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Eukaryotes are critical regulators of ecosystem functions in river systems. However, little is known about vertical micro-eukaryotic composition, diversity, and assembly patterns in urban aquaculture systems under vertical geochemical gradients. Molecular analysis based on 18S rRNA gene sequencing was performed to investigate the spatial variation in the eukaryotic community in six layers of different depths in water and sediments in the Jinchuan River of Nanjing, China. SAR and Opisthokonta were the most abundant groups, comprising 75.19% and 10.54% of the total sequences in water and 32.43% and 44.10% of the total sequences in sediment, respectively. The composition, diversity, and assembly patterns of eukaryotic microbes varied regionally and spatially in different layers. The abundance of Paramecium increased, and that of Ploimida decreased as the level of water pollution increased in water layers. In sediment layers, Cyclotella, Ochromonas, and Stentor were more abundant in less-polluted sites. In addition, depth-related differences could also be observed among eukaryotes in different layers. Paramecium was most common across all water layers with an average relative abundance of 19.64%, while LKM11 dominated the community in sediment (18.79%). Statistics of the beta-diversity patterns suggested that a higher level of river pollution probably leads to greater heterogeneity in eukaryotic community composition (BSOR-C > BSOR-A > BSOR-D > BSOR-B). Vertical beta-diversity patterns in the eukaryotic community primarily resulted from species replacement, which was mainly driven by spatial turnover (βSIM = 0.4471 in water and 0.6662 in sediment) rather than nestedness (βNES = 0.0292 in water and 0.0188 in sediment). [ABSTRACT FROM AUTHOR]

Tetrabromobisphenol A (TBBPA) and copper (Cu) are the main pollutants at e-waste recycling sites and the effects of their biotoxicity on microorganisms have drawn extensive attention. Nitrate-based bioremediation has been applied to organic pollutant-contaminated sediments since nitrate is a favorable electron acceptor for microbes. However, the effects of TBBPA and Cu on nitrogen (N)-cycling microorganisms and bioremediation in co-contaminated sediments remain unclear. Thus, our study examined the effects of TBBPA and Cu with/without nitrate addition on the TBBPA biodegradation efficiencies, microbial activities, and N functional genes. It was found the biodegradation efficiencies of TBBPA were improved by the nitrate addition from 34.7% to 59.3% and from 22.6% to 42.8% in the TBBPA and TBBPA-Cu contaminated groups, respectively. The inhibitions of the catalase activity increased with the nitrate addition because of the anaerobic respiration of the microorganisms. In addition, the potential denitrification rate exhibited an increasing trend from 6.46 to 8.23 mg-N kg−1 dry sediment day−1 during the period of 15–90 days after adding nitrate to the co-contaminated group, whereas the potential nitrification rate exhibited an opposite trend and decreased from 4.47 to 3.19 mg-N kg−1 dry sediment day−1. The denitrification gene abundances of the N-cycling genes were 107-108 orders of magnitude higher and significantly increased in the nitrate addition groups. The amo A gene abundances were lower than the denitrification gene abundances and were 105-106 orders of magnitude in the same groups. Moreover, the interaction types of the pollutants on the gene abundances were changed from synergistic to antagonistic as nitrate addition. Our study emphasized the gap of knowledge on nitrate addition affecting N-cycling microbes in the combined pollutants exposure sediments, and will be helpful for further bioremediation in different contaminated scenarios. Image 1 • The TBBPA biodegradation efficiencies and PDNR were enhanced by nitrate addition. • Nitrification gene abundances were always lower than denitrification genes. • Nitrification and denitrification gene copies were increased by nitrate addition. • The pollutants interaction types on genes were antagonistic more than synergistic. [ABSTRACT FROM AUTHOR]

