Biodegradation Of Glyphosate Research Articles

13C assimilation as well as functional gene abundance and expression elucidate the biodegradation of glyphosate in a field experiment

Glyphosate (N-phosphonomethylglycine; GLP) and its main metabolite AMPA (aminomethylphosphonic acid), are frequently detected in relatively high concentrations in European agricultural topsoils. Glyphosate has a high sorption affinity, yet it can be detected occasionally in groundwater. We hypothesized that shrinkage cracks occurring after dry periods could facilitate GLP transport to greater depths where subsoil conditions slow further microbial degradation. To test this hypothesis, we simulated a heavy rainfall event (HRE) on a clay-rich arable soil. We applied 2.1 kg ha−1 of 100% 13C3, 15N-labeled GLP one day before the simulated rainfall event. Microbial degradation of translocated GLP over a 21-day period was assessed by quantifying 13C incorporation into phospholipid fatty acids. Microbial degradation potential and activity were determined by quantifying the abundance and expression of functional genes involved in the two known degradation pathways of GLP; to AMPA (goxA) or sarcosine (sarc). We confirmed that goxA transcripts were elevated in the range of 4.23 x 105 copy numbers g−1 soil only one day after application. The increase in AMPA associated with a rise in goxA transcripts and goxA-harboring microorganisms indicated that the degradation pathway to AMPA dominated. Based on 13C-enrichment 3 h after the HRE, fungi appeared to initiate glyphosate degradation. At later time points, Gram+-bacteria proved to be the main degraders due to their higher 13C-incorporation. Once GLP reached the subsoil, degradation continued but more slowly. By comparing GLP distribution and its microbial degradation in macropores and in the bulk soil, we demonstrated different time- and depth-dependent GLP degradation dynamics in macropores. This indicates the need for field studies in which soil properties relevant to GLP degradation are related to limiting environmental conditions, providing a realistic assessment of GLP fate in soils.

Microbiomes and glyphosate biodegradation in edaphic and aquatic environments: recent issues and trends.

Glyphosate (N-(phosphonomethyl)glycine) has emerged as the top-selling herbicide worldwide because of its versatility in controlling annual and perennial weeds and the extensive use of glyphosate-resistant crops. Concerns related to the widespread use of glyphosate and its ubiquitous presence in the environment has led to a large number of studies and reviews, which examined the toxicity and fate of glyphosate and its major metabolite, aminomethylphosphonic acid (AMPA) in the environment. Because the biological breakdown of glyphosate is most likely the main elimination process, the biodegradation of glyphosate has also been the object of abundant experimental work. Importantly, glyphosate biodegradation in aquatic and soil ecosystems is affected not only by the composition and the activity of microbial communities, but also by the physical environment. However, the interplay between microbiomes and glyphosate biodegradation in edaphic and aquatic environments has rarely been considered before. The proposed minireview aims at filling this gap. We summarize the most recent work exploring glyphosate biodegradation in natural aquatic biofilms, the biological, chemical and physical factors and processes playing on the adsorption, transport and biodegradation of glyphosate at different levels of soil organization and under different agricultural managements, and its impact on soil microbial communities.

Characterization of a novel glyphosate-degrading bacterial species, Chryseobacterium sp. Y16C, and evaluation of its effects on microbial communities in glyphosate-contaminated soil

