Antibiotic resistance genes as emerging contaminants: studies in northern Colorado

This study explores antibiotic resistance genes (ARGs) as emerging environmental contaminants. The purpose of this study was to investigate the occurrence of ARGs in various environmental compartments in northern Colorado, including Cache La Poudre (Poudre) River sediments, irrigation ditches, dairy lagoons, and the effluents of wastewater recycling and drinking water treatment plants. Additionally, ARG concentrations in the Poudre River sediments were analyzed at three time points at five sites with varying levels of urban/agricultural impact and compared with two previously published time points. It was expected that ARG concentrations would be significantly higher in environments directly impacted by urban/agricultural activity than in pristine and lesser-impacted environments. Polymerase chain reaction (PCR) detection assays were applied to detect the presence/absence of several tetracycline and sulfonamide ARGs. Quantitative real-time PCR was used to further quantify two tetracycline ARGs (tet(W) and tet(O)) and two sulfonamide ARGs (sul(I) and sul(II)). The following trend was observed with respect to ARG concentrations (normalized to eubacterial 16S rRNA genes): dairy lagoon water > irrigation ditch water > urban/agriculturally impacted river sediments (p < 0.0001), except for sul(II), which was absent in ditch water. It was noted that tet(W) and tet(O) were also present in treated drinking water and recycled wastewater, suggesting that these are potential pathways for the spread of ARGs to and from humans. On the basis of this study, there is a need for environmental scientists and engineers to help address the issue of the spread of ARGs in the environment.     

Effect of various sludge digestion conditions on sulfonamide, macrolide, and tetracycline resistance genes and class I integrons

Wastewater treatment processes are of growing interest as a potential means to limit the dissemination of antibiotic resistance. This study examines the response of nine representative antibiotic resistance genes (ARGs) encoding resistance to sulfonamide (sulI, sulII), erythromycin (erm(B), erm(F)), and tetracycline (tet(O), tet(W), tet(C), tet(G), tet(X)) to various laboratory-scale sludge digestion processes. The class I integron gene (intI1) was also monitored as an indicator of horizontal gene transfer potential and multiple antibiotic resistance. Mesophilic anaerobic digestion at both 10 and 20 day solids retention times (SRTs) significantly reduced sulI, suII, tet(C), tet(G), and tet(X) with longer SRT exhibiting a greater extent of removal; however, tet(W), erm(B) and erm(F) genes increased relative to the feed. Thermophilic anaerobic digesters operating at 47 °C, 52 °C, and 59 °C performed similarly to each other and provided more effective reduction of erm(B), erm(F), tet(O), and tet(W) compared to mesophilic digestion. However, thermophilic digestion resulted in similar or poorer removal of all other ARGs and intI1. Thermal hydrolysis pretreatment drastically reduced all ARGs, but they generally rebounded during subsequent anaerobic and aerobic digestion treatments. To gain insight into potential mechanisms driving ARG behavior in the digesters, the dominant bacterial communities were compared by denaturing gradient gel electrophoresis. The overall results suggest that bacterial community composition of the sludge digestion process, as controlled by the physical operating characteristics, drives the distribution of ARGs present in the produced biosolids, more so than the influent ARG composition.     

Indirect evidence of transposon-mediated selection of antibiotic resistance genes in aquatic systems at low-level oxytetracycline exposures

Subinhibitory levels of antibiotics can promote the development of antibiotic resistance in bacteria. However, it is unclear whether antibiotic concentrations released into aquatic systems exert adequate pressure to select populations with resistance traits. To examine this issue, 15 mesocosms containing pristine surface water were treated with oxytetracycline (OTC) for 56 days at five levels (0, 5, 20, 50, and 250 microg L(-1)), and six tetracycline-resistance genes (tet(B), tet(L), tet(M), ted(O), tet(Q), and tet(W)), the sum of those genes (tet(R)), "total" 16S-rRNA genes, and transposons (Tn916 and Tn 1545) were monitored using real-time PCR. Absolute water-column resistance-gene abundances did not change at any OTC exposure. However, an increase was observed in the ratio of tet(R) to 16S-rRNA genes in the 250 microg L(-1) OTC units, and an increase in the selection rate of Tc(r) genes (relative to 16S-rRNA genes) was seen when OTC levels were at 20 microg L(-1). Furthermore, tet(M) and Tn916/1545 gene abundances correlated among all treatments (r2 = 0.701, p = 0.05), and there were similar selection patterns of tetR and Tn916/1545 genes relative to the OTC level, suggesting a possible mechanism for retention of specific resistance genes within the systems.     

