Bacterial Communities in the Rhizosphere of Common Bean Plants (Phaseolus vulgaris L.) Grown in an Arable Soil Amended with TiO₂ Nanoparticles

The use of nanoparticles, such as titanium dioxide (TiO2-NPs), has increased substantially over the years. Some of them will end up in the soil, where their effect on plants and the soil bacterial community needs to be studied to determine their possible environmental risks. In this paper, beans (Phaseolus vulgaris L.) were cultivated in soil with 0, 150, or 300 mg TiO2-NPs kg(-1). Plant development, nodule formation, chlorophyl content, and the bacterial community were monitored in uncultivated, non-rhizosphere, and rhizosphere soils. TiO2-NPs did not affect the beans' growth and their chlorophyl content, but they did increase bacterial diversity and had a significant effect on the bacterial community structure in the rhizosphere, but not in the bulk and non-rhizosphere soil. Although the relative abundance of most bacterial groups varied with the TiO2-NP application rate, the cultivation of the bean plants, or the exposure time, that of Acidobacteria decreased, while that of Planctomycetes increased in the TiO2-NP-amended soil. Many bacterial groups were affected by the cultivation of the bean plants, i.e., the relative abundance of Acidobacteria, Gemmatimonadetes, Deltaproteobacteria, and Firmicutes mostly decreased in the rhizosphere independent of the application of TiO2-NPs or the time of exposure, while most groups belonging to Actinobacteria, Bacteroidetes, Rhizobiaceae, Phyllobacteriaceae, and Sphingomonadaceae were enriched.