Widespread use of the herbicide glyphosate in agriculture has resulted in serious environmental problems. Thus, environment-friendly technological solutions are urgently needed for the removal of residual glyphosate from soil. Here, we successfully isolated a novel bacterial strain, Chryseobacterium sp. Y16C, which efficiently degrades glyphosate and its main metabolite aminomethylphosphonic acid (AMPA). Strain Y16C was found to completely degrade glyphosate at 400 mg·L-1 concentration within four days. Kinetics analysis indicated that glyphosate biodegradation was concentration-dependent, with a maximum specific degradation rate, half-saturation constant, and inhibition constant of 0.91459 d-1, 15.79796 mg·L-1, and 290.28133 mg·L-1, respectively. AMPA was identified as the major degradation product of glyphosate degradation, suggesting that glyphosate was first degraded via cleavage of its C–N bond prior to subsequent metabolic degradation. Strain Y16C was also found to tolerate and degrade AMPA at concentrations up to 800 mg·L-1. Moreover, strain Y16C accelerated glyphosate degradation in soil indirectly by inducing a slight alteration in the diversity and composition of soil microbial community. Taken together, our results suggest that strain Y16C may be a potential microbial agent for bioremediation of glyphosate-contaminated soil.

13c Assimilation as Well as Functional Gene Abundance and Expression Elucidate the Biodegradation of Glyphosate in a Field Experiment

13c Assimilation as Well as Functional Gene Abundance and Expression Elucidate the Biodegradation of Glyphosate in a Field Experiment

Improved oxygen reduction and simultaneous glyphosate degradation over iron phthalocyanine and reduced graphene oxide‒dispersed activated carbon fiber electrodes in a microbial fuel cell

Iron phthalocyanine (FePc) and reduced graphene oxide (rGO)‒dispersed activated carbon fiber (ACF) is used for the first time as electrodes in a microbial fuel cell (MFC) for energy generation and biodegradation of glyphosate in wastewater. The FePc-rGO/ACF is studied through various physicochemical characterization techniques. Electrochemical analysis shows a pseudocapacitive behaviour with consistent redox peaks of FePc-rGO/ACF, and a 4 e ‒ pathway for oxygen reduction reaction at the cathode. High maximum current and power densities (∼8050 mA m ˗2 and 1101 mW m ˗2 , respectively) are measured for the whole MFC, with simultaneous ∼80% degradation of glyphosate (30 mg L ˗1 ) and 79% reduction in chemical oxygen demand at the anode. The improved performance of MFC is attributed to the synergistic effects of FePc and rGO and biocompatibility of the material. The present study has successfully demonstrated the FePc-rGO/ACF electrode‒based MFC to be a green bioelectrochemical device for efficient bioelectricity generation and biodegradation of emerging herbicides in wastewater. • Successful inclusion of FePc in rGO and their effective coating over ACF. • FePc-rGO/ACF supported the formation of a dense and uniform biofilm. • 80% degradation of PMG and 79% COD reduction achieved in MFC. • The MFC based on FePc-rGO/ACF generated a high power density of 1101 mW/m 2 . • 4 e ‒ pathway indicated for ORR at the cathode of the MFC.

Glyphosate Pollution Treatment and Microbial Degradation Alternatives, a Review.

Glyphosate is a broad-spectrum herbicide extensively used worldwide to eliminate weeds in agricultural areas. Since its market introduction in the 70’s, the levels of glyphosate agricultural use have increased, mainly due to the introduction of glyphosate-resistant transgenic crops in the 90’s. Glyphosate presence in the environment causes pollution, and recent findings have proposed that glyphosate exposure causes adverse effects in different organisms, including humans. In 2015, glyphosate was classified as a probable carcinogen chemical, and several other human health effects have been documented since. Environmental pollution and human health threats derived from glyphosate intensive use require the development of alternatives for its elimination and proper treatment. Bioremediation has been proposed as a suitable alternative for the treatment of glyphosate-related pollution, and several microorganisms have great potential for the biodegradation of this herbicide. The present review highlights the environmental and human health impacts related to glyphosate pollution, the proposed alternatives for its elimination through physicochemical and biological approaches, and recent studies related to glyphosate biodegradation by bacteria and fungi are also reviewed. Microbial remediation strategies have great potential for glyphosate elimination, however, additional studies are needed to characterize the mechanisms employed by the microorganisms to counteract the adverse effects generated by the glyphosate exposure.