Antibiotic resistance gene abundances associated with waste discharges to the Almendares River near Havana, Cuba

Considerable debate exists over the primary cause of increased antibiotic resistance (AR) worldwide. Evidence suggests increasing AR results from overuse of antibiotics in medicine and therapeutic and nontherapeutic applications in agriculture. However, pollution also can influence environmental AR, particularly associated with heavy metal, pharmaceutical, and other waste releases, although the relative scale of the "pollution" contribution is poorly defined, which restricts targeted mitigation efforts. The question is "where to study and quantify AR from pollution versus other causes to best understand the pollution effect". One useful site is Cuba because industrial pollution broadly exists; antibiotics are used sparingly in medicine and agriculture; and multiresistant bacterial infections are increasing in clinical settings without explanation. Within this context, we quantified 13 antibiotic resistance genes (ARG; indicators of AR potential), 6 heavy metals, 3 antibiotics, and 17 other organic pollutants at 8 locations along the Almendares River in western Havana at sites bracketing known waste discharge points, including a large solid waste landfill and various pharmaceutical factories. Significant correlations (p < 0.05) were found between sediment ARG levels, especially for tetracyclines and β-lactams (e.g., tet(M), tet(O), tet(Q), tet(W), bla(OXA)), and sediment Cu and water column ampicillin levels in the river. Further, sediment ARG levels increased by up to 3 orders of magnitude downstream of the pharmaceutical factories and were highest where human population densities also were high. Although explicit links are not shown, results suggest that pollution has increased background AR levels in a setting where other causes of AR are less prevalent.     

Abundance and distribution of tetracycline resistance genes and mobile elements in an oxytetracycline production wastewater treatment system

We investigated the abundance and distribution of tetracycline resistance genes (tet genes) in an oxytetracycline (OTC) antibiotic production wastewater (APW) treatment system. Of sixteen tet genes and five mobile elements, nine tet genes (tet(A), tet(C), tet(G), tet(L), tet(M), tet(O), tet(Q), tet(W), and tet(X)) and two mobile elements (class 1 integron (intI1) and transposon Tn916/1545) were successfully quantified by real-time PCR. The relative abundance of tet genes in the effluent and activated sludge (1.2 × 10(-4) to 1.3 × 10(0)) of the APW treatment system were up to 2 orders of magnitude higher than those in the OTC fermentation residues (8.5 × 10(-5) to 6.7 × 10(-3)) (P < 0.01), and 1-4 orders of magnitude higher than those in sewage and nonantibiotic production wastewater treatment systems (P < 0.01), showing that the discharge of abundant tet genes from the APW treatment system is worthy of attention. The three most abundant genes in the APW treatment system, tet(A), tet(C), and tet(G), exhibited significant positive correlations with intI1 (R(2) = 0.73, 0.95, and 0.83, respectively; P < 0.05), suggesting that intI1 may be involved in their proliferation. This is the first study showing that some measures may be required to control the discharge of antibiotic resistance genes from treated APW and activated sludge.     

