Assessing soil extracellular DNA decomposition dynamics through plasmid amendment coupled with real-time PCR

Abstract

Determining soil extracellular DNA decomposition dynamics is essential to assessing lateral gene transfer possibility, nutrient-cycling efficiency, and the reliability of DNA-based methods for examining microbes in soils. The existing methods based on competent cell transformation and stable isotope probes are generally inefficient and not strictly quantitative. Therefore, this study aimed to establish a rigorously quantitative and efficient approach to monitor the decomposition dynamics of the soil extracellular DNA. A soil was collected from a Tibetan alpine meadow. Extracellular DNA was simulated by modified exogenous plasmids. The plasmid solution was sprayed onto the fresh soil and thoroughly homogenized. Then, the soil was incubated for 4 weeks, during which they were sampled and immediately stored at − 20 °C on days 0, 0.5, 1, 2, 4, 8, 16, and 28 of the incubation. Finally, the total soil DNA was extracted, and the exogenous plasmid copies remained in the soils were determined using real-time PCR. Additionally, another similar experiment was conducted with a sterilized soil to assess the abiotic influences on the changes in the exogenous plasmid copies. In the fresh soil, the exogenous plasmid DNA copies decreased quickly in the first 12 h of the incubation, remained stable in the following 36 h, and gradually dropped to 1.10–5.20% of the initial plasmid copies at sampling time 0 after being incubated for 4 weeks. The variations in the soil plasmid DNA copies fitted well with the modified exponential decay model. As for the sterilized soil, the exogenous plasmid copies remained stable during the 16 day’s incubation. However, they dramatically dropped after being incubated for 28 days, which was probably elicited by the recolonization of microbes in the soil. Collectively, the decrease in the exogenous plasmid DNA copies could be mainly attributed to biological activities. Extracellular DNA can persist in soil for more than 4 weeks. Exogenous plasmid amendment coupled with real-time PCR provides a convenient and rigorously quantitative approach for monitoring extracellular DNA degradation in soils.