A Systematic Study on the Microbial Degradation of Glyphosate: A Review

One of the most widely used herbicides for the control of annual and perennial weeds and grass worldwide is Glyphosate. Extensive use of this herbicide resulted in its ubiquitous presence in the environment and has an adverse effect on both human health and the ecosystem. The rapid binding of this herbicide to the soil particles makes it hazardous even for the soil organisms. There are several treatment technologies such as photocatalytic degradation, microbial approach or adsorption that removes glyphosate from the environment. Among both biotic and abiotic approaches, microbial degradation is the most effective and eco-friendly alternative for glyphosate removal. The microorganisms have the ability to use glyphosate as the sole source of carbon, phosphorus or nitrogen. There are several reviews pertaining to the fate and toxicity of both glyphosate and its intermediates while reviews on the biodegradation of glyphosate and its metabolic pathway are scarce. This minireview presents a collective summary on the environmental fate of glyphosate, different available treatment technologies for remediation from the environment. It focuses on the comparative assessment of the degradation of glyphosate by various methods considering their advantages and drawbacks.

Glyphosate Biodegradation Potential in Soil Based on Glycine Oxidase Gene (thiO) from Bradyrhizobium.

Despite the intensive use of glyphosate(GP) and its ubiquitous presence in the environment, studies addressing the presence of microbial genes involved in glyphosate degradation in natural conditions are scarce. Based on the agronomical importance of Bradyrhizobium genus and its metabolic versatility, we tested the hypothesis that species or genotypes of Bradyrhizobium could be a proxy for GP degrader potential in soil. A quantitative PCR assay was designed to target a specific region of the glycine oxidase gene (thiO), involved in the oxidation of glyphosate to AMPA, from known sequences of Bradyrhizobium species. The abundance of the thiO gene was determined in response to herbicide application in soils with different GP exposure history both under field and microcosm conditions. The gene coding for RNA polymerase subunitB (rpoB) was usedas a reference for the abundance of total Bradyrhizobia. The assay using the designed primers was linear over a very large concentration range of the target and showed high efficiency and specificity. In a field experiment, there was a differential response related to the history of glyphosate use and the native Bradyrhizobium genotypes. In a soil without previous exposure to herbicides, thiO gene increased over time after glyphosate application with most genotypes belonging to the B. jicamae and B. elkanni supergroups. Conversely, in an agricultural soil with more than 10 years of continuous glyphosate application, the abundance of thiO gene decreased and most genotypes belonged to B. japonicum supergroup. In a microcosm assay, the amount of herbicide degraded after a single application was positively correlated to the number of thiO copies in different agricultural soils from the Pampean Region. Our results suggest that Bradyrhizobium species are differently involved in glyphosate degradation, denoting the existence of metabolically versatile microorganisms which can be explored for sustainable agriculture practices. The relationship between the abundance of thiO gene and the GP degraded in soil point to the use of thiO gene as a proxy for GP degradation in soil.

Binding interaction of glyphosate with glyphosate oxidoreductase and C–P lyase: Molecular docking and molecular dynamics simulation studies

Widespread application of glyphosate poses a threat to living organisms. Microbial strains are able to degrade glyphosate via contrasting metabolic pathways with the help of enzymes. Glyphosate oxidoreductase (GOX) and C–P lyase are the key enzymes for the biodegradation of glyphosate and its intermediate metabolite aminomethylphosphonic acid (AMPA) in microbes. The microbial degradation of glyphosate has been reported, but the underlying molecular mechanism is still unclear. Therefore, in this study, the interaction mechanism of GOX and C–P lyase with glyphosate and AMPA were investigated by using molecular docking and molecular dynamics (MD) simulations. The results indicate that glyphosate contacts with the active site of GOX and C–P lyase by hydrogen bonds as well as hydrophobic and van der Waals interactions in aqueous solution to maintain its stability. The presence of glyphosate and AMPA in the active site significantly changes the conformation of GOX and C–P lyase. The results of the MD simulations confirm that GOX and C–P lyase complexes are stable during the catalytic reaction. This study offers a molecular level of understanding of the expression and function of GOX and C–P lyase for the bioremediation of glyphosate.

