PNNL

Abstract

The effects of humic acid (HA) on interactions between ZnO nanoparticles (ZnO NPs) and Pseudomonas putida KT2440 bio?lms at different maturity stages were investigated. Three stages of bio?lm devel-opment were identi?ed according to bacterial adenosine triphosphate (ATP) activity associated with bio?lm development process. In the initial bio?lm stage 1, the ATP content of bacteria was reduced by more than 90% when bio?lms were exposed to ZnO NPs. However, in the mature bio?lm stages 2 and 3, the ATP content was only slightly decreased. Bio?lms at stage 3 exhibited less susceptibility to ZnO NPs than bio?lms at stage 2. These results suggest that more mature bio?lms have a signi?cantly higher tolerance to ZnO NPs compared to young bio?lms. In addition, bio?lms with intact extracellular poly-meric substances (EPS) showed higher tolerance to ZnO NPs than those without EPS, indicating that EPS play a key role in alleviating the toxic effects of ZnO NPs. In both pure ZnO NPs and ZnO-HA mixtures, dissolved Zn2þ originating from the NPs signi?cantly contributed to the overall toxicity. The presence of HA dramatically decreased the toxicity of ZnO NPs due to the binding of Zn2þ on HA. The combined results from this work suggest that the bio?lm maturity stages and environmental constituents (such as humic acid) are important factors to consider when evaluating potential risks of NPs to ecological systems.