Extracellular antibiotic resistance genes (eARGs) contribute to antibiotic resistance, and as such, they pose a serious threat to human health. eARGs, regarded as an emerging contaminant, have been widely detected in various bodies of water. Degradation greatly weakens their distribution potential and environmental risks. Dissolved organic matter (DOM), mainly consisted of humic substances, carbohydrates, and organic acids, is ubiquitous in diverse waters and significantly affects the degradation of coexisting contaminants. However, the photodegradation of eARGs in natural water, especially regarding the roles of DOM in this process, remains unknown. Herein, we investigated the eARGs photodegradation in waters with and without DOM. Illumination has been found to effectively photodegrade eARGs, and this process was significantly enhanced by DOM. Further experiments revealed that photosensitization of DOM produced hydroxyl radicals (•OH) to enhance plasmid strand breaks and produced singlet oxygen (1O2) to accelerate the guanine oxidation, which in turn promoted the photodegradation of plasmid-carried eARGs. Transformation assays indicated that eARGs transformation efficiencies were reduced after their photodegradation. The presence of DOM accelerated the decreases of eARGs transformation efficiencies under illumination. DOM concentration and some ions (e.g., NO3 –, NO2 –, HCO3 –, Br–, and Fe3+) affected •OH or 1O2 levels, further influencing the photodegradation of eARGs. Overall, eARGs photodegradation in aquatic environments is a crucial process both in the reduction of eARGs concentrations and in transformation efficiencies. This work facilitated us to better understand the fate of eARGs in waters.

Organic phosphorus (OP), one of the main forms of phosphorus in effluent from biological wastewater treatment plants, may contribute to the bioavailable phosphorus pool as well as water eutrophication. However, little is known about the photomineralization of OP or the possible impacts on the phosphorus cycle in water bodies. Herein, the photomineralization of effluent OP was investigated. An increase in orthophosphate concentration was observed under illumination. The 31P liquid nuclear magnetic resonance spectra demonstrated that the release of orthophosphate resulted from photomineralization of OP. Furthermore, the photoproduced hydroxyl radicals (·OH) were proved to play a dominant role in the OP photomineralization. Nitrate, effluent organic matter (EfOM), and Fe­(III) presented in effluent were the main chromophores for ·OH photoproduction, and their contributions to ·OH production and photomineralization of OP followed the order: nitrate > EfOM > Fe­(III). Additionally, the carbonate (or bicarbonate) in the effluent and high pH were unfavorable for OP photomineralization. The present study revealed the photomineralization behavior of OP in actual effluent, suggesting that photomineralization of OP might contribute to eutrophication and may play a non-negligible role in phosphorus turnover in water bodies.

Extracellular DNA (eDNA), which is derived from lysis or secretion of cells, is ubiquitous in various environments and crucial for gene dissemination, bacterial metabolism, biofilm integrity, and aquatic monitoring. However, these processes are largely influenced by damage to eDNA. Photodamage to eDNA, one of the most important types of DNA damage in natural waters, thus far remains unclear. In particular, the roles of the ubiquitous dissolved organic matter (DOM) in this process have yet to be determined. In this study, eDNA photodamage, including both deoxynucleoside damage and strand breaks, proved to be significantly influenced by DOM. DOM competed with eDNA for photons to inhibit the direct photodamage of eDNA. Nevertheless, DOM was photosensitized to produce reactive oxygen species (ROS) (i.e., hydroxyl radicals (·OH) and singlet oxygen (1O2)) to enhance the indirect photodamage of eDNA. The ·OH induced damage to four deoxynucleosides and strand breaks, and the 1O2 substantially enhanced deoxyguanosine damage. The presence of DOM changed the main photodamage products of deoxynucleosides, additional oxidation products induced by ROS formed besides pyrimidine dimers caused by UV. Results indicate that DOM-mediated indirect photodamage contributed significantly to eDNA photodamage in most water bodies. This study revealed the previously unrecognized crucial role of DOM in the decay of eDNA in waters.