A new spectrophotometric method for the determination of glyphosate: statistical optimization and application in biodegradation studies

Glyphosate (N-phosphonomethylglycine) is extensively used as herbicide against annual and perennial weeds. In the present study, we report a new colorimetric method for the estimation of glyphosate which is based on formation of copper dithiocarbamic acid complex from glyphosate. Glyphosate formed yellow colored complex (λmax = 372 nm) when it was reacted with carbon disulfide (CS2) and copper(II)nitrate in alkaline medium. Using Box–Behnken design (BBD), glyphosate, CS2, copper(II) nitrate, and reaction time were found to influence the absorbance at 372 nm significantly (p < 0.05). The quadratic model developed for the absorbance at 372 nm fitted well to the data (p < 0.0001, F = 21.62, R2= 0.96, adjusted R2= 0.92, predicted R2= 0.75). The optimum concentration of glyphosate, CS2, copper (II) nitrate, and reaction time for maximum absorbance were found to be 500 µg/mL, 1.0% (v/v), 150 µg/mL, and 60 min, respectively. The predicted value of absorbance at 372 nm (0.450) agreed fairly well with the actual value of 0.480. The proposed method obeyed Beer’s law up to 500 µg/mL and showed limit of detection (LOD) and limit of quantitation (LOQ) of 28.9 and 96.4 µg/mL, respectively, with R2of 0.9947. Glycine, aminomethylphosphonic acid (AMPA), cultivation medium components, and soil extract did not interfere in the color complex formation. Using the method, under shake flask conditions, a bacterial isolate Stenotrophomonas maltophilia GP-1 exhibited biodegradation efficiency of 84% after 100 h suggesting that the above method is suitable for study of biodegradation of glyphosate.

Probabilistic indicators for soil and groundwater contamination risk assessment

Abstract Deterministic assessments of whether, when, and where environmental safety thresholds are exceeded by pollutants are often unreliable due to uncertainty stemming from incomplete knowledge of the properties of environmental systems and limited sampling. We present a global sensitivity analysis to rank the contribution of uncertain parameters to the probability, P, of a target quantity to exceed user-defined environmental safety thresholds. To this end, we propose a new index (AMAP) which quantifies the impact of a parameter on P and can be readily employed in probabilistic risk assessment. We apply AMAP, along with existing moment-based sensitivity indices, to quantify the sensitivity of soil and aquifer contamination following herbicide glyphosate (GLP) dispersal to soil hydraulic parameters. Target quantities are GLP and its toxic metabolite aminomethylphosphonic acid (AMPA) concentrations in the top soil as well as their leaching below the root zone. The global sensitivity analysis encompasses six scenarios of managed water amendments and rainfall events. The biodegradation of GLP and AMPA varies slightly across scenarios, while leaching below the root zone is greatly affected by the assumed hydrologic boundary conditions. AMAP shows that, among the tested uncertain parameters, absolute permeability, air-entry suction, and porosity have the greatest impact on GLP and AMPA probability to pollute the aquifer by exceeding the aqueous concentration thresholds. Our results show that AMAP is effective to thoroughly explore time histories arising from model-based predictions of environmental pollution hazards. The proposed methodology may support informed decision making in risk assessments and help assessing ecological indicators through threshold-based analyses.

