Effects of antibiotics on microbial nitrogen cycling and N2O emissions: A review.

Sulfonamides, quinolones, tetracyclines, and macrolides are the most prevalent classes of antibiotics used in both medical treatment and agriculture. The misuse of antibiotics leads to their extensive dissemination in the environment. These antibiotics can modify the structure and functionality of microbial communities, consequently impacting microbial-mediated nitrogen cycling processes including nitrification, denitrification, and anammox. They can change the relative abundance of nirK/norB contributing to the emission of nitrous oxide, a potent greenhouse gas. This review provides a comprehensive examination of the presence of these four antibiotic classes across different environmental matrices and synthesizes current knowledge of their effects on the nitrogen cycle, including the underlying mechanisms. Such an overview is crucial for understanding the ecological impacts of antibiotics and for guiding future research directions. The presence of antibiotics in the environment varies widely, with significant differences in concentration and type across various settings. We conducted a comprehensive review of over 70 research articles that compare various aspects including processes, antibiotics, concentration ranges, microbial sources, experimental methods, and mechanisms of influence. Antibiotics can either inhibit, have no effect, or even stimulate nitrification, denitrification, and anammox, depending on the experimental conditions. The influence of antibiotics on the nitrogen cycle is characterized by dose-dependent responses, primarily inhibiting nitrification, denitrification, and anammox. This is achieved through alterations in microbial community composition and diversity, carbon source utilization, enzyme activities, electron transfer chain function, and the abundance of specific functional enzymes and antibiotic resistance genes. These alterations can lead to diminished removal of reactive nitrogen and heightened nitrous oxide emissions, potentially exacerbating the greenhouse effect and related environmental issues. Future research should consider diverse reaction mechanisms and expand the scope to investigate the combined effects of multiple antibiotics, as well as their interactions with heavy metals and other chemicals or organisms.

Critical role of multiple antibiotics on the denitrification rate in groundwater: Field investigative proof.

The impact of antibiotics on denitrification has emerged as a significant topic; however, there is a dearth of mechanistic understanding regarding the effects of multiple antibiotics at the ng/L level on denitrification in groundwater. This study conducted five field samplings between March 2019 and July 2021 at two representative monitoring wells. The investigation utilized metagenomic sequencing to unveil the antibiotic mechanisms influencing denitrification. Results revealed the detection of 16 out of 64 antibiotics, with a maximum detection frequency and total concentration of 100 % and 187 ng/L, respectively. Additionally, both nitrate and total antibiotic concentrations exhibited a gradual decrease along the groundwater flow direction. Metagenomic evidence indicated that denitrification served as the dominant biogeochemical process controlling nitrate attenuation in groundwater. However, the denitrification capacity experienced significant inhibition in the presence of multiple antibiotics at the ng/L level. This inhibition was attributed to decreases in the relative abundance of dominant denitrifying bacteria (Candidatus_Scalindua, Herminiimonas and unclassified_p_Planctomycetes) and denitrifying functional genes (narGH, nirKS and norB), signifying the pressure exerted by antibiotics on denitrifying bacteria. The variation in antibiotic concentration (∆Cantibiotics) indicated a change in antibiotic pressure on denitrifying bacteria. A larger ∆Cantibiotics corresponded to a greater rebound in the relative abundance of denitrifying functional genes, resulting in a faster denitrification rate (Kdenitrification). Field observations further demonstrated a positive correlation between Kdenitrification and ∆Cantibiotics. Comparatively, a higher Kdenitrification observed at higher ∆Cantibiotics was primarily due to the enrichment of more nondominant denitrifying bacteria carrying key denitrifying functional genes. In conclusion, this study underscores that multiple antibiotics at the ng/L level in groundwater inhibited denitrification, and the degree of inhibition was closely related to ∆Cantibiotics.

Solution, exchangeable and fixed ammonium in natural diatomite as a simulated PRB material: effects of adsorption and bioregeneration processes.

Ammonia nitrogen (NH4+-N) is widely found in aquifers with strong reducibility or poor adsorptivity as a dissolved inorganic nitrogen pollutant. The application of adsorbents with effective long-term in situ bioregeneration as permeable reactive barrier (PRB) media for nitrogen removal has raised concern. In this study, the advantage of natural diatomite as a PRB material was investigated by exploring its NH4+-N adsorption and desorption characteristics, and the ability of diatomite and zeolite to be loaded nitrifying bacteria was also compared. The results showed that the exchangeable ammonium from chemical-monolayer adsorption was the main form of NH4+-N and was adsorbed by diatomite. Moreover, the adsorption process was limited with a maximum adsorption capacity of 0.677 mg g-1. However, diatomite demonstrated an excellent loading of aerobic-heterotrophic microorganisms, even stronger than zeolite. Compared with zeolite reactors, a higher OD600 value of nitrifiers, a faster NH4+-N degradation rate and more abundant functional genes were observed during the bioregeneration process of diatomite. Both the solution and exchangeable ammonium forms were bioavailable, and the regeneration of diatomite was more than 80.0% after two days. Moreover, desorption-biodegradation was systematically analysed to determine the bioregeneration mechanism of diatomite. Diatomite with good regeneration ability can be used as a competitive alternative to address sudden nitrogen pollution.

Prioritization of antibiotic contaminants in China based on decennial national screening data and their persistence, bioaccumulation and toxicity.

