Published online 8 September 2005
Published in J Environ Qual 34:1746-1754 (2005)
DOI: 10.2134/jeq2004.0182
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Bioremediation and Biodegradation
Transformation of Benzothiazole in Estuarine Sediments
W. James Catalloa,* and
T. Junkb
a Laboratory for Ecological Chemistry, CBS Department, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803
b Department of Chemistry, University of Louisiana, Monroe, LA 71209
* Corresponding author (jcatallo{at}mail.vetmed.lsu.edu)
Received for publication May 11, 2004.
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ABSTRACT
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Benzothiazole (BT) is a natural and synthetic compound occurring in aquatic sediments and wastewater. The purpose of this work was to investigate BT biogeochemistry in controlled Eh/pH microcosms (CEPMs) containing estuarine sediments of different particle sizes (coarse, intermediate, fine) under oxidized and reduced conditions vs. killed controls, and tide simulation mesocosms (TSMs) containing plants and meiofauna under well-drained (oxidized), consistently saturated/flooded (reduced), and tidal (alternating oxidized/reduced) conditions. Benzothiazole was transformed into complex product mixtures under all conditions. Benzothiazole transformation rates in CEPMs were slower under reduced conditions vs. oxidized conditions in the fine- and intermediate-grain sediments, but the same in the coarse sediment. Quiescent (unstirred) CEPMs showed greatly reduced BT transformation rates in all sediments, with half-lives on the order of 2200 to >4000 h (unstirred) vs. 640 to 1000 h in the continuously stirred systems. The TSM data showed that tidal and drained systems processed BT at identical rates, far exceeding those observed in statically flooded (reduced) TSMs. Mixing was found to be a more significant variable in BT transformation rate than either Eh or sediment particle size breakdown, with constant stirring increasing observed degradation appreciably. Otherwise, BT was transformed more readily on sediments of high surface area under oxidized conditions vs. coarser sediments and those under reducing electrochemical conditions.
Abbreviations: BT, benzothiazole • CEPM, controlled Eh/pH microcosm • TSM, tide simulation mesocosm
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INTRODUCTION
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A RANGE OF HETEROARENE COMPOUNDS containing nitrogen, sulfur, and oxygen are found in natural sediments and water, and at elevated levels near industrial activities, urban centers, and waste sites (Warshawsky, 1992; Bieko et al., 1987; Catallo et al., 1990; Turov et al., 1987; Furlong and Carpenter, 1982; Krone et al., 1986; Thompkins and Ho, 1982; Malins et al., 1985; Chou and Bohonos, 1979; Smith et al., 1977, 1978; Catallo, 1996). Benzothiazole (BT, I, CAS #95-16-9) is found in a broad range of natural and synthetic mixtures (Bieko et al., 1987; Catallo, 1996; Reddy and Quinn, 1997; Li et al., 2001; Seo et al., 2002; Reemtsma et al., 2002; Bellavia et al., 2000; Lopez et al., 1999; Tale, 2002; Kashiyama et al., 1999; Jaycox and Olsen, 2000; Chien et al., 2003; Kumata et al., 2000).
Benzothiazole (Fig. 1)
and several of its derivatives occur in crude oils and other geochemical mixtures (Catallo, 1996; Li et al., 2001). Other BT derivatives are biologically active and of interest in pharmacology and cancer biology (Kashiyama et al., 1999). Benzothiazole and the 2-mecaptoBT are used as vulcanization accelerators in polymer production and occur in automobile tires, asphalt, crumb rubber, plastics and their volatiles, and pyrogenic products (Reddy and Quinn, 1997; Jaycox and Olsen, 2000; Chien et al., 2003; Kumata et al., 2000). Some of these (e.g., 4-morpholinyl-2-BT) have left distinguishable geochronological signatures near urban industrial activity (Kumata et al., 2000). Benzothiazole also occurs in foodstuffs such as wines, dairy products, and parsley preparations (Bellavia et al., 2000; Lopez et al., 1999). Several BT derivatives are double photonic and/or nonlinear optical materials of possible value in high speed memory and information retrieval (Zheng et al., 2001; Abbotto et al., 2002). Benzothiazole and some of its derivatives have been identified in lacustrine, riverine, and estuarine water globally (Kumata et al., 2000; Scott et al., 1996, Louter et al., 1994; Bester et al., 1997; Bester, 1998), and as major constituents of organic nitrogen in agricultural soils (Leinweber and Schulten, 1998).
