Do antibiotics have environmental side-effects? Impact of synthetic antibiotics on biogeochemical processes

International audienceAntibiotic use in the early 1900 vastly improved human health but at the same time started an arms race of antibiotic resistance. The widespread use of antibiotics has resulted in ubiquitous trace concentrations of many antibiotics in most environments. Little is known about the impact of these antibiotics on microbial processes or “non-target” organisms. This mini-review summarizes our knowledge of the effect of synthetically produced antibiotics on microorganisms involved in biogeochemical cycling. We found only 31 articles that dealt with the effects of antibiotics on such processes in soil, sediment, or freshwater. We compare the processes, antibiotics, concentration range, source, environment, and experimental approach of these studies. Examining the effects of antibiotics on biogeochemical processes should involve environmentally relevant concentrations (instead of therapeutic), chronic exposure (versus acute), and monitoring of the administered antibiotics. Furthermore, the lack of standardized tests hinders generalizations regarding the effects of antibiotics on biogeochemical processes. We investigated the effects of antibiotics on biogeochemical N cycling, specifically nitrification, denitrification, and anammox. We found that environmentally relevant concentrations of fluoroquinolones and sulfonamides could partially inhibit denitrification. So far, the only documented effects of antibiotic inhibitions were at therapeutic doses on anammox activities. The most studied and inhibited was nitrification (25–100 %) mainly at therapeutic doses and rarely environmentally relevant. We recommend that firm conclusions regarding inhibition of antibiotics at environmentally relevant concentrations remain difficult due to the lack of studies testing low concentrations at chronic exposure. There is thus a need to test the effects of these environmental concentrations on biogeochemical processes to further establish the possible effects on ecosystem functionin

Environmental Controls on Nitrogen and Sulfur Cycles in Surficial Aquatic Sediments

Enhanced anthropogenic inputs of nitrogen (N) and sulfur (S) have disturbed their biogeochemical cycling in aquatic and terrestrial ecosystems. The N and S cycles interact with one another through competition for labile forms of organic carbon between nitrate-reducing and sulfate-reducing bacteria. Furthermore, the N and S cycles could interact through nitrate (NO3-) reduction coupled to S oxidation, consuming NO3-, and producing sulfate (SO42-). The research questions of this study were: (1) what are the environmental factors explaining variability in N and S biogeochemical reaction rates in a wide range of surficial aquatic sediments when NO3- and SO42- are present separately or simultaneously, (2) how the N and S cycles could interact through S oxidation coupled to NO3- reduction, and (3) what is the extent of sulfate reduction inhibition by nitrate, and vice versa. The N and S biogeochemical reaction rates were measured on intact surface sediment slices using flow-through reactors. The two terminal electron acceptors NO3- and SO42- were added either separately or simultaneously and NO3- and SO42- reduction rates as well as NO3- reduction linked to S oxidation were determined. We used redundancy analysis, to assess how environmental variables were related to these rates. Our analysis showed that overlying water pH and salinity were two dominant environmental factors that explain 58% of the variance in the N and S biogeochemical reaction rates when NO3- and SO42- were both present. When NO3- and SO42- were added separately, however, sediment N content in addition to pH and salinity accounted for 62% of total variance of the biogeochemical reaction rates. The SO42- addition had little effect on NO3- reduction; neither did the NO3- addition inhibit SO42- reduction. The presence of NO3- led to SO42- production most likely due to the oxidation of sulfur. Our observations suggest that metal-bound S, instead of free sulfide produced by SO42- reduction, was responsible for the S oxidation

Exposure to vancomycin causes a shift in the microbial community structure without affecting nitrate reduction rates in river sediments

International audienceAntibiotics and antibiotic resistance genes have shown to be omnipresent in the environment. In this study, we investigated the effect of vancomycin (VA) on denitrifying bacteria in river sediments of a Waste Water Treatment Plant, receiving both domestic and hospital waste. We exposed these sediments continuously in flow-through reactors to different VA concentrations under denitrifying conditions (nitrate addition and anoxia) in order to determine potential nitrate reduction rates and changes in sedimentary microbial community structures. The presence of VA had no effect on sedimentary nitrate reduction rates at environmental concentrations, whereas a change in bacterial (16S rDNA) and denitrifying (nosZ) community structures was observed (determined by polymerase chain reaction-denaturing gradient gel electrophoresis). The bacterial and denitrifying community structure within the sediment changed upon VA exposure indicating a selection of a non-susceptible VA population

