Influence of Season on Biodegradation Rates in Rivers.

Biodegradation plays a key role in the fate of chemicals in the environment. The variability of biodegradation in time can cause uncertainty in evaluating the environmental persistence and risk of chemicals. However, the seasonality of biodegradation in rivers has not yet been the subject of environmentally relevant testing and systematic investigation for large numbers of chemicals. In this work, we studied the biodegradation of 96 compounds during four seasons at four locations (up- and downstream of WWTPs located on two Swedish rivers). Significant seasonality (ANOVA, p < 0.05) of the first-order rate constant for primary biodegradation was observed for most compounds. Variations in pH and total bacterial cell count were not the major factors explaining the seasonality of biodegradation. Deviation from the classical Arrhenius-type behavior was observed for most of the studied compounds, which calls into question the application of this relationship to correct biodegradation rate constants for differences in environmental temperature. Similarities in magnitude and seasonality of biodegradation rate constants were observed for some groups of chemicals possessing the same functional groups. Moreover, reduced seasonality of biodegradation was observed downstream of WWTPs, while biodegradation rates of most compounds were not significantly different between up- and downstream.

Combined Surface-Subsurface Stream Restoration Structures Can Optimize Hyporheic Attenuation of Stream Water Contaminants.

There is a design-to-function knowledge gap regarding how engineered stream restoration structures can maximize hyporheic contaminant attenuation. Surface and subsurface structures have each been studied in isolation as techniques to restore hyporheic exchange, but surface-subsurface structures have not been investigated or optimized in an integrated manner. Here, we used a numerical model to systematically evaluate key design variables for combined surface (i.e., weir height and length) and subsurface (i.e., upstream and downstream baffle plate spacing) structures. We also compared performance metrics that place differing emphasis on hyporheic flux versus transit times. We found that surface structures tended to create higher flux, shorter transit time flowpaths, whereas subsurface structures promoted moderate-flux, longer transit time flowpaths. Optimal combined surface-subsurface structures could increase fluxes and transit times simultaneously, thus providing conditions for contaminant attenuation that were many times more effective than surface or subsurface structures alone. All performance metrics were improved by the presence of an upstream plate and the absence of a downstream plate. Increasing the weir length tended to improve all metrics, whereas the optimal weir height varied based on metrics. These findings may improve stream restoration by better aligning specific restoration goals with appropriate performance metrics and hyporheic structure designs.

Fate of trace organic compounds in the hyporheic zone: Influence of microbial metabolism.

The hyporheic zone (HZ) is considered a hydrodynamically-driven bioreactor with significant pollutant removal capacities and can therefore not only improve wholestream water quality but also preserve human and ecosystem health. Microbial metabolism is hypothesized to play a key role in pollutant transformation in hyporheic sediments of natural streams. However, previous work investigating the influence of microbial metabolism on pollutant transformation has been predominantly laboratory studies. The key challenge for field studies is the appropriate determination of net microbial metabolism, i.e. information on the actual exposure times to specific microbial processes in the investigated system. The present study uses reactive fluorescent tracers to determine microbial metabolism and ultimately its influence on pollutant transformation, e.g. for trace organic compounds, in hyporheic sediments under natural conditions. In particular, the reactive fluorescent tracers resazurin and its main transformation product resorufin were used to determine the microbial metabolism of facultative or obligate aerobes. The influence of the derived microbial metabolism on the transformation of 20 trace organic compounds, such as pharmaceuticals, including 3 parent-daughter pairs, was examined. The present findings validate laboratory results on the microbially-mediated transformation of the anticonvulsant gabapentin to its main transformation product gabapentin lactam under natural conditions. All other TrOCs investigated did not show a clear link between TrOC reactivity to the microbial metabolism informed by the resazurin-resorufin-system. Overall, the present study not only demonstrates the use of the fluorescent tracer-system resazurin and resorufin for determining microbial metabolism of facultative or obligate aerobes but also generally highlights the potential of reactive fluorescent tracers to disentangle specific reactive properties and ultimately their influence on the fate of pollutants in natural HZs.

Toxicometabolomics and Biotransformation Product Elucidation in Single Zebrafish Embryos Exposed to Carbamazepine from Environmentally-Relevant to Morphologically Altering Doses.