Recent work has provided detail on fate and transformation profiles as well as assessments of the significance of BT in the environment. Reddy and Quinn (1997) concluded that the benzothiazoles would be unlikely to adsorb to particles, settle to sediments, or be accumulated in organisms, while others (Catallo, 1996; Bester, 1998) conclude that BT should adsorb preferentially to sediments and organic mater and be less available for microbial and other transformation vs. solution. The predicted and measured log Kow of BT is 2.1 to 2.4 at pH 7, and this indicates a strong preference for hydrophobic environments (surfaces, lipids, micelles). At pH 7, the protonation and consequent increase of water solubility of the benzothiazolium ion BTH+ is trivial; the acid–base dissociation constant of BT/BTH+ is low (e.g., pKa of thiazole is 2.44, pKa of BT is 1.83; Fig. 1). Also, the BT molecule is polarizable and has a substantial quadrupolar moment which may contribute to its partitioning with surfaces and hydrophobic phases. Further, there is a lot of work on degradation of hydrocarbons, heterocycles, and PAHs in estuarine and marine sediments which suggests that, all other factors remaining the same, increasing log Kow is inversely proportional to observed transformation rates (Perrin, 1972; Perrin et al., 1981; Leo et al., 1971; Catallo, 1996; DeLaune et al., 1981; Hansch and Leo, 1995; Hansch et al., 2003). So, in absence of electrochemical or other reaction giving rise to stable water or more lipid soluble BT species, the model above suggests that BT would be expected to behave like a hydrocarbon of similar molecular weight: it should bind to sediment surfaces two orders of magnitude more than the partitioning with water. Further, BT in hydrophobic compartments and on particle surface would be expected to be sequestered from (relatively) rapid transformation in the aqueous phase.
The purpose of this work was to study BT in estuarine sediments under realistic electrochemical and hydrologic conditions and to resolve and identify to some degree the product mixtures resulting from biogeochemical transformation of BT. Both goals are germane to the use of BT and its homologs as anthropogenic (Kumata et al., 2000) or natural (Li et al., 2001) geochemical markers, remediation of BT in wastewater and slurries, and the significance of exotic products from chemical and biotransformation of BT.
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MATERIALS AND METHODS
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Benzothiazole and Synthesis of Deuterated Benzothiazole Analogs
Isotopic dilution mass spectrometry offers high precision and accuracy with respect to analyte concentration in extracts. It was expected that observed differences in BT transformation would be small between time points, and perhaps subtle also between treatments. As a result, resources were devoted to synthesis and analytical method development for BT and expected transformation products.
Benzothiazole is metallated readily in the 2 position by treatment with n-butyl lithium, and quenching with deuterium oxide furnishes benzothiazole-2d (Junk et al., 1997). The preparation BT-4,5,6,7d was done by cyclizing aniline-d7 to mercaptobenzothiazole, followed by oxidation to the sulfinic acid, which was permitted to decompose. Aniline-d7 was prepared by nitration and subsequent catalytic reduction with deuterium, or from aniline, by isotope exchange. For the latter approach, rapid access was achieved using supercritical D2O under alkaline conditions (Junk et al., 1997). Other compounds synthesized for use as MS standards included 2-mercaptoBT, 2-methylBT, 2-methylthioBT, bis-BT, and 2-aniloBT.