Sorption-desorption isotherms and biodegradation of glyphosate in two tropical soils aged with eucalyptus biochar

ABSTRACT The objective of this study was to evaluate the sorption-desorption process and biodegradation of glyphosate in two tropical soils aged with biochar derived from eucalyptus. The biochar aging period was 30 d. There was little difference between the amounts of sorbed glyphosate in Ultisol (96.8, 96.8 and 96.4%) and Alfisol (97.1, 97.5 and 97.4%) soils that were unamended or amended with biochar aged for 0 or 30 d, respectively. Similar amounts of desorbed herbicide occurred in Ultisol (3.3, 3.3 and 3.4%) and Alfisol (4.1, 4.2 and 3.9%) soils, respectively. The degradation time half-life (DT50) of glyphosate in Ultisol unamended and initial amended were higher (38 and 36 d, respectively) than DT50 in the amended soil with 30 d of biochar aging (27 d); and in the Alfisol DT50 was higher in unamended soil (38 d), and similar in soil unamended at 0 and 30 d of biochar aging (21 and 26 d, respectively). The addition of biochar to two tropical soils or its aging did not have any effect on the sorption and desorption of glyphosate and its biodegradation in relation to the unamended soils, and it can did not affect the transport and persistence of this herbicide in soil.

Toxicity, Biodegradability and Detection Methods of Glyphosate; the Most Used Herbicide: A Systematic Review

Introduction: Glyphosate is known as the most used world's herbicides and contradictions exist over its classification as a probably carcinogenic for the human. This study aimed to review the newest evidences in toxicity, biodegradability and detection methods of glyphosate. Materials and Methods: To conduct this systematic review, databases such as Scopus, Web of Science, PubMed, and Google Scholar were searched to extract studies on the non-target toxicity, biodegradability and detection methods of glyphosate from 2000 to 2018. The applied key words included glyphosate, herbicide, biodegradation, and bio decomposition. The number of articles retrieved and reviewed was 84 and 23, respectively. Results: Glyphosate could cause endocrine disrupting effects, dermal irritation, embryo toxicity, electrolyte abnormalities, apoptosis, cardiovascular collapse, teratogenicity, and mutagenic effects. High-performance liquid chromatography, UV-visible spectroscopy, gas chromatography/ mass spectrometry, and ion-exchange liquid chromatography were techniques used for detecting glyphosate in soil and water. The biodegradation of glyphosate was performed by various bacteria and fungi microorganisms. Conclusions: Given the high consumption and low rates of biodegradation of glyphosate, more attention should be paid to its toxicity potential in the human's environment and health

Kinetic study of the biodegradation of glyphosate by indigenous soil bacterial isolates in presence of humic acid, Fe(III) and Cu(II) ions

The present study was executed to investigate the kinetics of the biodegradation of glyphosate in an aqueous medium. Three bacterial strains (Streptomyces sp., Bacillus subtilis, and Rhizobium leguminosarum) with excellent glyphosate-degrading capabilities were isolated from the agricultural soils and characterized on the basis of 16S ribosomal RNA (rRNA) gene sequencing methods. Biodegradation of glyphosate was performed with and without applications of metal ions [Fe(III) and Cu(II)] and humic acid (HA). In thirty days experiment, three strains were utilized 85-90% (with half life periods from 8.36 to 9.12 days) of glyphosate. Applications of Fe(III) and Cu(II) have inhibited the biodegradation to 70-80% (with half life periods from 10.03 to 11.15 days). In the presence of HA, biodegradation was inhibited significantly (57-70%) with half life periods from 12.54 to 13.09 days. Overall, glyphosate’s biodegradation inhibition order under different conditions was as HA > Cu(II) > Fe (III). UV-visible and mass spectrometric results indicated two concurrent pathways of degradation of glyphosate with major metabolites included glycine, aminomethylphosphonic acid (AMPA), sarcosine, glyoxylate, metaphosphoric acid and phosphate. Glyphosate breakdown by these three strains was achieved by the C-P lyase and the glyphosate oxidoreductase activity. Hence, these strains could be used as potential biological agents in the effective biodegradation campaign for other pesticide-contaminated sites.

