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With the increase in silver (Ag)-based products in our lives,it is essential to test the potential toxicity of silver nanoparticles (AgNPs) and silver ions (Ag ions) on living organisms under various conditions.Here,we investigated the toxicity of AgNPs with Ag ions to Escherichia coli K-12 strain under various conditions.We observed that both AgNPs and Ag ions display antibacterial activities,and that Ag ions had higher toxicity to E.coli K-12 strain than AgNPs under the same concentrations.To understand the toxicity of AgNPs at a cellular level,reactive oxygen species (ROS) enzymes were detected for use as antioxidant enzymatic biomarkers.We have also studied the toxicity of AgNPs and Ag ions under various coexistence conditions including:fixed total concentration,with a varied the ratio of AgNPs to Ag ions;fixed the AgNPs concentration and then increased the Ag ions concentration;fixed Ag ions concentration and then increasing the AgNPs concentration.Exposure to AgNPs and Ag ions clearly had synergistic toxicity;however,decreased toxicity (for a fixed AgNPs concentration of 5 mg/L,after increasing the Ag ions concentration) to E.coli K-12 strain.AgNPs and Ag ions in the presence of L-cysteine accelerated the bacterial cell growth rate,thereby reducing the bioavailability of Ag ions released from AgNPs under the single and coexistence conditions.Further works are needed to consider this potential for AgNPs and Ag ions toxicity across a range of environmental conditions.Environmental Significance Statement:As silver nanoparticles (AgNPs)-based products are being broadly used in commercial industries,an ecotoxicological understanding of the AgNPs being released into the environment should be further considered.Here,we investigate the comparative toxicity of AgNPs and silver ions (Ag ions) to Escherichia coli K-12 strain,a representative ecotoxicological bioreporter.This study showed that toxicities of AgNPs and Ag ions to E.coli K-12 strain display different relationships when existing individually or when coexisting,and in the presence of L-cysteine materials.These findings suggest that the toxicology research of nanomaterials should consider conditions when NPs coexist with and without their bioavailable ions.