Abstract
Extracellular DNA (eDNA) cannot be effectively removed by most of the existing wastewater treatment technologies and can contribute to the gain of new functional traits when transformed into competent bacteria present in downstream environments. This study evaluates the contributions of solar and UV-C-254nm irradiation to the transformation of eDNA in Acinetobacter baylyi ADP1. Solar irradiation was evaluated because it is a natural environmental stressor to which eDNA would be exposed during wastewater reuse. UV-C-254nm was evaluated as an alternative to a chlorine-based disinfection strategy. Our findings showed that solar disinfection increased the natural transformation frequency by up to 2.0-fold after irradiance at 153 mJ/cm(2). This was largely mediated by reactive oxygen species generation, which was correlated with an upregulation of both DNA repair (recA and ddrR) and competence (comA and pilX) genes. In contrast, even though UV-C-254nm, exposure was accompanied by the upregulation of DNA repair (recA, ddrR, and uvrB) genes and, hence, possibly higher integration rates of eDNA, we observed a concentration-dependent decrease in transformation rates. This decrease in transformation was likely due to the UV dimerization of eDNA, which resulted in the integration of damaged genes that cannot be transcribed into any functional gene products. These results imply that even though sunlight stimulates eDNA uptake and integration in the natural environment, UV disinfection implemented at a treatment plant can potentially minimize subsequent detrimental effects by damaging the extracellular genetic material and ensuring that there is no substantial expression of these transformed genes.