Inorganic carbon utilization: A target of silver nanoparticle toxicity on a submerged macrophyte

Submerged macrophytes play an important role in the global carbon cycle through diversified pathways of inorganic carbon (Ci) utilization distinct from terrestrial plants. However, the effects of silver nanoparticles (AgNPs), an emerging contaminant, were unknown on the Ci utilization of submerged macrophytes. In Ottelia alismoides, the only known submerged macrophyte with three pathways of Ci utilization, before absorption, AgNPs inhibited the external carbonic anhydrase activity thus reducing the capacity of the plant to use HCO₃^-. After entering the plant, AgNPs mainly aggregated at the cell wall and in the chloroplast. The internalized AgNPs inhibited ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) activity blocking CO₂ fixation and disturbed C4 and crassulacean acid metabolism (CAM) by inhibiting phosphoenolpyruvate carboxylase (PEPC), pyruvate phosphate dikinase (PPDK), and NAD-dependent malic enzyme (NAD-ME) activities to alter intracellular malate biosynthesis and decarboxylation. Overall, our findings indicate that the Ci utilization of the submerged macrophyte is a target of AgNPs toxicity that might affect the carbon cycle in aquatic systems.