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.
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.
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.
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