Nitrifying Kinetics and the Persistence of Nitrite in the Seine River, France

International audienceAlthough a higher oxidation rate for nitrite than for ammonia generally prevents nitrite accumulation in oxic waters, nitrite concentrations in the Seine River (1-31 mM) exceed European norms. We investigated the kinetics of in situ ammonia-and nitrite-oxidizing communities in river water and wastewater treatment plant (WWTP) effluents to determine the role of pelagic nitrification in the origin and persistence of nitrite downstream of Paris. The main source of nitrite is the major Parisian WWTP, and its persistence, up to tens of kilometers downstream of the plant, is explained by low ammonia and nitrite oxidation rates and high river flow. Furthermore, similar nitrite and ammonia oxidation rates preclude a rapid consumption of both preexisting nitrite and nitrite produced by ammonia oxidation. Maximum ammonia oxidation rates are two to three times higher downstream than upstream of the WWTP, indicating the input of ammonia oxidizers and ammonia from the WWTP. In both river water and WWTP effluents, nitrite oxidizers were unable to oxidize all available nitrite. In the human-impacted Seine River, this phenomenon might be due to mixotrophy. This study highlights the low resilience of the river to nitrite contamination as well as the importance of managing nitrite, nitrifiers, and organic matter concentrations in WWTP effluents to avoid nitrite persistence in rivers

Estuarine benthic nitrate reduction rates: Potential role of microalgae?

International audienceThe ecological functioning of the Seine estuary is strongly affected by the input of nitrogen, especially in the form of nitrate, which contributes to the eutrophication of the Seine Bight (France). Elimination of nitrate by benthic denitrification in riparian zones or adjacent wetlands could significantly improve the water quality of the Seine estuary. The goal of this study was to investigate the potential for denitrification and the factors affecting these rates. To this end, we measured nitrate reduction and ammonium production rates using flow-through reactors in contrasted sediments collected along the Seine Estuary. Sediment and organic carbon characteristics (organic C, Corg:N ratio, bioavailable carbon, extracellular polymeric substances (EPS), chlorophyll a and phaeopigments and abundance of nitrogen transforming microorganisms were determined and related to the potential nitrate reduction rates. Nitrate reduction rates showed a large spatial and seasonal variation and showed a significant correlation with sediment phaeopigments, whereas overall microbial activity (ammonium production rates) were highly correlated to chlorophyll a and EPS fractions. Surprisingly, bacterial abundance was not correlated to nitrate reduction nor to ammonium production rates. The presence of microalgae appears to be an important driver for nitrate reduction rates in these riparian sediments and seems to have fueled the benthic nitrate reducing activity

How mineral induced antibiotic transformation products impact bacterial growth and denitrification activity

International audienceThe abiotic transformations of quinolones and tetracyclines facilitated by redox-active minerals has been studied extensively, however limited information is available regarding the antimicrobial activity and toxicity of their resultant transformation products. In this study, we first investigated the mechanisms underlying the transformation of two commonly used antibiotics, ciprofloxacin (CIP) and tetracycline (TC), by the ubiquitous redox soil mineral, birnessite (MnO2). Subsequently, we evaluated the impact of these transformation products on both the growth and activity of the environmental denitrifier Pseudomonas veronii. Following the reaction with birnessite, four transformation products for CIP and five for TC were identified. Remarkably, the antibacterial activity of both CIP and TC was lost upon the formation of transformation products during their interaction with birnessite. This loss of antimicrobial efficacy was associated with specific chemical transformations, such as the opening of the piperazine ring for CIP and hydroxylation and demethylation for TC. Interestingly, denitrifying activity, quantified in terms of nitrate reduction rates, remained unaffected by both CIP and TC at low concentrations that did not impact bacterial growth. However, under certain conditions, specifically at low concentrations of CIP, the second step of denitrification-nitrite reduction-was hindered, leading to the accumulation of nitrite. Our findings highlight that the transformation products induced by the mineral-mediated reactions of CIP or TC lose the initial antibacterial activity observed in the parent compounds. This research contributes valuable insights into the intricate interplay between antibiotics, redox-active minerals, and microbial activity in environmental systems

Modelling the fate of nitrite in an urbanized river using experimentally obtained nitrifier growth parameters