Toxicometabolomics and biotransformation product (bioTP) elucidation were carried out in single zebrafish (ZF) embryos exposed to carbamazepine (CBZ). Exposures were conducted in 96-well plates containing six CBZ concentrations ranging from 0.5 µg/L to 50 mg/L (n = 12 embryos per dose). In the 50 mg/L dose group, 33% of embryos developed edema during the exposure (120 hpf), while hatching was significantly delayed in three of the lower-dose groups (0.46, 3.85, and 445 µg/L) compared to the control at 48 hpf. Toxicometabolomic analysis together with random forest modeling revealed a total of 80 significantly affected metabolites (22 identified via targeted lipidomics and 58 via nontarget analysis). The wide range of doses enabled the observation of both monotonic and nonmonotonic dose responses in the metabolome, which ultimately produced a unique and comprehensive biochemical picture that aligns with existing knowledge on the mode of action of CBZ. The combination of high dose exposures and apical endpoint assessment in single embryos also enabled hypothesis generation regarding the target organ for the morphologically altering insult. In addition, two CBZ bioTPs were identified without additional exposure experiments. Overall, this work showcases the potential of toxicometabolomics and bioTP determination in single ZF embryos for rapid and comprehensive chemical hazard assessment.

Biodegradation of metoprolol in oxic and anoxic hyporheic zone sediments: unexpected effects on microbial communities.

Metoprolol is widely used as a beta-blocker and considered an emerging contaminant of environmental concern due to pseudo persistence in wastewater effluents that poses a potential ecotoxicological threat to aquatic ecosystems. Microbial removal of metoprolol in the redox-delineated hyporheic zone (HZ) was investigated using streambed sediments supplemented with 15 or 150 µM metoprolol in a laboratory microcosm incubation under oxic and anoxic conditions. Metoprolol disappeared from the aqueous phase under oxic and anoxic conditions within 65 and 72 days, respectively. Metoprolol was refed twice after initial depletion resulting in accelerated disappearance under both conditions. Metoprolol disappearance was marginal in sterile control microcosms with autoclaved sediment. Metoprolol was transformed mainly to metoprolol acid in oxic microcosms, while metoprolol acid and α-hydroxymetoprolol were formed in anoxic microcosms. Transformation products were transient and disappeared within 30 days under both conditions. Effects of metoprolol on the HZ bacterial community were evaluated using DNA- and RNA-based time-resolved amplicon Illumina MiSeq sequencing targeting the 16S rRNA gene and 16S rRNA, respectively, and were prominent on 16S rRNA rather than 16S rRNA gene level suggesting moderate metoprolol-induced activity-level changes. A positive impact of metoprolol on Sphingomonadaceae and Enterobacteriaceae under oxic and anoxic conditions, respectively, was observed. Nitrifiers were impaired by metoprolol under oxic and anoxic conditions. Collectively, our findings revealed high metoprolol biodegradation potentials in the hyporheic zone under contrasting redox conditions associated with changes in the active microbial communities, thus contributing to the attenuation of micropollutants. KEY POINTS: • High biotic oxic and anoxic metoprolol degradation potentials in the hyporheic zone. • Key metoprolol-associated taxa included Sphingomonadaceae, Enterobacteraceae, and Promicromonosporaceae. • Negative impact of metoprolol on nitrifiers.

Transformation of organic micropollutants along hyporheic flow in bedforms of river-simulating flumes.

Urban streams receive increasing loads of organic micropollutants from treated wastewaters. A comprehensive understanding of the in-stream fate of micropollutants is thus of high interest for water quality management. Bedforms induce pumping effects considerably contributing to whole stream hyporheic exchange and are hotspots of biogeochemical turnover processes. However, little is known about the transformation of micropollutants in such structures. In the present study, we set up recirculating flumes to examine the transformation of a set of micropollutants along single flowpaths in two triangular bedforms. We sampled porewater from four locations in the bedforms over 78 days and analysed the resulting concentration curves using the results of a hydrodynamic model in combination with a reactive transport model accounting for advection, dispersion, first-order removal and retardation. The four porewater sampling locations were positioned on individual flowpaths with median solute travel times ranging from 11.5 to 43.3 h as shown in a hydrodynamic model previously. Highest stability was estimated for hydrochlorothiazide on all flowpaths. Lowest detectable half-lives were estimated for sotalol (0.7 h) and sitagliptin (0.2 h) along the shortest flowpath. Also, venlafaxine, acesulfame, bezafibrate, irbesartan, valsartan, ibuprofen and naproxen displayed lower half-lives at shorter flowpaths in the first bedform. However, the behavior of many compounds in the second bedform deviated from expectations, where particularly transformation products, e.g. valsartan acid, showed high concentrations. Flowpath-specific behavior as observed for metformin or flume-specific behavior as observed for metoprolol acid, for instance, was attributed to potential small-scale or flume-scale heterogeneity of microbial community compositions, respectively. The results of the study indicate that the shallow hyporheic flow field and the small-scale heterogeneity of the microbial community are major controlling factors for the transformation of relevant micropollutants in river sediments.