Polyphosphate recovery by a native Bacillus cereus strain as a direct effect of glyphosate uptake

Seven bacterial strains isolated from a glyphosate-exposed orange plantation site were exposed to 1 mM N-(phosphonomethyl)glycine supplied as a phosphorus source. While some exhibited good biodegradation profiles, the strain 6 P, identified as Bacillus cereus, was the only strain capable of releasing inorganic phosphate to the culture supernatant, while accumulating polyphosphate intracellularly along the experimentation time. The composition and purity of the intracellular polyphosphate accumulated by the strain 6 P were confirmed by FTIR analysis. To date, the biological conversion of glyphosate into polyphosphate has not been reported. However, given the importance of this biopolymer in the survival of microorganisms, it can be expected that this process could represent an important ecological advantage for the adaptation of this strain to an ecological niche exposed to this herbicide. The polyphosphate production yield was calculated as 4 mg l−1, while the glyphosate biodegradation kinetic constant was calculated on 0.003 h−1 using the modified Hockey–Stick first-order kinetic model, with a half-life of 279 h. Our results suggest that B. cereus 6 P is a potential candidate for the generation of an innovative biotechnological process to produce polyphosphate through the biodegradation of the herbicide glyphosate.

Meta-analysis of glyphosate contamination in surface waters and dissipation by biofilms

One consequence of the intensive use of glyphosate is the contamination of rivers by the active substance and its metabolites aminomethyl phosphonic acid (AMPA) and sarcosine, inducing river eutrophication. Biofilms are the predominant lifestyle for microorganisms in rivers, providing pivotal roles in ecosystem functioning and pollutant removal. The persistence of glyphosate in these ecosystems is suspected to be mostly influenced by microbial biodegradation processes.The present study aimed to investigate the tripartite relationship among biofilms, phosphorus and glyphosate in rivers. The first part consists of a co-occurrence analysis among glyphosate, AMPA and phosphorus using an extensive dataset of measurements (n = 56,198) from French surface waters between 2013 and 2017. The second part investigated the capacity of natural river biofilms to dissipate glyphosate, depending on phosphorus availability and the exposure history of the biofilm, in a microcosm study.A strong co-occurrence among glyphosate, AMPA and phosphorus was found in surface waters. More than two-thirds of samples contained phosphorous with glyphosate, AMPA or both compounds. Seasonal fluctuations in glyphosate, AMPA and phosphorus concentrations were correlated, peaking in spring/summer shortly after pesticide spreading. Laboratory experiments revealed that natural river biofilms can degrade glyphosate. However, phosphorus availability negatively influenced the biodegradation of glyphosate and induced the accumulation of AMPA in water. An increase in alkaline phosphatase activity and phosphorus uptake was observed in glyphosate-degrading biofilms, evidencing the tight link between phosphorus limitation and glyphosate degradation by biofilms.The results of the present study show that phosphorus not only is a key driver of river eutrophication but also can reduce complete glyphosate degradation by biofilms and favour the accumulation of AMPA in river water. The predominant role of biofilms and the trophic status of rivers must therefore be considered in order to better assess the fate and persistence of glyphosate.

Microcosm experiments and kinetic modeling of glyphosate biodegradation in soils and sediments

Glyphosate (GLP) is one of the most widely-used herbicides globally and its toxicity to humans and the environment is controversial. GLP is biodegradable, but little is known about the importance of site exposure history and other environmental variables on the rate and pathway of biodegradation. Here, GLP was added to microcosms of soils and sediments with different exposure histories and these were incubated with amendments of glucose, ammonium, and phosphate. GLP concentrations were measured with a newly-developed HPLC method capable of tolerating high concentrations of ammonium and amino acids. GLP biodegradation occurred after a lag-time proportional to the level of GLP pre-exposure in anthropogenically-impacted samples (soils and sediments), while no degradation occurred in samples from a pristine sediment after 180 days of incubation. Exposure history did not influence the rate of GLP degradation, after the lag-time was elapsed. Addition of C, N, and P triggered GLP degradation in pristine sediment and shortened the lag-time before degradation in other samples. In all microcosms, GLP was metabolised into aminomethylphosphonic acid (AMPA), which was highly persistent, and thus appears to be a more problematic pollutant than GLP. Bacterial communities changed along the gradients of anthropogenic impacts, but in some cases, taxonomically very-similar communities showed dramatically different activities with GLP. Our findings reveal important interactions between agriculturally-relevant nutrients and herbicides.

