TiO2 and ZnO Nanoparticles Negatively Affect Soil Bacterial Communities.

Increased usage of engineered nanoparticles (ENPs) has led to concerns regarding their environmental impacts in soil. The sparse published literature to date shows either a near absence of effects of fullerenes on soil microbial communities or subtle, yet measurable, effects. However, still unknown are effects of metal oxide ENPs on soil bacterial processes and communities. This study is towards quantifying the impacts of metal oxide ENPs to soil bacterial communities as a function of ENP type and concentration. Soils collected from a California grassland were exposed to different concentrations of TiO₂ (0, 0.5, 1.0 and 2.0 mg g^-1 soil) and ZnO (0.05, 0.1 and 0.5 mg g^-1 soil) in microcosms for 15, 30, 45 or 60 days. The effects on soil microbial biomass were assessed by substrate induced respiration (SIR) and total extractable soil DNA. The effects on bacterial community composition were evaluated by terminal restriction fragment length polymorphism (T-RFLP) analysis. Total soil respiration was also measured to evaluate the impacts to overall microbial activities. We found that (1) both TiO₂ and ZnO negatively affect soil microbial communities, as reflected by the negative effects on SIR, the extractable soil DNA and the T-RFLP based richness, as well as the shifts of soil bacterial community compositions; (2) the negative effects of TiO₂ and ZnO were time dependent, as indicated by the greater change of the extractable soil DNA and the community succession over time; and (3) the effect of ZnO was stronger than that of TiO₂, as reflected by significantly lower DNA and more distinctive shifts of bacterial community composition for ZnO at the same exposure concentration (0.5 mg g^-1 soil). Thus, differently from prior studies of fullerenes, nanoparticulate metal oxides may measurably and negatively impact soil bacterial communities.