The riverine bioreactor: An integrative perspective on biological decomposition of organic matter across riverine habitats.

Riverine ecosystems can be conceptualized as 'bioreactors' (the riverine bioreactor) which retain and decompose a wide range of organic substrates. The metabolic performance of the riverine bioreactor is linked to their community structure, the efficiency of energy transfer along food chains, and complex interactions among biotic and abiotic environmental factors. However, our understanding of the mechanistic functioning and capacity of the riverine bioreactor remains limited. We review the state of knowledge and outline major gaps in the understanding of biotic drivers of organic matter decomposition processes that occur in riverine ecosystems, across habitats, temporal dimensions, and latitudes influenced by climate change. We propose a novel, integrative analytical perspective to assess and predict decomposition processes in riverine ecosystems. We then use this model to analyse data to demonstrate that the size-spectra of a community can be used to predict decomposition rates by analysing an illustrative dataset. This modelling methodology allows comparison of the riverine bioreactor's performance across habitats and at a global scale. Our integrative analytical approach can be applied to advance understanding of the functioning and efficiency of the riverine bioreactor as hotspots of metabolic activity. Application of insights gained from such analyses could inform the development of strategies that promote the functioning of the riverine bioreactor across global ecosystems.

Ibuprofen Degradation and Associated Bacterial Communities in Hyporheic Zone Sediments.

Ibuprofen, a non-steroidal anti-inflammatory pain reliever, is among pharmaceutical residues of environmental concern ubiquitously detected in wastewater effluents and receiving rivers. Thus, ibuprofen removal potentials and associated bacteria in the hyporheic zone sediments of an impacted river were investigated. Microbially mediated ibuprofen degradation was determined in oxic sediment microcosms amended with ibuprofen (5, 40, 200, and 400 µM), or ibuprofen and acetate, relative to an un-amended control. Ibuprofen was removed by the original sediment microbial community as well as in ibuprofen-enrichments obtained by re-feeding of ibuprofen. Here, 1-, 2-, 3-hydroxy- and carboxy-ibuprofen were the primary transformation products. Quantitative real-time PCR analysis revealed a significantly higher 16S rRNA abundance in ibuprofen-amended relative to un-amended incubations. Time-resolved microbial community dynamics evaluated by 16S rRNA gene and 16S rRNA analyses revealed many new ibuprofen responsive taxa of the Acidobacteria, Actinobacteria, Bacteroidetes, Gemmatimonadetes, Latescibacteria, and Proteobacteria. Two ibuprofen-degrading strains belonging to the genera Novosphingobium and Pseudomonas were isolated from the ibuprofen-enriched sediments, consuming 400 and 300 µM ibuprofen within three and eight days, respectively. The collective results indicated that the hyporheic zone sediments sustain an efficient biotic (micro-)pollutant degradation potential, and hitherto unknown microbial diversity associated with such (micro)pollutant removal.

Bacterial Diversity Controls Transformation of Wastewater-Derived Organic Contaminants in River-Simulating Flumes.

Hyporheic zones are the water-saturated flow-through subsurfaces of rivers which are characterized by the simultaneous occurrence of multiple physical, biological, and chemical processes. Two factors playing a role in the hyporheic attenuation of organic contaminants are sediment bedforms (a major driver of hyporheic exchange) and the composition of the sediment microbial community. How these factors act on the diverse range of organic contaminants encountered downstream from wastewater treatment plants is not well understood. To address this knowledge gap, we investigated dissipation half-lives (DT50s) of 31 substances (mainly pharmaceuticals) under different combinations of bacterial diversity and bedform-induced hyporheic flow using 20 recirculating flumes in a central composite face factorial design. By combining small-volume pore water sampling, targeted analysis, and suspect screening, along with quantitative real-time PCR and time-resolved amplicon Illumina MiSeq sequencing, we determined a comprehensive set of DT50s, associated bacterial communities, and microbial transformation products. The resulting DT50s of parent compounds ranged from 0.5 (fluoxetine) to 306 days (carbamazepine), with 20 substances responding significantly to bacterial diversity and four to both diversity and hyporheic flow. Bacterial taxa that were associated with biodegradation included Acidobacteria (groups 6, 17, and 22), Actinobacteria (Nocardioides and Illumatobacter), Bacteroidetes (Terrimonas and Flavobacterium) and diverse Proteobacteria (Pseudomonadaceae, Sphingomonadaceae, and Xanthomonadaceae). Notable were the formation of valsartan acid from irbesartan and valsartan, the persistence of N-desmethylvenlafaxine across all treatments, and the identification of biuret as a novel transformation product of metformin. Twelve additional target transformation products were identified, which were persistent in either pore or surface water of at least one treatment, indicating their environmental relevance.