Sertraline is a widely used antidepressant that becomes an aquatic pollutant through metabolic excretion and improper disposal. Determining the impact of sertraline on benthic microbial ecosystems is important for the transformation of river biogenic elements. However, the molecular initiating event induced by sertraline is more readily observed at higher levels, such as the individual or population level of larger organisms, and the effect is not pronounced in benthic organisms, which are directly involved in nitrogen transformation. Therefore, this study used DNA metabarcoding to analyze the effect of sertraline on the microbial ecosystem and material cycles in river sediment through the lens of a microbial food web. The presence of sertraline in the river sediment enhanced the mineralization capacity of nitrogen and increased the accumulation of nitrate in the sediment. Sertraline affected the structure of the microbial food web by stimulating different successions of bacteria and eukaryotes. A structural equation model revealed that sertraline affected the microbial food web model through top-down forces (predation) by reducing the trophic transfer efficiency from metazoans to protozoans. This effect resulted in decreases in the trophic transfer efficiency from protozoans to bacteria and increases in nitrogen mineralization capacity. This was followed by a gradual increase in the nitrification reaction under the action of nitrifying bacteria, increasing the threat to the ecological health of rivers. The results show that sertraline affects the material cycle of river ecosystems and emphasizes that the assessment of the ecological risks of sertraline needs to be considered from the perspective of the material cycle of ecosystems. Image 1 • Effects of sertraline on nitrogen transformation and microbial food web in sediment were studied in microcosms. • The impact of sertraline on benthic organisms decreased at lower trophic levels. • Sertraline influenced microbial food web through top-down forces (predation). • Sertraline increased nitrogen mineralization capacity in sediment and promotes nitrification processes. The effect of sertraline on the microbial food web in river sediment was studied, and sertraline promoted nitrification in sediment by inhibiting predation. [ABSTRACT FROM AUTHOR]

The combined contamination of brominated flame retardants (BFRs) and heavy metals in electronic waste (e-waste) recycling and disposal areas has been a serious concern owing to their environmental persistence and chronic toxicities. Ammonia oxidizers, e.g., ammonia-oxidizing archaea (AOA) and bacteria (AOB) play essential roles in nitrogen cycling and can serve as ideal indicators that reflect the changes in sediment health in response to environmental variables. There is currently very little information available on the combined toxic effects of BFRs and heavy metals on AOA and AOB communities. In this study, two typical e-waste pollutants, tetrabromobisphenol A (TBBPA) and copper (Cu), were selected as target contaminants to investigate the individual and combined effects of both pollutants on AOA and AOB communities in river sediments. Respective treatments of TBBPA (1, 10, and 20 mg/kg wet weight), Cu (100 mg/kg wet weight) and their combined treatments (weight ratios of 1:100, 1:10, and 1:5) were performed in laboratory experiments. High-throughput sequencing was applied to explore the response of ammonia oxidizers to TBBPA and Cu. The interaction types of TBBPA and Cu were calculated by the directional classification system to reveal the individual and combined toxicities of both contaminants to the ammonia oxidizers. On days 15 and 30, the dominant interaction type of TBBPA and Cu was synergistic (62.50%), and the combined contamination exacted selective pressure and inhibition on the AOB and AOA communities. On days 45 and 90, the interaction type shifted to be antagonistic (83.33%), with both the AOB and AOA communities gradually reaching stable population equilibria. The alteration of the interaction type is attributed to the elevated TBBPA/Cu tolerance as the incubation time increased. This study disclosed the interaction types of TBBPA and Cu in contaminated river sediments, and revealed that the combined effect could potentially manipulate AOB and AOA communities. Image 1 • The co-contamination of TBBPA and Cu markedly inhibited the biodiversity of AOB. • AOA showed higher capacity to tolerate the toxicity of TBBPA and Cu. • The dominant interaction type of TBBPA and Cu turned from synergism to antagonism. • The transition of TBBPA and Cu interaction types influenced AOB and AOA communities. Main finding: The interaction type of TBBPA and Cu was found to be related to the combined effect of both contaminants, and manipulated the AOB and AOA communities in co-contaminated river sediments. [ABSTRACT FROM AUTHOR]

The hydrodynamics in the diverging area become complicated because of the basin hydrological conditions, making the distribution of antibiotics largely uncertain and thus bringing uncertain ecological risks of antibiotics. Through field sampling, experiments and numerical simulations, the distribution of antibiotics, its responses to hydrological conditions were studied. Antibiotics in the bifurcated river sediments was mainly distributed in the branch mouth. The hydrodynamic regions were affected by the hydrological frequency. Notably, the center of the low-velocity area moved upstream and gradually expands to the entire tributary as the hydrological frequency shifted from high to low. ENRO (enrofloxacin) and OFC (ofloxacin) were the key hazardous antibiotics affecting the ecological health in the diverging area, and their concentrations are mainly affected by sediment particle size (D < 0.15 mm) and oxygen content. The ecological risk of antibiotics in the diverging area were gradually decreased with the increase of the distance from the central area. The water physical and chemical properties, altered by the river basin hydrological conditions, play an important role in influencing the distribution of antibiotic concentrations, and ultimately posing great threat to aquatic ecosystem. The research provides a scientific basis for antibiotic risk control in the diverging area under different hydrological conditions. [Display omitted] • The distribution of sediment antibiotics was mainly affected by hydrodynamics. • Influence mechanism of hydrological changes on the sediment distribution was identified. • ENRO and OFC were hazardous and were affected by sediment particle size and DO. • The ecological risk of antibiotics gradually decreased from the center to the outside. [ABSTRACT FROM AUTHOR]

