Evidence for positive response of soil bacterial community structure and functions to biosynthesized silver...

The environmental impacts of biosynthesized nanoparticles on the soil bacterial community assemblage and functions are not sufficiently understood. Given the broad application of silver nanoparticles (AgNPs), the present study aims to reveal the effects of biosynthesized AgNPs (~12 nm) on the soil bacterial community structure and functions. Specifically, we used a quantitative real-time PCR (qPCR) approach to quantify the relative abundance of bacterial taxon/group and representative functional genes (AOA, AOB, NirK, NirS, NosZ, and PhoD). Results showed high relative abundance of Actinobacteria (1.53 × 10⁷, p = 0.000) followed by Alphaproteobacteria (1.18 × 10⁶, p = 0.000) and Betaproteobacteria (2.01 × 10⁶, p = 0.000) in the soil exposed to biosynthesized AgNPs (100 mg/kg soil) after 30 days of treatment. Bacteroidetes group was observed to be negatively affected by AgNPs treatment. In the case of functional genes abundance, more pronounced impact was observed after 30 days of application. The biosynthesized AgNPs treatment accounted for significant increase in the relative abundance of all targeted functional genes except NirS. We conclude that the biosynthesized AgNPs did not cause toxic effects on nitrifiers, denitrifiers and organic phosphorus metabolizing bacterial community. While AgNO₃ caused higher toxicity in the soil bacterial community structure and function. Based on our findings, we propose two key research questions for further studies; (i) is there any adaptation strategy or silver resistance embraced by the soil microbial community? and (ii) are biosynthesized nanoparticles environmentally safe and do not pose any risk to the soil microbial community? There is a necessity to address these questions to predict the environmental safety of biosynthesized AgNPs and to apply appropriate soil management policies to avoid nanotoxicity. Since this study provides preliminary evidence for the positive response of the soil bacterial community structure and functions to biosynthesized AgNPs, additional investigations under different soil conditions with varying soil physico-chemical properties are required to authenticate their environmental impact.