In-plate toxicometabolomics of single zebrafish embryos.

Toxicometabolomic studies involving zebrafish embryos have become increasingly popular for linking apical endpoints to biochemical perturbations as part of adverse outcome pathway determination. These experiments involve pooling embryos to generate sufficient biomass for metabolomic measurement, which adds both time and cost. To address this limitation, we developed a high-throughput toxicometabolomic assay involving single zebrafish embryos. Incubation, microscopy, embryo extraction, and instrumental metabolomic analysis were all performed in the same 96-well plate, following acquisition of conventional toxicological endpoints. The total time for the assay (including testing of 6 doses/n = 12 embryos per dose plus positive and negative controls, assessing conventional endpoints, instrumental analysis, data processing and multivariate statistics) is <14 days. Metabolomic perturbations at low dose were linked statistically to those observed at high dose and in the presence of an adverse effect, thereby contextualizing omic data amongst apical endpoints. Overall, this assay enables collection of high resolution metabolomic data in a high throughput manner, suitable for mode of action hypothesis generation in the context of pharmaceutical or toxicological screening.

Using recirculating flumes and a response surface model to investigate the role of hyporheic exchange and bacterial diversity on micropollutant half-lives.

Enhancing the understanding of the fate of wastewater-derived organic micropollutants in rivers is crucial to improve risk assessment, regulatory decision making and river management. Hyporheic exchange and sediment bacterial diversity are two factors gaining increasing importance as drivers for micropollutant degradation, but are complex to study in field experiments and usually ignored in laboratory tests aimed to estimate environmental half-lives. Flume mesocosms are useful to investigate micropollutant degradation processes, bridging the gap between the field and batch experiments. However, few studies have used flumes in this context. We present a novel experimental setup using 20 recirculating flumes and a response surface model to study the influence of hyporheic exchange and sediment bacterial diversity on half-lives of the anti-epileptic drug carbamazepine (CBZ) and the artificial sweetener acesulfame (ACS). The effect of bedform-induced hyporheic exchange was tested by three treatment levels differing in number of bedforms (0, 3 and 6). Three levels of sediment bacterial diversity were obtained by diluting sediment from the River Erpe in Berlin, Germany, with sand (1 : 10, 1 : 1000 and 1 : 100 000). Our results show that ACS half-lives were significantly influenced by sediment dilution and number of bedforms. Half-lives of CBZ were higher than ACS, and were significantly affected only by the sediment dilution variable, and thus by bacterial diversity. Our results show that (1) the flume-setup is a useful tool to study the fate of micropollutants in rivers, and that (2) higher hyporheic exchange and bacterial diversity in the sediment can increase the degradation of micropollutants in rivers.

Author Correction: Environmental filtering and community delineation in the streambed ecotone.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Fate of Trace Organic Compounds in the Hyporheic Zone: Influence of Retardation, the Benthic Biolayer, and Organic Carbon.

The fate of 28 trace organic compounds (TrOCs) was investigated in the hyporheic zone (HZ) of an urban lowland river in Berlin, Germany. Water samples were collected hourly over 17 h in the river and in three depths in the HZ using minipoint samplers. The four relatively variable time series were subsequently used to calculate first-order removal rates and retardation coefficients via a one-dimensional reactive transport model. Reversible sorption processes led to substantial retardation of many TrOCs along the investigated hyporheic flow path. Some TrOCs, such as dihydroxy-carbamazepine, O-desmethylvenlafaxine, and venlafaxine, were found to be stable in the HZ. Others were readily removed with half-lives in the first 10 cm of the HZ ranging from 0.1 ± 0.01 h for iopromide to 3.3 ± 0.3 h for tramadol. Removal rate constants of the majority of reactive TrOCs were highest in the first 10 cm of the HZ, where removal of biodegradable dissolved organic matter was also the highest. Because conditions were oxic along the top 30 cm of the investigated flow path, we attribute this finding to the high microbial activity typically associated with the shallow HZ. Frequent and short vertical hyporheic exchange flows could therefore be more important for reach-scale TrOC removal than long, lateral hyporheic flow paths.

Spatial and Temporal Variability in Attenuation of Polar Organic Micropollutants in an Urban Lowland Stream.

