The effect of silver nanoparticles on aerobic denitrifying bacteria during biological nitrogen removal: A new perspective based on morphological effects

Abstract

The morphological effect-dependent biotoxicity of silver nanoparticles (AgNPs) toward aerobic denitrifying bacteria is unclear. In this study, the behavior and biotoxicity of spherical and flaky AgNPs on aerobic denitrifier Pseudomonas stutzeri were investigated based on morphological effects. The three-dimensional fractal dimension of flaky AgNPs with a rough surface and complex structure was 2.73, which was higher than that of spherical AgNPs. Flaky AgNPs more significantly inhibited the growth and nitrogen removal of P. stutzeri than spherical AgNPs, with average inhibition rates of 50.4% and 76.4%, respectively. Flaky AgNPs resulted in higher secretion of extracellular polymeric substances, lower activity of electron transport system and higher concentrations of reactive oxygen species and lactate dehydrogenase in P. stutzeri than spherical AgNPs, indicating that flaky AgNPs displayed stronger inhibitory effects on P. stutzeri. The results of flow cytometry and scanning electron microscopy revealed that flaky AgNPs caused more damage to the cell surface of P. stutzeri, resulting in a higher proportion of cell death. Furthermore, flaky AgNPs more significantly inhibited the expression of napAB and nirS in P. stutzeri than spherical AgNPs, up to 67.7%, resulting in lower nitrate and nitrite reductase activities and worse nitrogen removal efficiency. The results of this study can elucidate the morphology-dependent toxicity of AgNPs and provide new insights into the negative impacts of AgNPs on aerobic denitrification wastewater treatment.