Sediments
Bulk sediment-water samples were collected from brackish (salinity 18–23 g/L) wetland areas in Bay St. Louis (Mississippi) using a box core. These were passed through 1-mm sieves to remove detritus and other solids. Sediment particle sizes were determined using a hydrometer method (Catallo, 1996) which gave mass percent distributions in three particle size ranges (Table 1). The three sediment mixes used were designated I, II, and III to reflect respectively the fine, intermediate, and coarser distributions. The relative amount of total surface area in Sediments I, II, and III was approximately 5:2:1 (Table 1). The three sediments were added to estuarine water and sterile artificial sea water (15 g/L, Instant Ocean; Marineland, Moorpark, CA) at 1:8 sediment to water ratios (w/w). Fresh ground smooth cordgrass (Spartina alterniflora Loisel.) plant and seed material was added to give an organic matter level of 5 wt. %. Benzothiazole was added to the sediments neat or as a concentrated stock solution in acetone followed by extended shaking and mixing. "Killed controls" were established by aseptic addition of BT solutions to sediment slurries previously sterilized by 
-radiation (2.5 Mrad) from a submerged Co60 source (Catallo, 1999). Oxidized and reduced conditions in the killed control sediments were established as above before irradiation, so that the sterilized sediments would have chemical conditions similar to normal oxidized and reduced sediments, in the absence of microflora and fauna.
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Table 1. Measured particle size breakdown (mass in each size range) and calculated total surface area for each experimental sediment treatment.
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Environmental Micro- and Mesocosms
Three systems were established to study BT biogeochemistry in estuarine sediment-water (Fig. 2)
: (i) stirred, controlled Eh/pH microcosms (CEPMs) for determination of redox potential and total particle surface area effects on BT transformation rates and product mixtures (Catallo, 1996, 1999); (ii) quiescent (unstirred) CEPMs, established to determine the effects of stirring on BT transformation; and (iii) tide simulation mesocosms (TSMs) with plants, meiofauna, and simulated diurnal tide hydrology for BT fate assessment under more realistic environmental conditions. The sediments used in these test systems were handled so as to preserve the natural estuarine microbial community sampled at the time of collection, without contamination.
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Fig. 2. Schematic diagrams of the controlled Eh/pH microcosms (CEPMs, top) and tide simulation mesocosms (TSMs, bottom) used in the current study. Top: (1) gas (air nitrogen) inlet; (2) Pt working electrode (two per CEMP); (3) saturated calomel reference electrode; (4) Agar/KCl salt bridge; (5) thermometer; (6) solid and liquid phase volatile organic traps; (7) combination pH electrode; (8) Teflon stir bar. Bottom: (1) light banks with timers; (2) plants; (3) sediment; (4) agricultural mesh and burlap; (5) oyster shells; (6) pumps; (7) cobble stones; (8) reservoirs; (9) simulated tide range; (10) 300-gallon enclosure; (11) data loggers; (12) shielded wire lead bundles; (13) data output to computer (BNC).
