Behavior and toxicity of silver nanoparticles to Chlorella vulgaris: A new perspective based on surface charges

The toxicity of silver nanoparticles (AgNPs) to algae is influenced by their surface charges. This study investigated the behavior and toxicity of two AgNPs modified by branched polyethyleneimine (BPEI-AgNPs with a positive charge) and citrate (CIT-AgNPs with a negative charge) on Chlorella vulgaris (C. vulgaris) to elucidate the surface charge-dependent toxicity of AgNPs to C. vulgaris. The results indicated that the BPEI-AgNPs exhibited a stronger inhibitory effect on the growth and photosynthetic activity of C. vulgaris, compared with the CIT-AgNPs. The reduction in the final superoxide dismutase content of 0.5 mg/L BPEI-AgNPs (48%) was higher than that of the CIT-AgNPs group (39%). The C. vulgaris exhibited less tolerance to the positively charged BPEI-AgNPs. The BPEI-AgNPs induced higher secretion of extracellular polymeric substances (EPS) and higher concentrations of lactate dehydrogenase (LDH) in C. vulgaris than CIT-AgNPs. The BPEI-AgNPs had a more significant up-regulation of gene expression for oxidative stress (sod2) than CIT-AgNPs, up to 2.57-fold. Genes encoding photosynthesis pathways were down-regulated, resulting in serious damage to photosynthetic system. Furthermore, the CIT-AgNPs inhibited the citrate cycle and oxidative phosphorylation metabolism, while BPEI-AgNPs disrupted the photophosphorylation pathways. These findings offer valuable insights into the effects of AgNPs with different surface charges on C. vulgaris.