Effects of Engineered Nanoparticles on Crops, their Symbionts, and Soil Microbial Communities

Abstract

Engineered nanoparticles (NPs) are small particles (< 100 nm) that are widely used in electronics, paints, cosmetics, and composite materials. As a result of the production and use of NP containing materials, NPs are released into the environment. For future risk assessment it is, therefore, important to investigate whether NPs negatively impact the environment and if they interact with plants and soil microorganisms.In this thesis we addressed the effects of three of the ten most produced NPs on plants, their symbiotic microorganisms and on soil microbial communities. In a first experiment we exposed nitrogen fixing bacteria (Rhizobium trifolii) grown in liquid cultures to different titanium oxide (TiO2) NPs, i.e. P25 and E171. The relative growth rate of R. trifolii was not affected by one TiO2 NP (P25; 23 mg l-1), while it decreased with the other TiO2 (23 mg l-1 E171). To assess whether the decreased growth of R. trifolii influences nodulation on red clover roots, and whether red clover is affected by TiO2 NPs, we exposed red clover and R. trifolii in a hydroponic system. All of the tested TiO2 treatments decreased shoot biomass (by 27-53%) and root length (by 30-44%) of red clover exposed for four weeks.To test whether the effects on red clover and R. trifolli also occur in soils, we mixed TiO2 NPs (10 to 1000 mg kg-1) into an agricultural soil and grew red clover for three months. We also tested CeO2 NPs and multiwalled carbon nanotubes (MWCNT). Red clover biomass was not affected by any of the treatments. However, flower number decreased in plants grown in pots with MWCNTs (3 mg kg-1) and biological nitrogen fixation increased by 8% when treated with MWCNT (3000 mg kg-1). Wheat was exposed to different TiO2 NPs in the same soil type used for the red clover experiment. Wheat biomass was not affected by any of the treatments. Microbial community structures were assessed using Illumina Miseq paired-end sequencing of ribosomal markers targeting prokaryotes and micro-eukaryotes (16SV3-V4 and ITS2, respectively) for both experiments. The community structure of prokaryotic microorganisms changed when treated with TiO2 NPs in the wheat experiment and MWCNT in the red clover experiment but was not affected by TiO2 NPs in the red clover experiment. In both experiments, eukaryotic microorganism community structure was not affected by any of the tested NPs. In conclusion, this work indicates that red clover and its symbiont R. trifolii are more sensitive to TiO2 NPs in hydroponic systems than in soils, and prokaryotic microorganisms were more sensitive to engineered NPs than eukaryotic microorganisms under the tested conditions. For further experiments it is recommended to directly test the plants and soil microorganisms in soil systems.