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Controlled Eh/pH Microcosms
Controlled Eh/pH microcosms (Fig. 2, top) were established for each of the three estuarine sediments (i.e., I–III). A CEPM is a 4-L first-order feedback control system that can utilize constant stirring and analog signal monitoring of Eh at Pt electrodes in the sediment slurry. The slurries were brought to a desired Eh range by intermittent bubbling of air (for oxidized, aerobic conditions) or N2 (for reducing, methanogenic conditions) through the slurry. Air and nitrogen were introduced to the systems automatically to maintain the oxidized or reduced conditions (respectively), and this was governed by a continuous feedback control loop including a potentiometric cell, readout, relay, and gas manifold. The Eh in each system was maintained within a preselected range of potentials, that is, "oxidized": +100 to +300 mV vs. SCE (saturated KCl-calomel electrode), and "reduced": –300 to –500 mV vs. SCE. These conditions are found in aerobic and anaerobic sediments in coastal wetlands (Catallo, 1999). After Eh conditions were established for each sediment in the CEPMs, BT was added and Eh, pH, and temperature data were acquired at 12-h intervals for 105 to 166 d. Identical CEPMs were established for Sediments I–III, but were not continuously stirred. Instead these systems were mixed briefly once daily with a Teflon propeller. Samples were taken from the slurries in the CEPMs using glass syringes with 10 gauge spikes. The samples then were processed immediately for extraction and chemical analyses (below). Concentration reported are corrected for volatilized BT, which was captured in C18 solid phase vapor traps arranged on the CEPM gas outlet ports. All CEPM experiments, stirred and quiescent, included killed controls to monitor BT chemical transformations in the absence of microflora. All stirred sand, silt, and clay CEPMs and TSMs were run in duplicate with each sampled at each time point. Psuedo-first order rate constants were estimated for the duplicate stirred CEPMs by calculating the slope of the regression Ln(Ct/Ci) versus time, where Ci is initial BT concentration and Ct is the concentration at time t. The unstirred CEPMs and the killed controls were run singly.
Tide Simulation Mesocosms
The designs of the few published tidal systems (e.g., Beyers and Odum, 1993) provided insight in the development of these TSMs (Catallo, 1999; Catallo and Junk, 2003) (Fig. 2, bottom). The approach was to keep moving parts to a minimum, strive for simplicity of design so that failure modes would be limited, constrain costs, and use available components. The TSMs used in the current study were 1135 L (300 gallons). Each system has a pump controlled by an on–off timer. The pump is pulsed so as to move water out of the mesocosm into elevated storage tanks over a 12-h period simulating a falling tide. When the storage tanks are full, a siphon is activated, which returns the water to the mesocosm with simple flow restriction used to control the recharge time (12 h). The other managed input was artificial sunlight, which shone for 10 h per day followed by 14 h of relative darkness. The lights were synchronized with normal daytime. The mesocosms were located in a laboratory annex containing no climate control and a large wall fan open to the outside environment. Sediments and plants (smooth cordgrass) were collected from Louisiana salt marshes and were replaced with greenhouse-reared smooth cordgrass. Only the dominant plant was collected: no attempt was made to reproduce the vegetative diversity of the marsh by collecting the other typical salt marsh species. The plants were transplanted to six TSMs which comprised duplicates of each hydrologic condition to be examined: flooded-anaerobic (oxygen deficient) with water exchanged weekly; drained-aerobic (well oxygenated) with water flushed into the system weekly; and tidal (pulsed by a simulated diurnal tide). Data indicated that the tidal sediments alternated between mildly anaerobic and aerobic during the course of the of the flood drain cycle. The "ocean water" used was a commercial material consisting of evaporated seawater for aquaria inoculated with authentic estuarine water at 20% by volume. These systems were run continuously for 2 yr allowing the annual grass smooth cordgrass to complete its entire growth cycle including producing viable seeds and extensive root mass.
Microbial Inocula
Mixed microbial populations were obtained from 4 L of estuarine water which was centrifuged at 2000 rpm. The resulting pellets were resuspended in 100 mL sterile artificial seawater (18 g/L w/w) amended with a trace inorganic nutrients mixture. Aliquots of this were added to the same seawater/trace salts mixture in conical tubes containing 0 (control) to 1.0 mL of BT as a submerged droplet. These were incubated at 27°C and checked daily for turbidity, changes in BT appearance, obvious microbial growth (determined visually and light microscope), and BT transformation by extraction and gas chromatography–mass spectrometry (GC–MS) analysis (below).