基于SmartChip HT-qPCR分析不同食品中抗生素抗性基因的多样性及丰度

抗生素抗性基因（Antibiotics Resistance Genes,ARGs）广泛存在于各种环境中,被认为是一种新型环境污染物,对环境及人类健康具有潜在威胁。但是目前对于不同食品中多种ARGs赋存状况的研究仍然有限。该研究采用SmartChip高通量实时定量PCR方法（High-Throughput Quantitative Real-Time PCR,HT-qPCR）对水产品、发酵食品、肉类、蔬菜四大类样品中的ARGs及可移动遗传元件（Mobile Genetic Elements,MGEs）进行了定量分析。结果显示,被分析的15个样品中共检出9个ARGs大类,涉及234个ARGs亚类。所有样品中共有的ARGs亚类为79个,特有的为51个;水产品和肉类样品富集多种ARGs,这些基因表现出较高的丰度（最高可达3.6 copies/16S rRNA gene）。MGEs的检出数量达10个,其中所有样均检出整合子基因intI-1(clinic)和转座酶基因tnpA-01。ARGs抵抗机制分析结果表明,抗生素灭活机制、外排泵机制和细胞保护机制为主要的ARGs抵抗机制,抵抗机制贡献大小依次为：抗生素灭活机制>外排泵机制>细胞保护机制。该研究进一步丰富了对不同食品中ARGs赋存情况的认识,可为食品中ARGs的风险分析提供依据。     

Tertiary-treated municipal wastewater is a significant point source of antibiotic resistance genes into Duluth-Superior Harbor

In this study, the impact of tertiary-treated municipal wastewater on the quantity of several antibiotic resistance determinants in Duluth-Superior Harbor was investigated by collecting surface water and sediment samples from 13 locations in Duluth-Superior Harbor, the St. Louis River, and Lake Superior. Quantitative PCR (qPCR) was used to target three different genes encoding resistance to tetracycline (tet(A), tet(X), and tet(W)), the gene encoding the integrase of class 1 integrons (intI1), and total bacterial abundance (16S rRNA genes) as well as total and human fecal contamination levels (16S rRNA genes specific to the genus Bacteroides ). The quantities of tet(A), tet(X), tet(W), intI1, total Bacteroides , and human-specific Bacteroides were typically 20-fold higher in the tertiary-treated wastewater than in nearby surface water samples. In contrast, the quantities of these genes in the St. Louis River and Lake Superior were typically below detection. Analysis of sequences of tet(W) gene fragments from four different samples collected throughout the study site supported the conclusion that tertiary-treated municipal wastewater is a point source of resistance genes into Duluth-Superior Harbor. This study demonstrates that the discharge of exceptionally treated municipal wastewater can have a statistically significant effect on the quantities of antibiotic resistance genes in otherwise pristine surface waters.     

Impact of treated wastewater irrigation on antibiotic resistance in agricultural soils

Antibiotic resistance (AR) is a global phenomenon with severe epidemiological ramifications. Anthropogenically impacted natural aquatic and terrestrial environments can serve as reservoirs of antibiotic resistance genes (ARG), which can be horizontally transferred to human-associated bacteria through water and food webs, and thus contribute to AR proliferation. Treated-wastewater (TWW) irrigation is becoming increasingly prevalent in arid regions of the world, due to growing demand and decline in freshwater supplies. The release of residual antibiotic compounds, AR bacteria, and ARGs from wastewater effluent may result in proliferation of AR in irrigated soil microcosms. The aim of this study was to assess the impact of TWW-irrigation on soil AR bacterial and ARG reservoirs. Tetracycline, erythromycin, sulfonamide, and ciprofloxacin resistance in soil was assessed using standard culture-based isolation methods and culture-independent molecular analysis using quantitative real-time PCR (qPCR). High levels of bacterial antibiotic resistance were detected in both freshwater- and TWW-irrigated soils. Nonetheless, in most of the soils analyzed, AR bacteria and ARG levels in TWW-irrigated soils were on the whole identical (or sometimes even lower) than in the freshwater-irrigated soils, indicating that the high number of resistant bacteria that enter the soils from the TWW are not able to compete or survive in the soil environment and that they do not significantly contribute ARG to soil bacteria. This strongly suggests that the impact of the TWW-associated bacteria on the soil microbiome is on the whole negligible, and that the high levels of AR bacteria and ARGs in both the freshwater- and the TWW-irrigated soils are indicative of native AR associated with the natural soil microbiome.     

