Chlorine oxide radical (ClO) enables the enhanced degradation of antibiotic resistance genes during UV/chlorine treatment by selectively inducing base damage.

Compared to individual UV or chlorine disinfection, the combined UV and chlorine (i.e., UV/chlorine) can substantially promote the degradation of antibiotic resistance genes (ARGs) in the effluent by generating radicals. However, the mechanisms of ARG degradation induced by radicals during UV/chlorine treatment remain largely unknown, limiting further enhancement of ARG degradation by process optimization. Herein, we aimed to uncover the role of different radicals in ARG degradation and the molecular mechanisms of ARG degradation by radicals in UV/chlorine process. The ClO was proven to be responsible for the enhanced ARG degradation during UV/chlorine treatment, while the other radicals (OH, Cl, and Cl2-) played a minor role. This is because ClO possessed both high steady-state concentration and high reactivity toward ARGs (rate constant: 4.29 × 1010 M-1 s-1). The ClO might collaborate with free chlorine to degrade ARG. The ClO degraded ARGs by selectively attacking guanine and thymine but failed to induce strand breakage, while chlorine could break the strand of ARGs. Ultimately, ClO cooperated with chlorine to degrade ARGs quickly by hydroxylation and chlorination of bases and produce many chlorine- and nitrogen-containing products as revealed by high-resolution mass spectrometry. The uncovered degradation mechanisms of ARGs by UV/chlorine provide useful guidelines for process optimization to achieve deep removal of effluent ARGs.

Lewis Acid-Base Interaction Triggering Electron Delocalization to Enhance the Photodegradation of Extracellular Antibiotic Resistance Genes Adsorbed on Clay Minerals.

The transformation of extracellular antibiotic resistance genes (eARGs) is largely influenced by their inevitable photodegradation in environments where they tend to be adsorbed by ubiquitous clay minerals instead of being in a free form. However, the photodegradation behaviors and mechanisms of the adsorbed eARGs may be quite different from those of the free form and still remain unclear. Herein, we found that kaolinite, a common 1:1-type clay, markedly enhanced eARG photodegradation and made eARGs undergo direct photodegradation under UVA. The decrease in the transformation efficiency of eARGs caused by photodegradation was also promoted. Spectroscopy methods combined with density functional theory calculations revealed that the Lewis acid-base interaction between P-O in eARGs and Al-OH on kaolinite delocalized electrons of eARGs, thus resulting in increased photon absorption ability of eARGs. This ultimately led to enhanced photodegradation of kaolinite-adsorbed eARGs. Additionally, divalent Ca2+ could reduce the Lewis acid-base interaction-mediated adsorption of eARGs by kaolinite, thereby weakening the enhanced photodegradation of eARGs caused by electron delocalization. In contrast, the 2:1-type clay montmorillonite without strong Lewis acid sites was unable to delocalize the electrons to enhance the photodegradation of eARGs. This work allowed us to better evaluate eARGs' fate and risk in real aqueous environments.

Dissolved organic matter dominating the photodegradation of free DNA bases in aquatic environments.

Free DNA bases are widely present in the environments, and can be utilized by bacteria for their nucleic acids synthesis or as nutrition sources. In sunlit natural waters, these free bases probably undergo photodegradation which would change the bioavailable bases contents. Though the photodegradation of DNA has been investigated, the photodegradation behaviors of free bases may be quite different from those of DNA-confined bases in consideration of their different chemical environments. Herein, the photodegradation of four free bases (guanine, adenine, thymine and cytosine) was investigated. Results show that direct photodegradation of free bases in phosphate buffer caused by UV was slow. However, the photodegradation of these free bases were greatly enhanced in dissolved organic matter (DOM) solution. In the presence of 10-50 mg/L DOM, the photodegradation rates of free bases were increased by 1.85-14.6 times compared to the controls without DOM. DOM could result in indirect photodegradation by producing hydroxyl radical (•OH) and singlet oxygen (1O2) under irradiation, and this indirect photodegradation enhanced and dominated the free bases photodegradation. The •OH was involved in all four bases photodegradation, while the 1O2 only participated in guanine photodegradation. In phosphate buffer, the fastest photodegradation bases were pyrimidine, however, guanine became the fastest photodegradation base in DOM solution due to the selective oxidation of guanine by 1O2. In summary, DOM may be a determinant for free bases photodegradation in natural waters and thereby deeply influence free bases fates in aquatic environments.

Redox state of microbial extracellular polymeric substances regulates reduction of selenite to elemental selenium accompanying with enhancing microbial detoxification in aquatic environments.

In nature, many microorganisms show resistance to toxic selenite by reducing selenite to non-soluble and low toxic elemental selenium. Extracellular polymeric substances (EPS), a high-molecular-weight biopolymers originated from microbial metabolism, contain many reducing groups and can induce reductive transformation of pollutants. However, the roles of EPS and its redox state in reductive detoxification or reduction removal of selenite, respectively, remain unknown yet. Herein, the reduction of selenite by different sources of EPS was investigated. Selenite was proved to be reduced by EPS and partly transformed to elemental selenium. The formed elemental selenium was mainly selenium nanoparticles confirmed by transmission electron microscopy coupled with energy dispersive spectroscopy. The redox state of EPS governed selenite reduction and elemental selenium formation, and the reduced state of EPS was in favor of selenite reduction. Dissolved oxygen concentration in water regulated EPS redox state and influenced selenite reduction. The thiols, aldehyde and phenolic groups in EPS were responsible for selenite reduction. Under selenite stress, EPS was capable of increasing cell survivability by enhancing microorganisms-mediated selenite reduction. This work revealed the previously undiscovered roles of EPS in selenite reduction and elemental selenium formation in aquatic environments and also suggested a possible crucial role of EPS in selenium biogeochemical cycle.

Unrecognized Contributions of Dissolved Organic Matter Inducing Photodamages to the Decay of Extracellular DNA in Waters.

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.

Enhanced Photodegradation of Extracellular Antibiotic Resistance Genes by Dissolved Organic Matter Photosensitization.

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.