Hydrilla verticillata's physiological reaction and carbon-nitrogen metabolism to elevated CO2 and NH4+ stress

Elevated CO₂ and NH 4 + stress disrupt aquatic ecosystem stability by affecting the carbon-nitrogen metabolism of aquatic plants, but the underlying mechanisms remain unclear. This study used H. verticillata as the research subject and measured water quality parameters, physiological and biochemical indices, transcriptome, and metabolome under different CO₂ and NH 4 + concentrations. The results showed whereas EC, SAL, and HCO₃⁻ levels rose, higher CO₂ considerably decreased DO, pH, and ORP in water. The amount of chlorophyll showed a downward trend, and high CO₂ circumstances markedly inhibited the activities of important carbon-nitrogen metabolic enzymes and antioxidant enzymes. Notably, PPDK and PEPC activity rose at first before declining. The five major modules that WGCNA identified were strongly linked to the metabolism of carbon-nitrogen, specifically the production and breakdown pathways of glutamate, glutamine, proline, arginine, and serine. Moreover, arginine production and the metabolic pathways of alanine, aspartate, and glutamate showed substantial reactivity to elevated CO₂, with the former having the greatest metabolic effect. In accordance with the study, HCO₃⁻ increases at elevated CO₂ levels. By activating key amino acid processes, this aids in maintaining the carbon-nitrogen balance. Conversely, sustained input of CO₂ decreases the pH levels of water. This results in NH₄⁺ stress, which eventually halts H. verticillata 's growth and kills the plant. This study offers a scientific foundation for the conservation of aquatic ecosystems by presenting the first mechanistic insights into the adaptive responses of submerged macrophytes under combined CO₂ elevation and NH₄⁺ release pressures. These findings also improve our knowledge of how climate change affects aquatic ecology and help guide restoration efforts in the future. • Dual stresses alters water physicochemical properties and restructure the C-N balance. • Key genes for NH₄⁺ and carbon metabolism show rise-then-fall expression patterns. • WGCNA identifies 5 core modules linked to glutamate, proline, and arginine metabolism. • Glutamate serves as a central hub in metabolic pathways, coordinating C-N synergy. • Putrescine and D-proline accumulation enhances adaptive defense against stress.