The Identification of Susceptible Soil Bacteria to TiO2 and ZnO Nanoparticles Using a Bar-Coded Pyrosequencing Approach.

Considering the reality that soil serves as a major sink of engineered nanoparticles (ENPs) released to the environment, the effects of ENPs on soil processes and the organisms that carry them out must be addressed. However, knowledge on ENP effects on soil communities remains largely unknown. The sparse literature uses DNA fingerprinting techniques to examine the effects of either fullerenes or metal oxide nanoparticles on soil bacterial communities (Tong et al. 2007; Johansen et al. 2008; Ge et al. 2011). While such studies are helpful to answer whether communities change, they do not directly show what changes have occurred. To address this question, we used a bar-coded pyrosequencing approach to evaluate the effects of two widely used ENPs, TiO₂ and ZnO nanoparticles, at varying doses (0, 0.5, 1.0 and 2.0 mg g^-1 soil for TiO₂, 0.05, 0.1 and 0.5 mg g^-1 soil for ZnO) on a California grassland soil in incubated microcosms for up to 60 days. Our results reveal a marked taxa-specific response of soil bacteria to TiO₂ and ZnO nanoparticles. Of the taxa that occur in all samples, sixteen taxa were positively correlated with at least one ENP at both sampling times, while twenty-eight taxa were negatively correlated with TiO₂ or ZnO concentrations. Notably, both TiO₂ and ZnO  had significant positive linear relationships, after both 15 and 60 days of exposure, with families Sphingomonadaceae and Streptomycetaceae, and the genus Streptomyces; significant negative linear relationships with the class Alphaproteobacteria, the order Rhizobiales, families Bradyrhizobiaceae, Geodermatophilaceae, Methylobacteriaceae, Micromonosporaceae and Rhodospirillaceae, and genera Actinoplanes, Balneimonas, Blastococcus, Bradyrhizobium and Skermanella. Our results clearly demonstrate that the abundances of soil bacterial taxa vary in the direction and magnitude with certain nanoparticles, providing insights into the fine responses of soil microbial communities to ENPs.