Evaluating response mechanisms of soil microbiomes and metabolomes to Bt toxin additions

The accumulation and persistence of Bt toxins in soils from Bt plants and Bt biopesticides may result in environmental hazards such as adverse impacts on soil microorganisms. However, the dynamic relationships among exogenous Bt toxins, soil characteristics, and soil microorganisms are not well understood. Cry1Ab is one of the most commonly used Bt toxins and was added to soils in this study to evaluate subsequent changes in soil physiochemical properties, microbial taxa, microbial functional genes, and metabolites profiles via 16S rRNA gene pyrosequencing, high-throughput qPCR, metagenomic shotgun sequencing, and untargeted metabolomics. Higher additions of Bt toxins led to higher concentrations of soil organic matter (SOM), ammonium (NH⁺₄-N), and nitrite (NO₂^--N) compared against controls without addition after 100 days of soil incubation. High-throughput qPCR analysis and shotgun metagenomic sequencing analysis revealed that the 500 ng/g Bt toxin addition significantly affected profiles of soil microbial functional genes involved in soil carbon (C), nitrogen (N), and phosphorus (P) cycling after 100 days of incubation. Furthermore, combined metagenomic and metabolomic analyses indicated that the 500 ng/g Bt toxin addition significantly altered low molecular weight metabolite profiles of soils. Importantly, some of these altered metabolites are involved in soil nutrient cycling, and robust associations were identified among differentially abundant metabolites and microorganisms due to Bt toxin addition treatments. Taken together, these results suggest that higher levels of Bt toxin addition can alter soil nutrients, probably by affecting the activities of Bt toxin-degrading microorganisms. These dynamics would then activate other microorganisms involved in nutrient cycling, finally leading to broad changes in metabolite profiles. Notably, the addition of Bt toxins did not cause the accumulation of potential microbial pathogens in soils, nor did it adversely affect the diversity and stability of microbial communities. This study provides new insights into the putative mechanistic associations among Bt toxins, soil characteristics, and microorganisms, providing new understanding into the ecological impacts of Bt toxins on soil ecosystems.