Chemical Analysis
Sediment slurries were collected from stirred CEPMs at regular intervals using a syringe with a 12 gauge stainless steel needle, amended with surrogate standards 4-fluorobiphenyl and hexachlorobenzene, and liquid-liquid extracted with dichloromethane (DCM). Sediments from the TSMs were collected with a clean glass corer, amended with the isotopic dilution (ID) standard BT-4,5,6,7d (above) and micro Soxhlet extracted. These extracts were dried with anhydrous Na2SO4. The volume was reduced and then analyzed by GC–MS for BT and its products. Semi-quantitation was achieved on the CEPM slurry samples using deuterated internal standards. Full quantitation was obtained on BT recovered from the TSM core samples by the ratio of the peak MS integral corresponding to BT (at m/z 135) to the corresponding peak integral from the ID standard, BT-4,5,6,7d (m/z 139). The BT and the ID standard were baseline resolved. The GC–MS conditions were: Shimadzu (Kyoto, Japan) 5000 GC–MS and HP (Palo Alto, CA) 5890 GC-5871 MSD; full scan mode 50–500 daltons, DB-5 30-m quartz capillary, temperature profile 50°C/5 min, 4°C/min ramp to 250°C/5 min. Mass spectra of resolved BT products were interpreted by (i) comparison with authentic standards, (ii) interpretation of molecular ions and logical fragment losses, and (iii) reference to mass spectra from published libraries and databases.
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RESULTS AND DISCUSSION
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Transformation data for BT from duplicate CEPMs containing the three experimental sediments and the corresponding unstirred CEPMs are shown in Fig. 3 and 4
, respectively. Stirring increased the observed transformation of BT by approximately 2- to >10-fold, depending on the sediment type. Basically, BT was reduced to <50% of its original concentration in the stirred CEPMs by 0900 h in all sediment types and redox conditions (Fig. 3). It is clear from Fig. 3 and Table 2 that BT was transformed in stirred marine sediments, with rates significantly higher under oxidized conditions except for the course-grained Sediment III, which showed similar transformation rates under both oxidized and reduced conditions. Under oxidized conditions, the rate of BT transformation is well-correlated to surface area; a threefold increase in apparent transformation rate was observed with a fivefold increase in particle surface area for Sediment I versus III (Tables 1 and 2). Under reduced conditions in stirred slurries, BT was transformed at a rate equivalent to BT transformation in the oxidized Sediment III, irrespective of grain size. Both trends were unexpected: many heteroarene, aromatic, and PAH compounds show slower rates of transformation as particle surface area increases and redox potentials decrease (Catallo, 1996, 1999; DeLaune et al., 1981). Inspection of the data from the unstirred CEPMs indicates that only the low surface area Sediments II and III under reduced conditions exhibited BT transformation of 
50% within 2400 h, with the other approximate half times of >3600 h (Fig. 4).
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Fig. 3. Benzothiazole transformation profiles in the continuously stirred controlled Eh/pH microcosms (CEPMs) for Sediments I–III under oxidized and reduced conditions.
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Fig. 4. Benzothiazole transformation profiles in the unstirred controlled Eh/pH microcosms (CEPMs) for Sediments I–III under oxidized and reduced conditions.
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Table 2. Apparent psuedo-first order rate constants for of benzothiazole (BT) transformation in duplicate stirred controlled Eh/pH microcosms (CEPMs).
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Data from the TSMs (Fig. 5)
showed the same general redox effect as the CEPMs: the tidal and drained sediments processed BT nearly identically (Fig. 5), while very little transformation was observed under flooded/reduced conditions over the same time frame. On the other hand, oxidized (drained and tidal) TSM sediments transformed BT far more than the corresponding unstirred oxidized CEPMs (Fig. 4). The reason for this could be related to the effects of bioturbation and plant rhizosphere oxidation in the drained TSMs, which were not present in the unstirred CEPMs.
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Fig. 5. Benzothiazole transformation profiles in the tide simulation mesocosms (TSMs) under drained (oxidized), flooded (reduced), and tidal (pulsed) conditions by isotopic dilution mass spectrometry.