Influence of autochthonous dissolved organic carbon and nutrient limitation on alachlor biotransformation in aerobic aquatic systems

Much work has suggested that the rate of attenuation of water-soluble organic contaminants in aerobic aquatic systems is dependent on the level of secondary nutrients in the water column. For example, the decay rate of alachlor, a common herbicide, was over 10 times higher under hypereutrophic compared with oligotrophic water conditions. It has been presumed that higher water column nutrient levels produce larger microbial communities, resulting in higher rates of alachlor cometabolism. However, most earlier field studies only assessed alachlor fate in systems with full light exposure (FLE). Therefore, new experiments were performed to assess how variations in light level affect alachlor cometabolism in such systems. Twelve tank mesocosms were maintained using identical nitrogen (N) and phosphorus (P) supply conditions: four units with full light exposure (100% FLE), four with partial shading (19.3% FLE), and four with near complete shading (0.5% FLE). Alachlor half-lives were found to vary broadly, from 50 to 60 days in higher light units to > 180 days in the 0.5% FLE units. Nutrient analysis indicated that the low light units were severely carbon (C)-limited for microbial decomposition, whereas the other units had excess C relative to N and P. Apparently, reduced light levels cause decreased production of bioavailable C for decomposition, which significantly reduces alachlor cometabolism. The data suggest that water column nutrient levels only correlate with the alachlor decay rate when light levels are high, and that the biodegradable carbon supply must be considered when the fate of water-soluble contaminants in aerobic aquatic systems is assessed.     

Response of antibiotic resistance genes (ARG) to biological treatment in dairy lagoon water

To explore the response of antibiotic resistance genes (ARG) to biological treatment, dairy lagoon water was incubated anaerobically or aerobically at 20 degrees C or 4 degrees C. Three conditions were compared: Antibiotic (Ab) Spiked, Ab Spiked and Killed, and Background (unamended). For Ab Spiked conditions, oxytetracycline, sulfamethoxazole, tylosin, and monensin were each added at 20 mg/L. Antibiotics and ARG were monitored using high-performance liquid chromatography/tandem mass spectrometry and quantitative real-time polymerase chain reaction, respectively. Biological degradation of antibiotics in all treatments and varied responses of different ARG was observed. Aerobic versus anaerobic treatment had no effect on tet(W), with an overall pattern of increase in the presence of antibiotics followed by decrease to initial levels. tet(O) responded differently under aerobic versus anaerobic treatment, increasing to highest levels at 4 degrees C under aerobic treatment and at 20 degrees C under anaerobic treatment before returning to initial levels. sul(I) and sul (II) showed similar patterns and increased in all Ab Spiked conditions, failing to return to initial levels at 4 degrees C and in some of the 20 degrees C treatments. ere(A) and msr(A) were lower than the other two ARG classes and remained constant in all treatments.     

Prevalence of clinically relevant antibiotic resistance genes in surface water samples collected from Germany and australia

The prevalence and proliferation of antibiotic resistant bacteria is profoundly important to human health, but the extent to which aquatic environments contribute toward the dissemination of antibiotic resistant genes (ARGs) is poorly understood. The prevalence of 24 ARGs active against eight antibiotic classes (β-lactams, aminoglycosides, glycopeptides, chloramphenicols, tetracycline, macrolides, trimethoprim, and sulfonamides) was evaluated in surface water samples collected from Germany and Australia with culture independent methods. The ARGs most frequently detected both in Germany and Australia were sulI, sulII (77-100%), and dfrA1 (43-55%) which code for resistance to sulfonamide and trimethoprim. Macrolides resistance gene ermB was relatively more prevalent in the surface water from Germany (68%) than Australia (18%). In contrast, the chloramphenicol resistance gene catII was more frequently detected in Australia (64%) than Germany (9%). Similarly, β-lactams resistance gene ampC was more prevalent in the samples from Australia (36%) than Germany (19%). This study highlights wide distribution of ARGs for sulfonamide, trimethoprim, macroline, β-lactams and chloramphenicol in the aquatic ecosystems. Aquatic ecosystems can therefore be reservoirs of ARGs genes which could potentially be transferred from commensal microorganisms to human pathogens.     

