimized conditions, CPP accumulated intracellular polyP and Spd at 247.56 mg/g and 102.71 mg/g, representing 2.44- and 2.99-fold increases over pre-optimization levels, with corresponding P and N valorization rates reaching 76.76% and 51.16%, respectively, substantially exceeding the productivity of conventional polyphosphate-accumulating organisms. Driven by the co-accumulation, polyP and Spd phase-separated into insoluble granules termed stabilisomes with diameters of up to 181 nm and a composition of 44.6% polyP and 20.5% Spd, which maintained intracellular homeostasis and sustaining their continuous co-production. Leveraging the sustained co-production of polyP and Spd, the product value far exceeds that of conventional single-nutrient recovery techniques. Calculations indicate that the heat-inactivated CPP-derived biostimulant (CPPB) has a unit production cost of approximately ¥1.97/g. Applied as a foliar spray at 1.8 g/L, increased Brassica chinensis dry weight by 121.14% and plant height by 31.08%, outperforming commercial microbial fertilizers tested. This work establishes a practical biorefinery route for simultaneous P and N valorization from municipal wastewater, advancing the transition toward a circular bioeconomy.