The effects of environmental concentration silver nanoparticles (ecAgNPs) on microbial communities and the nitrogen cycling in river sediments remain largely uncharacterized. As a fundamental component of sediments, Fe(III) can interact with AgNPs and participate in nitrogen transformation processes. N 2 O is an important intermediate in nitrogen transformation processes and can be a potent greenhouse gas with significant environmental effects. However, the impacts of the co-existence of AgNPs and Fe(III) on microbial communities and N 2 O emission in river sediments are still unclear. In the present study, mesocosm experiments were conducted to assess the changes of microbial communities and N 2 O emission in response to the co-existence of AgNPs and environmental concentration Fe(III). Our results revealed that the microbial community diversity and N 2 O emission in river sediments responded differently to ecAgNPs (0.05 mg/kg) and high-polluting concentration AgNPs (hcAgNPs, 5 mg/kg), which was further regulated by the environmental concentration Fe(III) (1 mg/g and 10 mg/g). After ecAgNPs treatments, a marked increase was observed in microbial diversity compared to hcAgNPs treatments, regardless of the Fe(III) concentration in the sediment. The β-NTI index indicated that AgNPs had stronger impacts on phylogenetic distance of bacterial communities in sediments containing 1 mg/g Fe(III) than that containing 10 mg/g Fe(III). In sediments containing 1 mg/g Fe(III), ecAgNPs did not affect N 2 O emission, but hcAgNPs significantly inhibited the emission of N 2 O. However, in sediments containing 10 mg/g Fe(III), N 2 O emission was significantly stimulated upon exposure to ecAgNPs, but the inhibition effect of hcAgNPs was barely observed. Functional prediction and real-time PCR analyses indicated that AgNPs and Fe(III) predominantly affected N 2 O emissions by affecting the abundance of the nirK gene. Our results provide new insights into the ecological impacts of the co-existence of environmental concentration AgNPs and Fe(III) in altering microbial communities and nitrogen transformation functions in river sediments. Unlabelled Image • Environmental concentration AgNPs increased the microbial community diversity. • Microbial community composition showed changes in response to Fe(III) and AgNPs. • High environmental concentration Fe(III) and AgNPs stimulated N 2 O emission. • nirK gene exhibited remarkable sensitivity to the co-existence of AgNPs and Fe(III). [ABSTRACT FROM AUTHOR]

[Display omitted] • Redox gradients were identified in the riverbed sediments of target rivers. • DON in surface water and sediment porewater reached 0.47 and 5.02 mg-N/L. • DON properties were monitored by DOM spectral footprints via ML method. • C1 was the most important variable for prediction of winter samples in both models. • SUVA 260 and SUVA 280 were vital predictors for summer samples in suboxic zone. An evaluation on the impacts of effluent discharge to receiving rivers is in demand. Discharged dissolved organic nitrogen (DON) with high eutrophication potentials tends to accumulate in the shallow riverbed sediments especially for effluent-dominated rivers. Since DON characterization requires for advanced analytical tools, there is thereby a need but it is still a challenge to evaluate their distribution and transformation in the riverbed sediments. Considering the fact that DON is an important part of dissolved organic matter (DOM), which has plentiful spectral information, the authors hypothesize if it is feasible to predict DON variations in riverbed sediments via the DOM properties in surface water or sediment porewater? Here, the spatiotemporal spectral characteristics of DOM in river water and sediment porewater of two effluent-dominated rivers were investigated. Compared to surface water, more abundant relationships among fluorephores and spectra-derived parameters in porewater were observed. The relatively high DON contents in both surface water (0.16–0.47 mg-N/L) and sediment porewater (0.35–5.02 mg-N/L) compared to those reported in other natural waters highlighted the importance in predicting DON in effluent-dominated rivers. The DON in the oxic zone from winter samples carried more humus-like signals, while the fractions in summer samples recognized protein-like and humus-like signals. Two machine learning methods, i.e., random forest (RF) and extreme gradient boosting (XGBoost) were employed to provide evidence-based information on how DON contents and properties differed at variable depths. RF (R2: 0.920, 0.475) outperformed XGBoost (R2: 0.488, 0.316) in predicting DON in winter-oxic zone and winter-suboxic zone. While XGboot (R2: 0.515, 0.787) is superior than RF (R2: 0.317, 0.567) in summer-oxic zone and summer-suboxic zone. Fluorophore of C1 served as the most important variable for predicting DON in winter samples in both models. SUVA 260 and SUVA 280 were verified most impressive predictor for samples from suboxic zone in summer in both models. HIX represented the most important variable predicting DON in summer samples in XGBoost model. These findings are critical for understanding the transformation processes of DON in the shallow riverbed sediments in receiving rivers, and could provide important implications to improve wastewater discharge management and river health. [ABSTRACT FROM AUTHOR]