Effect of temperature on the fate of genes encoding tetracycline resistance and the integrase of class 1 integrons within anaerobic and aerobic digesters treating municipal wastewater solids

The objective of this research was to investigate the ability of anaerobic and aerobic digesters to reduce the quantity of antibiotic resistant bacteria in wastewater solids. Lab-scale digesters were operated at different temperatures (22 °C, 37 °C, 46 °C, and 55 °C) under both anaerobic and aerobic conditions and fed wastewater solids collected from a full-scale treatment facility. Quantitative PCR was used to track five genes encoding tetracycline resistance (tet(A), tet(L), tet(O), tet(W), and tet(X)) and the gene encoding the integrase (intI1) of class 1 integrons. Statistically significant reductions in the quantities of these genes occurred in the anaerobic reactors at 37 °C, 46 °C, and 55 °C, with the removal rates and removal efficiencies increasing as a function of temperature. The aerobic digesters, in contrast, were generally incapable of significantly decreasing gene quantities, although these digesters were operated at much shorter mean hydraulic residence times. This research suggests that high temperature anaerobic digestion of wastewater solids would be a suitable technology for eliminating various antibiotic resistance genes, an emerging pollutant of concern.     

Occurrence, abundance, and diversity of tetracycline resistance genes in 15 sewage treatment plants across China and other global locations

Activated sludge was sampled from 15 sewage treatment plants (STPs) across China and other global locations to investigate the occurrence, abundance and diversity of tetracycline resistance genes (tet) in the STPs. Occurrence and abundance of 14 tet genes were determined using polymerase chain reaction (PCR) and quantitative real time PCR. Six genes (tet(A), tet(C), tet(G), tet(M), tet(S), and tet(X)) were detected in all the STPs, while no sludge sample contained tet(Q). Total concentration of the 14 genes was significantly different among the STPs and average tet abundance of the STPs varied greatly among the tet types (p<0.05). Tet(G) had the highest concentration in the STPs, followed by tet(C), tet(A) and tet(S). Phylogenetic diversity of the genes was investigated using DNA cloning. BLAST analysis showed that all of the 450 cloned sequences matched known tet genes, except for tet(G). The 56 tet(G) clones were grouped into 14 genotypes, among which type G24 had an identical sequence to tet(G) carried by Salmonella enterica or Acinetobacter baumannii, while the other sequences had low similarity to the known genes in GenBank. The results of this study might be useful to understand the diversity of these resistance genes in STPs.     

Effect of loading rate on the fate of mercury in littoral mesocosms

The effects of changes in atmospheric mercury (Hg) deposition on aquatic ecosystems are poorly understood. In this study, we examined the biogeochemical cycling of Hg in littoral mesocosms receiving different loading rates (7-107 microg Hg m(-2) year(-1)). We added a 202Hg-enriched preparation to differentiate the experimentally added Hg from the ambient Hg in the environment. This approach allowed us to follow the distribution and methylation of the isotopically enriched ("spike") Hg in the mesocosms. Within 3 weeks, spike Hg was distributed throughout the main environmental compartments (water, particles, periphyton, and sediments) and began to be converted to methylmercury (MeHg). Concentrations of spike total Hg and MeHg in these compartments, measured after 8 weeks, were directly proportional to loading rates. Thus, Hg(II) availability was the limiting factor for the major processes of the biogeochemical Hg cycle, including methylation. This is the first study to demonstrate a proportional response of in situ MeHg production to atmospherically relevant loading levels. On the basis of mass balances, we conclude that loading rate had no effect on the relative distribution of spike Hg among the main compartments or on the fraction of spike Hg converted to MeHg. Therefore, loading rate did not change the relative magnitude of biogeochemical pathways competing for Hg within the mesocosms. These data suggest that reductions of Hg deposition to lake surfaces would be equally effective across a broad range of deposition rates.     

Fate and effects of enrofloxacin in aquatic systems under different light conditions