International audienceMaintaining low nitrite concentrations in aquatic systems is a major issue for stakeholders due to nitrite's high toxicity for living species. This study reports on a cost-effective and realistic approach to study nitrite dynamics and improve its modelling in human-impacted river systems. The implementation of different nitrifying biomasses to model riverine communities and waste water treatment plant (WWTP)-related communities enabled us to assess the impact of a major WWTP effluent on in-river nitrification dynamics. The optimal kinetic parameters and biomasses of the different nitrifying communities were determined and validated by coupling laboratory experiments and modelling. This approach was carried out in the Seine River, as an example of a large human-impacted river with high nitrite concentrations. The simulation of nitrite fate was performed at a high spatial and temporal resolution (Δt = 10 min, View the MathML sourcedx¯ = 500 m) including water and sediment layers along a 220 km stretch of the Seine River for a 6-year period (2007–2012). The model outputs were in good agreement with the peak of nitrite downstream the WWTP as well as its slow decrease towards the estuary. Nitrite persistence between the WWTP and the estuary was mostly explained by similar production and consumption rates of nitrite in both water and sediment layers. The sediment layer constituted a significant source of nitrite, especially during high river discharges (0.1–0.4 mgN h−1 m−2). This points out how essential it is to represent the benthic layer in river water quality models, since it can constitute a source of nitrite to the water-column. As a consequence of anthropogenic emissions and in-river processes, nitrite fluxes to the estuary were significant and varied from 4.1 to 5.5 TN d−1 in low and high water discharge conditions, respectively, over the 2007–2012 period. This study provides a methodology that can be applied to any anthropized river to realistically parametrize autochthonous and WWTP-related nitrifier communities and simulate nitrite dynamics. Based on simulation analysis, it is shown that high spatio-temporal resolution hydro-ecological models are efficient to 1) estimate water quality criteria and 2) forecast the effect of future management strategies. Process-based simulations constitute essential tools to complete our understanding of nutrient cycling, and to decrease monitoring costs in the context of water quality and eutrophication management in river ecosystems

Chronic exposure of river sediments to environmentally relevant levels of tetracycline affects bacterial communities but not denitrification rates

International audienceThe effects of tetracycline (TC) at chronic sub-inhibitory exposure concentrations on benthic denitrification rates and bacterial communities were explored. River sediments were continuously exposed to different TC concentrations (0.5, 20 and 10,000 mu g L-1) for 2 weeks in flow-through reactors allowing denitrification and bacterial growth conditions. Bacterial communities were fingerprinted by Denaturing Gradient Gel Electrophoresis of 16S rRNA gene amplification products. Cultivable denitrifiers enriched from the sediment were tested for TC resistance (2-128 mg L-1). Denitrification rates were unaffected by exposure to TC, regardless of concentration. In contrast, the bacterial community composition changed significantly from sub-inhibitory (ng-mu g L-1) to therapeutic (mg L-1) exposure concentrations. Furthermore the cultivable denitrifiers showed a high TC sensitivity (< 4 mg L-1). Maintenance of efficient benthic denitrification rates, even at the highest level of TC exposure most likely originated from an adaptation of the autochthonous bacterial community where dominant species become those that acquire, or already have resistance to antibiotics

Analytical pitfalls when using inhibitors in specific nitrification assays

International audienceCharacterisation of the reaction steps involved in nitrification can help determine the processes that produce potentially harmful environmental pollutants such as nitrite, nitrate and nitrous oxide (N2O). The use of nitrification inhibitors can uncouple the reactions and therefore assist in their mechanistic and isotopic characterisation. However, nitrification inhibitors can interfere with the methods for determining the concentrations and stable isotope ratios of ammonium, nitrite and nitrate. The interference of allylthiourca, hydrazine or sodium chlorate in colorimetric methods and stable isotope measurements were assessed. Ammonium concentrations were measured with the salicylate method. Nitrite and nitrate were measured with the Griess reaction, with nitrate first being reduced to nitrite with vanadium (III) chloride. For the stable isotope analysis, nitrite was reduced to N2O in a 1 : 1 sodium azide and acetic acid buffer solution; preceded, when necessary, by ammonium oxidation to nitrite by hypobromite or nitrate reduction to nitrite on an activated cadmium column. Sodium chlorate did not interfere with any of the analyses and none of the inhibitors interfered with the stable isotope ratios determination of nitrate. Allylthiourea interfered with ammonium and nitrate quantification. Both allylthiourea and hydrazine also clearly interfered in the determination of the nitrogen stable isotope ratio of ammonium, while only allylthiourea interfered in the determination of nitrogen and oxygen stable isotope ratios of nitrite. Although we suggest methods to overcome some of these interferences, our study demonstrated that the analytical methods used in combination with allylthiourea or hydrazine as nitrification inhibitors should be considered with caution when designing experiments

Carbon availability limits potential denitrification in watercress farm sediment

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