Ammonia oxidation plays a fundamental role in river nitrogen cycling ecosystems, which is normally governed by both ammonia oxidizing archaea (AOA) and ammonia oxidizing bacteria (AOB). Co-contamination of typical emerging pollutant Polybrominated diphenyl ethers (PBDEs) and heavy metal on AOA and AOB communities in river sediments remains unknown. In this study, multiple analytical tools, including high-throughput pyrosequencing and real-time quantitative PCR (qPCR), were used to reveal the ammonia monooxygenase (AMO) activity, subunit alpha ( amo A) gene abundance, and community structures of AOA and AOB in river sediments. It was found that the inhibition of AMO activities was increased with the increase of decabromodiphenyl ether (BDE 209, 1–100 mg kg −1 ) and copper (Cu, 50–500 mg kg −1 ) concentrations. Moreover, the synergic effects of BDE 209 and Cu resulted in a higher AMO activity reduction than the individual pollutant BDE 209. The AOA amo A copy number declined by 75.9% and 83.2% and AOB amo A gene abundance declined 82.8% and 90.0% at 20 and 100 mg kg −1 BDE 209 with a 100 mg kg −1 Cu co-contamination, respectively. The pyrosequencing results showed that both AOB and AOA community structures were altered, with a higher change of AOB than that of AOA. The results demonstrated that the AOB microbial community may be better adapted to BDE 209 and Cu pollution, while AOA might possess a greater capacity for stress resistance. Our study provides a better understanding of the ecotoxicological effects of heavy metal and micropollutant combined exposure on AOA and AOB in river sediments. [ABSTRACT FROM AUTHOR]

• Potential PE/PP degrading bacterial community was enriched via an 1150-day culture. • Core and resuscitated PE/PP degrading bacteria were firstly screened from rivers. • Diversity of PE/PP degrading bacteria was higher than that of indicatory bacteria. • Potential PE/PP degrading bacterial community widely distributes in natural rivers. • Eutrophic sediments of rivers have great microplastic degradation potential. Carbon chain microplastics, polyethylene (PE), and polypropylene (PP) are the main types of refractory organics. Compared to heterochain microplastics, PE/PP degrading bacterial community and their distribution characteristics in natural rivers are unclear. In this study, the field in situ experiment and indoor enrichment experiment with PE/PP resin as only carbon sources were conducted for a total period of 1150 days. The microbial degradation of pure PE/PP resin was determined by SEM, FTIR, CLSM, GC-MS, and GPC. The Chao 1 index and Invsimpson index of the bacterial community significantly reduced after a series of incubation, demonstrating that the bacterial community was selectively enriched. Empirical core PE/PP degrading bacteria (C-bacteria) and resuscitated PE/PP degrading bacteria (R-bacteria) were screened based on the variation of the abundance of OTUs, and co-occurrence analysis displayed that C-bacteria presented higher betweenness centrality than R-bacteria. The higher abundance and diversity of R-bacteria in biofilms suggest the presence of many rare or low abundance bacteria in natural rivers that may be potential PE/PP degrading bacteria or PE/PP degrading bacteria to be activated, while the lower abundance and diversity of C-bacteria support the slow degradation rate of PE and PP in waters. Compared to the isolated and indicatory PE/PP degrading bacterial genera, the C-bacteria OTUs or genera enriched in this study displayed higher richness and abundance. Enriched PE/PP degrading bacteria occurred in all sampled sites of the Qinhuai River with higher abundance and standard betweenness centrality in sediments (averaging 0.01354 and 0.44421, respectively) than those in overlying water (averaging 0.00536 and 0.17571, respectively), while the highest abundance of degrading bacteria presented in the eutrophic sediments. Inorganic nitrogen was determined to be significantly correlated with the distribution of PE/PP degrading bacteria in sediments via redundancy analysis. This study provides a new perspective on the natural degradation potential of carbon chain microplastics by microbial communities in rivers. [Display omitted] [ABSTRACT FROM AUTHOR]