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Product analyses from the microcosms treated with BT showed complex suites of transformation products, many of which were of higher molecular weight than the starting material (Fig. 6 and 7)
. Interestingly, the BT transformation products were very similar in all sediments and redox conditions. The major products included aniline, N-methylaniline, alkylated benzothiazoles, oxygenated benzothiazoles, addition products (bisbenzothiazole, anilobenzothiazole), and high nitrogen compounds (e.g., triphenylguanidine) (Fig. 6). There also was evidence of suites of glucoaldehyde substituted BTs (based on interpretation of mass spectra and reference library matches). Some of the alkylated benzothiazoles also were identified in killed controls, indicating that they were products of chemical rather than biological transformation (Fig. 6 and 7). The results of in vitro metabolic studies using mixed inocula from estuarine water showed that although benzothiazole as a sole organic C/N source supported the growth of some bacterial species, there was only a limited number of distinct products that accumulated in culture medium vs. sterile controls, for example, benzothiazole-2-one and 2-hydroxybenzothiazole (De Wever and Verachtert, 1994, 2001; Haroune et al., 2002, Reemtsma et al., 2002). These product distributions were considerably simpler than those obtained in sediments, presumably containing a wider array of microflora and possible sites for abiotic transformation.
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Fig. 6. Chemical structures for selected benzothiazole (BT) transformation products found in sediment extracts in both live treatments and killed controls.
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CONCLUSIONS
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The current work has shown that BT is transformed in estuarine sediments, yielding mixtures of degradation products (phenol, aniline, N-methylaniline) and BT derivatives including structural analogs of biologically active BT compounds (e.g., aniloBT), as well as suites of higher molecular weight compounds including the aldehydes, alkyl-BTs, bisbenzothiazoles, and anilobenzothiazoles. Benzothiazole degradation was fastest in stirred CEPMs, intermediate in drained and tidal TSMs, and slowest under reduced conditions in all sediments. Benzothiazole degradation in situ apparently is surface-enhanced, with increased rates of transformation observed as total particle surface area increased. At least one species of estuarine bacterium was able to grow directly on a submerged mass of BT as a sole organic C/N source. Both the stirred CEPMs and the TSMs showed that physical turbation (stirring and tidal pulsing) was the most significant variable affecting BT transformation rate in both cases. Another important variable was sediment grain size (inversely proportional to surface area), which affected BT transformation most significantly under oxidized conditions, with transformation rates increasing with increasing surface area. This is the opposite of what is normally encountered for microbial transformation of organic chemicals in sediments. Bulk phase redox potential was only significant in BT transformation rates in sediments of high surface area (clay and silt). Benzothiazole introduced to estuarine water and sediments can be expected to be mobile in polar and nonpolar phases through processes including dispersive partitioning, redox reactions, complexation, and by various solution routes that have recently been described for aromatic hydrocarbons (e.g., formation of aromatic hydrocarbon micelles and nanodroplets) (Li and Lee, 2001). In general, sediment grain size appeared to affect transformation rates most significantly under oxidized conditions, where high particle surface area (sediment I) gave clearly increased rates vs. rates from lower particle surface area (Sediment III). Irrespective of sediment type or redox characteristics, transformation of BT was incomplete, resulting in mixtures of products. The toxicological significance of these product mixtures is unknown, but of interest for future study.
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ACKNOWLEDGMENTS
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This work was supported by grants from the USEPA (STAR), U.S. Department of Interior (MMS-LSU Coastal Marine Institute), and the USFWS National Wetlands Research Center. The authors are indebted to Dr. R.D. DeLaune (LSU Wetland Biogeochemistry Institute) and Dr. L.S. Lee (Purdue University, Agronomy Crop, Soil and Environmental Sciences Department) for advice, substantive critical review, and material support. This work is dedicated to the memory of Dr. William H. Patrick, Boyd Professor and Founding Director of the LSU Wetland Biogeochemistry Institute.
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