Silver nanoparticles inhibit nitrogen fixation in soybean (Glycine max) root nodules

Antimicrobial silver nanoparticles (AgNPs) are popular in consumer and industrial products, leading to increasing concentrations in the environment. We tested whether exposure to AgNPs could be detrimental to a microbe, its host plant, and their symbiotic relationship. When subjected to 10 mu g/mL AgNPs, growth of Bradyrhizobium japonicum USDA 110 was halted. Axenic nitrogen-fertilized Glycine max seedlings were unaffected by 2.5 mu g/mL of 30 nm AgNPs, but growth was inhibited with the same dose of 16 nm AgNPs. With 2.5 mu g/mL AgNPs, biomass of inoculated plants was 50% of the control. Bacteroids were not found in nodules on plants treated with 2.5 mu g/mL AgNPs and plants given 0.5-2.5 mu g/mL AgNPs had 40-65% decreased nitrogen fixation. In conclusion, AgNPs not only interfere with general plant and bacterial growth but also inhibit nodule development and bacterial nitrogen fixation. We should be mindful of not releasing AgNPs to the environment or to agricultural land.