The potential adverse impacts of antibiotic contamination on environmental quality are generating increasing concern. Given that an alarming amount and variety of antibiotics have been used in China, a list of priority antibiotics is urgently needed to develop regulatory frameworks to control antibiotic use and monitor environmental pollution. This study established a new method of ranking priority antibiotics based on their prevalence (Pv), occurrence (O), persistence, and bioaccumulation, and toxicity (PBT) in the environment. The Pv and O criteria were weighted and quantified using the decennial national screening datasets (>15,000 concentration values for 105 candidate antibiotics in eight environmental compartments), and quantitative structure-activity relationships were used to estimate PBT values. A total of 26 high priority antibiotics were identified using the PvOPBT method, including 8 quinolones, 5 sulfonamides, 5 macrolides, 4 tetracyclines, 3 from other classes, and 1 unclassified antibiotic. For individual antibiotic classes, the ß-lactams and aminoglycosides were ranked from no priority to low priority, whereas the macrolides and tetracyclines were ranked from medium to high priority. Although the PvOPBT ranking scores for the aqueous and solid phases demonstrated an apparent difference for some candidate antibiotics, eighteen antibiotics were ranked as high priority in both aqueous phases and solid phases and are suggested as the top priorities worthy of immediate attention. These top priority antibiotics are primarily utilized in animal husbandry within China. Therefore, urgent action is needed to limit the use of these top priority antibiotics in the animal industry.

Effects of norfloxacin on nitrate reduction and dynamic denitrifying enzymes activities in groundwater.

The impact of antibiotics on denitrification has attracted widespread attention recently. Norfloxacin, as a representative of fluoroquinolone antibiotics, is extensively detected in groundwater. However, whether the release of norfloxacin into the groundwater poses potential risks to denitrification remains unclear. In this study, effect of norfloxacin on denitrification was investigated. The results showed that increasing norfloxacin from 0 to 100 µg/L decreased nitrate removal rate from 0.68 to 0.44 mg/L/h, but enhanced N2O emission by 177 folds. Additionally, 100 µg/L of norfloxacin decreased nitrite accumulation by 50.6%. Corresponding inhibition of norfloxacin on bacterial growth, carbon source utilization, electron transport system activity and genes expression was revealed. Furthermore, denitrifying enzyme dynamic monitoring results showed that norfloxacin inhibited nitrate reductase activity, and enhanced nitrite reductase activity to some extent in denitrification process, which was consistent with the variations of nitrate and nitrite. Meanwhile, sensitivity analysis demonstrated that nitrate reductase was more easily affected by norfloxacin than nitrite reductase. Overall, this study suggests that multiple regulation of denitrifying enzyme activity contributes to evaluating the comprehensive effects of antibiotics on groundwater denitrification.

The role of different functional groups in a novel adsorption-complexation-reduction multi-step kinetic model for hexavalent chromium retention by undissolved humic acid.

Undissolved humic acid (HA) has a great retention effect on the migration of hexavalent chromium [Cr(VI)] in soil, and HA functional groups play a predominant role in this process. However, the coupled mode between Cr(VI) retention and HA functional groups reaction is still unclear. In this study, it was found that a fair amount of Cr on HA existed in the forms of ion exchangeable and binding Cr(VI) during the reaction resulting from the ion exchange adsorption and complexation of Cr(VI). According to the results of two-dimensional correlation spectroscopic analysis (2DCOS), HA functional groups participated in the reaction with Cr(VI) in the order of carboxyl ≈ chelated carboxyl > phenol > polysaccharide > methyl, and all the functional groups were more likely to be located at aromatic domains. Based on the results of XPS spectra, rather than to be oxidized by Cr(VI), carboxyl more tended to be complexed by chromium, which is regarded as the precondition for Cr(VI) reduction. Phenol, polysaccharide and methyl with distinct reaction activities successively acted as major electron donors for Cr(VI) reduction in different reaction stages. Consequently, it was determined that the retention of Cr(VI) by undissolved HA followed an adsorption-complexation-reduction mechanism, and based on this, a multi-step kinetic model with multiple types of complexation/reduction sites was developed to simulate the retention processes resulting in a much better fitting effect (R2 > 0.99) compared with traditional first-order and second-order kinetic models (R2 < 0.95). This demonstrated that the multi-step kinetic model is of great potential in accurately simulating the migration and transformation of Cr(VI) in soil environment.

Mechanism study of humic acid functional groups for Cr(VI) retention: Two-dimensional FTIR and 13C CP/MAS NMR correlation spectroscopic analysis.

Undissolved humic acid (HA) is known to substantially effect the migration and transformation of hexavalent chromium [Cr(VI)] in soils. The mechanisms of Cr(VI) retention in soils by undissolved HA have been reported; however, past studies are inconclusive about the types of HA functional groups that are involved in Cr(VI) retention and the retention mechanisms. Utilizing a two-dimensional correlation spectroscopy (2DCOS) analysis for FTIR and 13C CP/MAS NMR, this study investigated the variations of HA function groups and molecular structures after reactions with aqueous Cr(VI) under different pH conditions. Based on the changing sequence of functional groups interpreted from the 2DCOS results, a four-step mechanism for Cr(VI) retention was determined as follows: (1) electrostatic adsorption of Cr(VI) to HA surface, (2) complexation of adsorbed Cr(VI) by carboxyl and ester, (3) reduction of complexed Cr(VI) to Cr(III) by phenol and polysaccharide, and (4) complexation of reduced Cr(III) by carboxylic groups. These functional groups that are involved in Cr(VI) retention were determined to occur in aromatic domains.