Potential Mechanisms and Environmental Controls of TiO₂ Nanoparticle Effects on Soil Bacterial Communities

It has been reported that engineered nanoparticles (ENPs) alter soil bacterial communities, but the underlying mechanisms and environmental controls of such effects remain unknown. Besides direct toxicity, ENPs may indirectly affect soil bacteria by changing soil water availability or other properties. Alternatively, soil water or other environmental factors may mediate ENP effects on soil bacterial communities. To test, we incubated nano-TiO₂-amended soils across a range of water potentials for 288 days. Following incubation, the soil water characteristics, organic matter, total carbon, total nitrogen, and respiration upon rewetting (an indicator of bioavailable organic carbon) were measured. Bacterial community shifts were characterized by terminal restriction fragment length polymorphism (T-RFLP). The endpoint soil water holding had been reported previously as not changing with this nano-TiO₂ amendment; herein, we also found that some selected soil properties were unaffected by the treatments. However, we found that nano-TiO₂ altered the bacterial community composition and reduced diversity. Nano-TiO₂-induced community dissimilarities increased but tended to approach a plateau when soils became drier. Taken together, nano-TiO₂ effects on soil bacteria appear to be a result of direct toxicity rather than indirectly through nano-TiO₂ affecting soil water and organic matter pools. However, such directs effects of nano-TiO₂ on soil bacterial communities are mediated by soil water

Solar Inactivation of Enterococci and <i>Escherichia coli</i> in Natural Waters: Effects of Water Absorbance and Depth

The decay of sewage-sourced <i>Escherichia coli</i> and enterococci was measured at multiple depths in a freshwater marsh, a brackish water lagoon, and a marine site, all located in California. The marine site had very clear water, while the waters from the marsh and lagoon contained colored dissolved organic matter that not only blocked light but also produced reactive oxygen species. First order decay rate constants of both enterococci and <i>E. coli</i> were between 1 and 2 d^–1 under low light conditions and as high as 6 d^–1 under high light conditions. First order decay rate constants were well correlated to the daily average UVB light intensity corrected for light screening incorporating water absorbance and depth, suggesting endogenous photoinactivation is a major pathway for bacterial decay. Additional laboratory experiments demonstrated the presence of colored dissolved organic matter in marsh water enhanced photoinactivation of a laboratory strain of <i>Enterococcus faecalis</i>, but depressed photoinactivation of sewage-sourced enterococci and <i>E. coli</i> after correcting for UVB light screening, suggesting that although the exogenous indirect photoinactivation mechanism may be active against <i>Ent. faecalis,</i> it is not for the sewage-source organisms. A simple linear regression model based on UVB light intensity appears to be a useful tool for predicting inactivation rate constants in natural waters of any depth and absorbance

Soybean Plants Modify Metal Oxide Nanoparticle Effects on Soil Bacterial Communities

Engineered nanoparticles (ENPs) are entering agricultural soils through land application of nanocontaining biosolids and agrochemicals. The potential adverse effects of ENPs have been studied on food crops and soil bacterial communities separately; however, how ENPs will affect the interacting plant–soil system remains unknown. To address this, we assessed ENP effects on soil microbial communities in soybean-planted, versus unplanted, mesocosms exposed to different doses of nano-CeO₂ (0–1.0 g kg^–1) or nano-ZnO (0–0.5 g kg^–1). Nano-CeO₂ did not affect soil bacterial communities in unplanted soils, but 0.1 g kg^–1 nano-CeO₂ altered soil bacterial communities in planted soils, indicating that plants interactively promote nano-CeO₂ effects in soil, possibly due to belowground C shifts since plant growth was impacted. Nano-ZnO at 0.5 g kg^–1 significantly altered soil bacterial communities, increasing some (e.g., <i>Rhizobium</i> and <i>Sphingomonas</i>) but decreasing other (e.g., <i>Ensifer</i>, <i>Rhodospirillaceae</i>, <i>Clostridium</i>, and <i>Azotobacter</i>) operational taxonomic units (OTUs). Fewer OTUs decreased from nano-ZnO exposure in planted (41) versus unplanted (85) soils, suggesting that plants ameliorate nano-ZnO effects. Taken together, plantspotentially through their effects on belowground biogeochemistrycould either promote (i.e., for the 0.1 g kg^–1 nano-CeO₂ treatment) or limit (i.e., for the 0.5 g kg^–1 nano-ZnO treatment) ENP effects on soil bacterial communities