Glyphosate dispersion, degradation, and aquifer contamination in vineyards and wheat fields in the Po Valley, Italy

Biodegradation of glyphosate (GLP) and its metabolite aminomethylphosphonic acid (AMPA) was numerically assessed for a vineyard and a wheat field in the Po Valley, Italy. Calculation of the Hazard Quotient suggested that GLP and AMPA can pose a risk of aquifer contamination in the top 1.5 m depth within 50 years of GLP use. Numerical results relative to soil GLP and AMPA concentrations, and GLP age, half life, and turnover time show that GLP was equivalently removed through hydrolysis and oxidation, but the latter produced AMPA. Biodegradation processes in the root zone removed more than 90% of applied GLP and more than 23% of the produced AMPA between two consecutive applications. Doubling organic carbon availability enhanced GLP and AMPA biodegradation, especially GLP hydrolysis to sarcosine. This work highlights that GLP and AMPA removal is controlled by soil water dynamics that depend on ecohydrological boundary conditions, and by carbon sources availability to biodegraders.

Kinetic study on biodegradation of glyphosate with unacclimated activated sludge

ABSTRACTThis article is concerned with the study of biodegradation of an organophosphorus herbicide (glyphosate) using unacclimated activated sludge. Glyphosate at different concentrations (0.1, 0.5, 1, 2 and 5 g/L) was tested for cellular growth. On the other hand, the effect of glyphosate on its own biodegradation was studied by evaluating the fittings of different kinetic models (Andrews, Aiba et al., Han and Levenspiel, Luong, Tessier, Webb, Tseng and Wayman, Yano and Koga). According to the obtained results, the activated sludge was able to use glyphosate as the sole carbon source; however, 2 and 5 g/L glyphosate seemed to inhibit cellular growth. Moreover, glyphosate at initial concentrations of 0.1, 0.5 and 1 g/L was completely degraded within 4, 13 and 18 h of incubation, respectively. Yano and Koga model was the best-fit model (R2 = 0.999, F = 173,106 and P = 0.000006).

High Permeation Rates in Liposome Systems Explain Rapid Glyphosate Biodegradation Associated with Strong Isotope Fractionation.

Bacterial uptake of charged organic pollutants such as the widely used herbicide glyphosate is typically attributed to active transporters, whereas passive membrane permeation as an uptake pathway is usually neglected. For 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes, the pH-dependent apparent membrane permeation coefficients (Papp) of glyphosate, determined by nuclear magnetic resonance (NMR) spectroscopy, varied from Papp (pH 7.0) = 3.7 (±0.3) × 10–7 m·s–1 to Papp (pH 4.1) = 4.2 (±0.1) × 10–6 m·s–1. The magnitude of this surprisingly rapid membrane permeation depended on glyphosate speciation and was, at circumneutral pH, in the range of polar, noncharged molecules. These findings point to passive membrane permeation as a potential uptake pathway during glyphosate biodegradation. To test this hypothesis, a Gram-negative glyphosate degrader, Ochrobactrum sp. FrEM, was isolated from glyphosate-treated soil and glyphosate permeation rates inferred from the liposome model system were compared to bacterial degradation rates. Estimated maximum permeation rates were, indeed, 2 orders of magnitude higher than degradation rates of glyphosate. In addition, biodegradation of millimolar glyphosate concentrations gave rise to pronounced carbon isotope fractionation with an apparent kinetic isotope effect, AKIEcarbon, of 1.014 ± 0.003. This value lies in the range typical of non-masked enzymatic isotope fractionation demonstrating that glyphosate biodegradation was not subject to mass transfer limitations and glyphosate exchange across the cell membrane was rapid relative to enzymatic turnover.