Ammonium-directed strategy shift in duckweed (Spirodela polyrhiza) synchronizes tunable wastewater treatment with industrial biomass valorization

Ammonium pollution, a major threat to aquatic ecosystems, requires efficient and sustainable remediation approaches. However, most previous studies have focused on either toxicity thresholds or single‑level treatments, leaving the integrated physiological and metabolic responses across a continuous wide concentration gradient largely unexplored. Here, we systematically examined the physiological and metabolic responses of duckweed ( Spirodela polyrhiza ) across a 0–2062.5 mg/L (NH₄)₂SO₄ gradient to elucidate its adaptive strategies for ammonium remediation and industrial biomass valorization. Results indicate that ammonium concentration fundamentally shapes duckweed’s physiological priorities. At low concentrations (0–82.5 mg/L), duckweed exhibits a growth-remediation mode characterized by high nutrient removal efficiency, with total nitrogen and phosphorus removal at 69.23%–78.25% and 83.91%–86.83%, respectively. At high concentrations (412.5–2062.5 mg/L), duckweed shifts to a storage-tolerance mode, suppressing growth and redirecting carbon flux to valuable energy-rich compounds, accumulating starch (to 217.39 mg/g dry weight) and triglycerides (to 21.27 nmol/g dry weight) while maintaining cellular integrity. Importantly, low ammonium concentrations cause more severe membrane damage than high ammonium concentrations, challenging a simple linear dose‑response assumption. Mineral analysis reveals adaptive nutrient management with enhanced phosphorus and zinc retention under stress. These findings demonstrate that the ammonium‑dominated treatment conditions act as a bioprocess switch, directing metabolism toward wastewater treatment or starch‑ and lipid‑rich biomass accumulation. This metabolic plasticity positions S. polyrhiza as a promising industrial crop and provides the first systematic study of a concentration‑dependent strategy shift, offering a tunable framework that links wastewater nutrient recovery with directed feedstock production for the circular bioeconomy. • Ammonium stress switches S. polyrhiza from growth-remediation to storage-tolerance mode. • Extreme ammonium stress boosts starch (217.39 mg/g) and TG (21.27 nmol/g) accumulation. • Over 86% TP and 78% TN are removed in the growth-remediation mode. • Superior membrane integrity at 412.5 mg/L ammonium reveals effective stress acclimation. • High ammonium stress enhances P and Zn accumulation but reduces K, Ca, Mg, and B uptake.