Investigation of the influence of different nanoparticles on the growth of soil microorganisms and organic mung bean

Abstract

Silver and titanium dioxide nanoparticles (NPs) are the most produced NPs worldwide due to their antimicrobial properties. These NPs are widely used in many fields including agriculture, where they can inhibit the growth of plant pathogens and stimulate plant growth. However, there are concerns about nanoparticles reaching the environment, as they can also manifest adverse effects. In this study, the effects of TiO2 and two types of Ag NPs (synthesized by chemical reduction and “green” by dextrane) on soil microorganisms and organic beans were investigated. Two different species of rhizospheric soil microorganisms, namely actinomycetes Streptomyces microflavus and nitrogen-fixing soil bacteria Rhizobium sp. were exposed to the low concentration of NPs. For that purpose, microorganisms were grown in nutrient media supplemented with 0.2 mM NPs. After 24 h of exposure, the numbers of viable cells of both tested microorganisms were higher in the samples compared to the controls (media without NPs) for all three NPs, suggesting their stimulatory effect on the soil microorganisms' growth. The highest positive effect was observed for Rhizobium sp. treated with TiO2 NPs (the number of viable cells increased for two orders of magnitude, from ~106 to ~108 CFU/mL). Ag NPs showed equal influence on both tested microorganisms, with dextrane-coated Ag NPs were more effective. In the experiment with organic mung beans, the same colloid solutions of Ag NPs and TiO2 NPs had a stimulating effect on the germination and root length of this plant, also. The number of germinated seeds was the same as in the control, while the average root length was higher in the samples than in the control. The average root length was 1.47 cm in the control. Seeds treated with dextrane-coated Ag NPs had an average root length even 82 % higher compared to the control. The stimulatory effect was also observed in the case of seeds treated with Ag NPs synthesized by chemical reduction and TiO2 NPs, where the average root lengths were higher for 40% and 76 %, respectively than in the control. Germination index values, which are higher than 100%, suggesting that tested nanoparticles can be used as fertilizers in agriculture. Moreover, the obtained results show that silver nanoparticles and titanium dioxide nanoparticles have a stimulatory effect on the soil bacteria and organic mung bean, which prove their safety for this kind of application.