Contamination of rivers by trace organic compounds (TrOCs) poses a risk for aquatic ecosystems and drinking water quality. Spatially- and temporally varying environmental conditions are expected to play a major role in controlling in-stream attenuation of TrOCs. This variability is rarely captured by in situ studies of TrOC attenuation. Instead, snap-shots or time-weighted average conditions and corresponding attenuation rates are reported. The present work sought to investigate this variability and factors controlling it by analysis of 24 TrOCs over a 4.7 km reach of the River Erpe (Berlin, Germany). The factors investigated included sunlight and water temperature as well as the presence of macrophytes. Attenuation rate constants in 48 consecutive hourly water parcels were tracked along two contiguous river sections of different characteristics. Section 1 was less shaded and more densely covered with submerged macrophytes compared to section 2. The sampling campaign was repeated after macrophyte removal from section 1. The findings show, that section 1 generally provided more favorable conditions for both photo- and biodegradation. Macrophyte removal enhanced photolysis of some compounds (e.g., hydrochlorothiazide and diclofenac) while reducing the biodegradation of metoprolol. The transformation products metoprolol acid and valsartan acid were formed along the reach under all conditions.

Hyporheic Exchange Controls Fate of Trace Organic Compounds in an Urban Stream.

First-order half-lives for 26 trace organic compounds (TrOCs) were determined in the hyporheic zone (HZ) and along a 3 km reach of a first-order stream in South Australia during both dry and wet seasons. Two salt tracer experiments were conducted and evaluated using a transient storage model to characterize seasonal differences in stream residence time and transient storage. Lagrangian and time-integrated surface water sampling were conducted to calculate half-lives in the surface water. Half-lives in the HZ were calculated using porewater samples obtained from a modified mini-point sampler and hyporheic residence times measured via active heat-pulse sensing. Half of the investigated TrOCs (e.g., oxazepam, olmesartan, candesartan) were not significantly removed along both the investigated river stretch and the sampled hyporheic flow paths. The remaining TrOCs (e.g., metformin, guanylurea, valsartan) were found to be significantly removed in the HZ and along the river stretch with relative removals in the HZ correlating to reach-scale relative removals. Using the modeled transport parameters, it was estimated that wet season reach-scale removal of TrOCs was predominately caused by removal in the HZ when the intensity of hyporheic exchange was also higher. Factors that increase HZ exchange are thus likely to promote in-stream reactivity of TrOCs.

Environmental filtering and community delineation in the streambed ecotone.

A current controversy in ecology is whether biological communities are discrete biological entities or simply study units created for convenience; a debate that becomes even more heated when delimiting communities along ecotones. Here, we report an interdisciplinary study designed to address the interplay between environmental drivers and community ecology in a typical ecotone ecosystem: the streambed. Environmental filtering at a micro-scale determined how diversity, productivity and composition of the whole streambed assemblage varied with depth and with the direction of vertical water exchange. Biomass and production decreased with increasing depth, and were lower under upwelling than downwelling conditions. However, the rate at which biomass and production decreased with increasing depth differed significantly for different taxonomic groups. Using quantitative biocenosis analysis, we also showed that benthic and hyporheic zone assemblages (assemblages in close juxtaposition) could be clearly distinguished as discrete communities with individual integrity. Vertical hydrodynamic conditions also influenced the demarcation between both communities; the benthic community reached greater depths in downwelling than in upwelling zones.

Determination of polar organic micropollutants in surface and pore water by high-resolution sampling-direct injection-ultra high performance liquid chromatography-tandem mass spectrometry.

Hyporheic zones (HZs) are dynamic and complex transition regions between rivers and aquifers which are thought to play an important role in the attenuation of environmental micropollutants. Non-steady state and small-scale hyporheic processes which affect micropollutants in the HZ are poorly characterized due to limitations in existing analytical methodologies. In this work we developed a method for high spatio-temporal resolution analysis of polar organic micropollutants (POMs) in hyporheic pore- and surface waters by combining (semi-) automatic low volume sampling techniques with direct-injection ultra-high performance liquid chromatography tandem mass spectrometry. The method is capable of quantifying 25 parent compounds and 18 transformation products (TPs) using only 0.4 mL of water and few preparation steps. Application of the method to both surface and pore water revealed significant (i.e. > an order of magnitude) differences in POM concentrations over small time and spatial scales (i.e. < a few hours and tens of cm, respectively). Guanylurea, a TP of the antidiabetic drug metformin was detected at unprecedentedly high concentrations. Collectively, this method is suitable for in situ characterization of POMs at high spatial and temporal resolution and with minimal disturbance of natural flow paths and infiltration of surface water.