Silver nanoparticles impair photosynthesis via inhibiting particle-sensitive NADP-dependent GAPDH activity

As an emerging contaminant, silver nanoparticles (AgNPs) show significant toxicity to the photosynthesis of aquatic plants. However, photosynthesis is a complex process, and it remains unclear which specific steps are critical targets for AgNP toxicity. Here, Spirodela polyrhiza, was used to investigate the effects of AgNPs on various steps of the photosynthetic process. Results showed that AgNPs significantly induced Ag accumulation in chloroplasts and this Ag co-occurrence with the S element further altered the chloroplast morphology and structure. Then, this accumulated Ag significantly decreased the photosynthetic pigments content, reduced the light absorption and conversion, blocked the electron transportation, and inhibited the activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and NADP-dependent glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH) to suppress the Calvin cycle. Furthermore, evidence showed that Rubisco was sensitive to Ag⁺, while NADP-GAPDH was more sensitive to nanoparticles themselves. Based on the effective concentration at 50 % (EC₅₀) value and the structural equation modeling, the inhibition of the dark reactions, particularly the inhibition of NADP-GAPDH activity, was the key trigger in the AgNPs toxicity to photosynthesis in S. polyrhiza. Given the key role of aquatic plants, the inhibition of their photosynthesis potentially disrupts primary productivity and energy flow in aquatic ecosystems, and threatens food web stability.