The fate and effects of fluoroquinolone antibacterials (FQ) in the environment is of significance because of apparent increased FQ resistance in environmental and clinical organisms. Here we simultaneously assessed the fate and effects of enrofloxacin (enro), an FQ often used in agriculture, on the chemistry and in situ microbial communities in receiving waters. We added enro to 25 microg/L in nine outdoor mesocosms maintained under three light conditions (in triplicate): full sunlight typical of the upper epilimnion (100% full-light exposure, FLE), partial shading typical of the lower epilimnion (28% FLE), and near-complete shading typical of the hypolimnion (0.5% FLE). Enro disappearance and ciprofloxacin (cipro) formation were monitored over time using LC/MS, and water chemistry and ambient microbial communities (using denaturing gradient gel electrophoresis; DGGE) were characterized. Enro half-lives were 0.8, 3.7, and 72 days for the 100%, 28%, and 0.5% FLE treatments, respectively, creating three distinct FQ exposure scenarios. Although FQ exposures ranged from approximately 6 microg/L for 24 h to approximately 21 microg/L for 30 days, no statistically significant exposure effects were noted in water quality or microbial communities (as indicated by whole-community 16S rDNA DGGE analysis and specific amplification of the QRDR region of gyrase A). Small changes in water chemistry were noted over time; however, changes could not be specifically attributed to FQs. In general, enro addition had minimal effect on water column conditions at the levels and durations used here; however, further investigation is needed to assess effects in aquatic sediments.     

Occurrence and solid-liquid partition of sulfonated naphthalene-formaldehyde condensates in the aquatic environment

Sulfonated naphthalene-formaldehyde condensates (SNFC) are high production volume chemicals used in a variety of applications, for example, as concrete plasticizers, tanning agents, or dye dispersants. They enter the aquatic environment primarily by the wastewater path. The occurrence and fate of the monomers, which are different isomers of mono- and disulfonated naphthalene, was intensively investigated in previous studies. However, the environmental fate of the persistent higher molecular SNFC is so far widely unknown. This paper describes an ultrasonic extraction under alkaline conditions, followed by ion-pair HPLC with fluorescence detection for the analysis of SNFC oligomers from solid environmental matrixes such as sewage sludge, suspended solids, and river sediments. Limits of quantification of about 0.1 mg kg-1 d.m. were well below the measured concentrations in environmental samples. SNFC were adsorbed to suspended solids and river sediments in three major German rivers (Rhine, Neckar, and Danube) in concentrations typically up to several mg kg(-1) d.m. A total content of about 4 g kg(-1) d.m. was measured in a sewage sludge of a municipal wastewater treatment plant, which receives wastewater from a textile dyeing plant. Furthermore, the first quantitative field data on the partition of SNFC and their monomers between the aqueous phase and solid environmental compartments are presented. Solid-liquid partition coefficients (Kd) of oligomers with a chain-length ranging from three to six naphthalenesulfonate units were derived from the analysis of corresponding wastewater and sewage sludge samples and from suspended solids and river water samples, respectively. Determined Kd values were in the range from 10(2) to 10(4) L kg(-1).     

市售牛奶中4种四环素类抗生素抗性基因的比较研究

目的研究市售牛奶商品中4种代表性四环素类抗生素抗性基因(antibiotic resistance genes,ARGs)的分布特征。方法以12种商品牛奶为研究对象,通过对样品中的4种四环素类ARGs和16S r RNA进行实时荧光定量PCR测定,分析ARGs的分布特征,并对样品的保存条件、保质期、成分含量做相关性分析。结果牛奶样品中普遍检出抗性基因Tet(A)、Tet(W),检出率为100%,其抗性机制分别是外排泵和细胞保护;Tet(32 s)和Tet(L)均未检出。比较样品中ARGs的相对丰度,Tet(A)最高达到2.28,最低为0.06;Tet(W)最高达到0.24,最低为0.03。牛奶的产品类型、包装形式、保存条件和保质期与Tet(W)的相对丰度显著相关。结论市售牛奶样品中存在ARGs,不同牛奶样品中ARGs的富集程度存在差异。牛奶样品中ARGs相对丰度受到多种因素影响,值得从食品安全的角度深入研究。     

Environmental exposure and risk assessment of fluoroquinolone antibacterial agents in wastewater and river water of the Glatt Valley Watershed,Switzerland