Prioritizing the relationship between heterogeneity of sediment habitats and river bends is critical when planning and reconstructing urban rivers. However, the exact relationship between ecological heterogeneity and river bends remains ambiguous. Therefore, this research proposed a new approach to quantify and predict bend-induced ecological heterogeneity, incorporating the bacteria-based index of biotic integrity (Ba-IBI), path model, and random forest regression model. The developed Ba-IBI quantified heterogeneity in sediment microbial communities, ranging from low (1.40) to high (3.97). A path model was developed and validated in order to further investigate the relative contributions of environmental factors to the Ba-IBI. The established path model, which was considered acceptable with a CMIN/df = 1.949 < 4, suggested that primary environmental factors affecting the sediment bacterial communities were flow velocity and ammonium concentration in sediment. To further characterize the relationship between environmental factors and the Ba-IBI, a function was constructed using the random forest regression model that predicts the responses of sediment bacterial communities to environmental factors with R2 = 0.6126. The proposed approach and prediction tools will provide knowledge to improve natural channel design and post-project evaluations in river restoration projects. [Display omitted] • A novel way for the identification of ecological heterogeneity was established. • Ba-IBI scores revealed a significant difference between straight and bent channels. • Ba-IBI score was influenced by flow velocity, particle size and NH 4 +-N in sediment. • A function reflecting habitat heterogeneity with environmental paraments was found. [ABSTRACT FROM AUTHOR]

• Existence of alternative microbiota states in rivers were identified by field data. • NH 4 + − N and NO 3 − − N were identified as main environmental stress. • Clostridiales, Nitrospirales and Myxococcales were discerned as key taxa. • Alternative stable states theory can guide river biodiversity conservation. Catastrophic shifts in river ecosystems can abruptly degrade their structures and functions, often reducing the efficacy of traditional remediation targeting physicochemical properties. Alternative stable states theory can not only explain this phenomenon but also provide a new insight into river restoration; however, little is known about the existence and implications of alternative stable states in a river. Considering the important role of benthic microbiota in sustaining river ecosystem structures and functions, ecological theory and high-throughput sequencing were combined to firstly investigate multi-stability in microbial communities and its relationship with environmental factors in river sediments. The Nanjing reach of the Yangtze River was selected as the study area because of its huge spatial heterogeneity and varying degrees of pollution. Bimodal distributions combined with temporal variations of microbiota status provided direct evidence of bistability by showing the instability at the intermediate. In addition, environmental stress, particularly concentrations of NH 4 + − N and NO 3 − − N , was identified as an important driver of alternative microbiota states from the perspectives of the behavior of bistable ecosystems. Comparison of α-diversity indices and network properties between two alternative microbiota states revealed that the diversity and co-occurrence pattern of microbial communities will be high if they are settled in favorable environments (i.e., comprehensive sediment quality identification index > 3.7). Key taxa, including Clostridiales, Nitrospirales and Myxococcales, were discerned by combining LEfSe and network analysis, and their strong interspecies interactions were believed to be an important factor in triggering alternative microbiota states. This study suggests alternative stable states theory should be considered in river remediation to better understand the response of river ecosystems to environmental stress and the effect of hysteresis, benefiting the implementation of effective monitoring and restoration strategies in a river of urban area. [Display omitted] [ABSTRACT FROM AUTHOR]

Antidepressants may influence the food web and alter the nitrogen cycle through top-down forces. However, the effect of antidepressants on the key nitrogen-using species in the benthic microbial food web remains unclear, particularly the resulting changes in the nitrogen transformation process within the microecosystems. Therefore, in this study, we employed DNA stable-isotope probing to detect nitrogen-converting organisms at various trophic levels and quantify the nitrogen transformation process for the first time. The input of sertraline greatly increased nitrogen-transforming microorganisms and promoted more species to participate in the nitrogen transformation process. 100 μg/L sertraline was observed to stimulate the predation of bacteria via protozoa and metazoan, increasing the total nitrogen flow flux through the microbial food web to 31.50%, 1.32 times that of the natural condition. The results confirm that at sertraline concentrations close to the lowest observable effect concentration in the meiobenthos (100 μg/L), key components in the microbial food web were largely interfered and exerted a long-term interference on the nutrient cycle in the river sediment ecosystem. These findings confirm that sertraline has negative effects on river ecosystems from the perspective of microbial food webs and open a new line of inquiry into assessing ecological risks of antidepressants. [Display omitted] • DNA-SIP discovered dominant Nitrogen-transforming bacteria. • Nitrogen flow pathways through microbial food web were formed. • DNA-SIP and uptake activity measurements quantified nitrogen flow pathways. • Sertraline concentration near the LOEC will disturb nutrient cycle in sediment. [ABSTRACT FROM AUTHOR]