The mass flows of fluoroquinolone antibacterial agents (FQs) were investigated in the aqueous compartments of the Glatt Valley Watershed, a densely populated region in Switzerland. The major human-use FQs consumed in Switzerland, ciprofloxacin (CIP) and norfloxacin (NOR), were determined in municipal wastewater effluents and in the receiving surface water, the Glatt River. Individual concentrations in raw sewage and in final wastewater effluents ranged from 255 to 568 ng/L and from 36 to 106 ng/L, respectively. In the Glatt River, the FQs were present at concentrations below 19 ng/L. The removal of FQs from the water stream during wastewater treatment was between 79 and 87%. During the studied summer period, FQs in the dissolved fraction were significantly reduced downstream in the Glatt River (15-20 h residence time) (66% for CIP and 48% for NOR). Thus, after wastewater treatment, transport in rivers causes an additional decrease of residual levels of FQs in the aquatic environment. Refined predicted environmental concentrations for the study area compare favorably with the measured environmental concentrations (MEC) obtained in the monitoring study. Total measured FQ concentrations occurring in the examined aquatic compartments of the Glatt Valley Watershed were related to acute ecotoxicity data from the literature. The risk quotients obtained (MEC/PNEC < 1) following the recommendations of the European guidelines or draft documents suggest a low probability for adverse effects of the occurring FQs, either on microbial activity in WWTPs or on algae, daphnia, and fish in surface waters.     

Long-term transformation and fate of manufactured ag nanoparticles in a simulated large scale freshwater emergent wetland

Transformations and long-term fate of engineered nanomaterials must be measured in realistic complex natural systems to accurately assess the risks that they may pose. Here, we determine the long-term behavior of poly(vinylpyrrolidone)-coated silver nanoparticles (AgNPs) in freshwater mesocosms simulating an emergent wetland environment. AgNPs were either applied to the water column or to the terrestrial soils. The distribution of silver among water, solids, and biota, and Ag speciation in soils and sediment was determined 18 months after dosing. Most (70 wt %) of the added Ag resided in the soils and sediments, and largely remained in the compartment in which they were dosed. However, some movement between soil and sediment was observed. Movement of AgNPs from terrestrial soils to sediments was more facile than from sediments to soils, suggesting that erosion and runoff is a potential pathway for AgNPs to enter waterways. The AgNPs in terrestrial soils were transformed to Ag(2)S (~52%), whereas AgNPs in the subaquatic sediment were present as Ag(2)S (55%) and Ag-sulfhydryl compounds (27%). Despite significant sulfidation of the AgNPs, a fraction of the added Ag resided in the terrestrial plant biomass (~3 wt % for the terrestrially dosed mesocosm), and relatively high body burdens of Ag (0.5-3.3 μg Ag/g wet weight) were found in mosquito fish and chironomids in both mesocosms. Thus, Ag from the NPs remained bioavailable even after partial sulfidation and when water column total Ag concentrations are low (<0.002 mg/L).     

Influence of flow conditions and system geometry on nitrate use by benthic biofilms: implications for nutrient mitigation

The effects of substratum geometry and overlying velocity on nitrate use by periphyton were assessed. Periphyton was cultivated at an average current velocity of 0.5 cm s(-1) in laboratory mesocosms (120 cm long, 60 cm wide) on polyethylene nets of three different geometries, "1-lay er", "3-layer", and "bedform" structures, overlaying a thin bed of sand. Bulk nitrate use was then measured as the reduction of nitrate concentration in the overlying water under average velocities of 0.005, 0.05, and 0.5 cm s(-1). Periphyton structural characteristics were quantified as algal/bacterial biomass, algal species composition, and bacterial densities. Accrual of microbial biomass increased monotonically with increasing benthic net surface area, with upper sections of structures supporting the highest biomass. Maximum rates of nitrate removal were measured in the bedform geometry at intermediate velocity (173 mg NO3-N m(-2) d(-1)), and the lowest was measured with 1-layer geometry at the fastest velocity (11 mg NO3-N m(-2) d(-1)). Oxygen microprofiles within biofilms demonstrated that hydrodynamic conditions and benthic structure both play a key role in the regulation of microbial processing of nitrate delivered from the water column by promotion of denitrification in downstream sections of bedform substrata. Interactions between hydrodynamic conditions and substratum geometry are expected to regulate microbial activity in all surficial natural and engineered environments and must be parameterized to forecast long-term average biochemical transformation rates in rivers and other dynamic aquatic systems.