• Microplastic concentration increased from shallow to deep layers of sediments • Smaller microplastic particles were dominant in deeper layers • Bacterial community on microplastics has less stability than that in sediments • More indicative plastic-degrading bacteria colonizing microplastics than sediments • More indicative plastic-degrading bacteria colonizing microplastics in deeper layers Sediments have been found to be one of the most important reservoirs for microplastics, providing abundant indigenous microbes. The processes involved in the distribution and degradation behavior of microplastics are complex. This study investigated the vertical distribution of microplastics (with the size < 5 mm) and the bacterial community assemblages colonizing microplastics in urban river sediments at a depth from 0 to 50 cm. The results showed that both microplastics and associated microbial communities presented vertical profiles in river sediments. The mean concentration of microplastics increased from the shallow layers to the deep layers of sediments, and smaller microplastic particles were dominant in deeper layers. A greater degradation of microplastics in deeper layers was confirmed by contact angle measurements, scanning electron microscopy and Fourier transform infrared spectroscopy-attenuated total reflectance analyses. Unlike the surrounding sediments, the whole bacterial communities on microplastics exhibited higher frequency of positive correlations in the bacterial co-occurrence network, which indicated a less stability of bacterial communities on microplastics. The indicative plastic-degrading bacteria with an average abundance of 4.33% was found in the surrounding sediments, while on the microplastics 21.37% was found. From shallow layers to deep layers, the indicative plastic-degrading bacteria significantly increased both in the abundance and in their betweenness centrality in the co-occurrence network, which suggested a potentially primary role of these bacteria in the degradation of microplastics in deep layers. This study provides new insight into the vertical distribution and the potential microbial degrading characteristics of microplastics in urban river sediments, which expanded our understanding of the fate of microplastics in aquatic environments. The observed results implied a great risk that microplastics might become smaller and more in deepened sediments and finally migrate into groundwater. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Understanding the differences of biogeochemical processes between straight and bent channel is important for weighting them in urban river planning and reconstruction. Shifts in the assembly of the sediment microbial community of bent channels are key, but understudied, component of bend-induced increases in biogeochemical reaction rates. Here, the assembly of microbial community and its feedback to nitrogen transformation in urban river bends were firstly studied by coupling ecological theory, aqueous biogeochemistry, DNA sequencing, and hydrodynamic profiling. It was found that the sediment particle size was the main driving force for producing the significant difference of microbial community structure in river bends. Homogeneous selection, quantified by β-nearest taxon index (βNTI), emerged in the urban river bends and accounted for 79.2% of all ecological processes. Moreover, a significant positive relationship between βNTI and the sediment particle size indicated that shifts in particle size were associated with shifts in deterministic selective pressures, which govern the composition of the microbial community. The significant correlation between the βNTI and changes in nitrate concentration also indicated that nitrate leads to deterministic processes, which select microbial taxa. These microbial taxa which are governed by deterministic processes show specific nitrogen transformation traits, and react on the nitrate concentration. A multiphase transport model allowed the separation of the effects of deterministic processes on nitrogen concentration from measured concentration influenced by complex biogeochemical processes. The results indicated that both the ammonia transformation and microbial nitrogen removal were stimulated in coarse sediment regions of the river bends, and were confirmed by abundant differences of microbial taxa that could promote ammoxidation and denitrification. The coarse sediment benefits microbial nitrogen removal in urban river bends, a discovery that should inform urban river reconstruction designs and the efforts to assess the environmental water capacity of urban rivers. Image 1 • Microbial community distribution and assembly in urban river bends was analyzed. • Sediment particle size governs microbial community distribution in the river bends. • Microbial community affects nitrogen transformation by deterministic processes. • Microbial nitrogen removal in coarse sediment region of river bends was enhanced. [ABSTRACT FROM AUTHOR]