Denitrification Research Articles

Rapid conversion of heterotrophic denitrification to autotrophic denitrification in mariculture wastewater treatment: Denitrification performance and microbial communities under antibiotic stress

Mariculture wastewater, characterized by a low COD/Nitrogen (C/N) ratio and the occurrence of antibiotics, is commonly treated by a heterotrophic denitrification (HD) process with supplying substantial carbon sources. Compared to HD, sulfur-based autotrophic denitrification (AD) requires less external carbon sources and is more favorable for low-carbon wastewater treatment. However, the AD process often consumes more startup time. Thus, the present study was designed to explore a rapid startup strategy for the AD process by switching from HD to AD, treating mariculture wastewater. A rapid transformation from HD to AD was achieved by inoculating mixotrophic denitrifying sludge, reducing carbon sources and adding sulfur sources under the antibiotic sulfamethoxazole (SMX). From the functional microbial community point of view, our results showed that autotrophic bacteria exhibited greater adaptability to sulfamethoxazole than heterotrophic bacteria. During the transition from HD to AD, nitrate removal improved from 53.13 % to 68.57 %, and the secretion of tightly bound extracellular polymeric substances (TB-EPS) increased by 29.94 mg/g VSS. Meanwhile, in the heterotrophic phase, the presence of sulfate in mariculture wastewater maintained a high abundance of sulfate-reducing bacteria such as Desulfuromusa (0.8 %) and sulfur-based autotrophic denitrifying bacteria such as Sedimenticola (36.7 %) and Thiogranum (15.2 %). This increases the activity of sulfur metabolism during the heterotrophic phase and ensures a rapid transition from HD to AD. The rapid shift from HD to AD in seawater under antibiotic stress provides theoretical support for the improvement of mariculture wastewater treatment processes.

Promising membrane photobioreactor for continuous flow treatment of Low C/N wastewater: Treating process, community structure, and cooperation mechanism

Low carbon/nitrogen (C/N) wastewater treatment is an emerging issue for conventional treatment technologies. To achieve this goal, a novel membrane photobioreactor (MPBR), integrating microalgae consortium (MC) and activated sludge (AS) phases, was investigated. The separation of the MC and AS phases could improve the removal of pollutants in low C/N wastewater, which could achieve the rapid denitrogenation through NH4+-N assimilation in MC phase, and effectively reduce organic matter concentration in AS phase, resulting in better nutrient recovery, lower costs, and stable pH. The results in phase I showed that the pollutants, such as chemical oxygen demand (COD), ammonium (NH4+-N), and total phosphorus (TP), met the Environmental Quality Standards for Surface Water (GB3838–2002) by MPBR. The AS phase with low DO, which could enrich heterotrophic bacteria (e.g., Bacteroidetes vadinHA17, Spirochaeta and Ahniella) and nitrogen cycling bacteria (e.g., Mycobacterium, Bacillus and Ellin6067), could promote the removal of COD and NO3--N in the continuous effluent. Based on the co-occurrence pattern analysis, the main influencing factor of MC and AS phases was the reduction of COD, which also illustrated the synergistic removal of organic matter by the cooperative interaction of microalgae and heterotrophic bacteria. From a genetic perspective, the internal circulation mode also enhanced microbial nitrogen metabolism in the MPBRs, with higher abundances of nitrification- and denitrification-related functional genes in the MC phase, and higher abundances of nitrification, dissimilatory nitrate reduction, and denitrification genes in the AS phase. This study provided an experimental and theoretical basis for MPBR in low-C/N wastewater treatment.

Iron-scrap and pyrite mediated autotrophic denitrification in constructed wetland for enhancing nitrate-polluted groundwater remediation: Performance and mechanism

Nitrate (NO3−-N) is ubiquitously present in groundwater, constituting a growing threat to human health. In this study, iron-scrap and pyrite-based constructed wetlands (CWs) were constructed to investigate the treatment performance and removal mechanisms in purifying nitrate-contaminated groundwater. Results showed that the NO3−-N removal efficiency of iron-scrap-based CWs (87.1%) was higher than that of pyrite-based CWs (59.5%), and the higher N2O emission (28.46 μg/(m2•h)) was also obtained in CW with iron-scrap. Nevertheless, total nitrogen removal did not exhibit a significant difference between two CWs due to the accumulation of ammonium nitrogen (NH4+-N) in iron-scrap-based CWs. Additionally, sulfate was produced in CW with pyrite, as a by-product, and caused oxidative stress damage to plants. Metagenomic analysis showed that the addition of iron-scrap could increase microbial diversity and enrich autotrophs denitrifies (e.g., Ferritrophicum), which explained the higher NO3−-N removal reasonably. Additionally, the abundance of dissimilatory nitrate reduction bacterium was enhanced in iron-scrap-based CW, explaining the NH₄⁺-N accumulation. Further metabolic analysis confirmed the occurrence of Fe cycle in CW with iron-scrap, and functional genes related to NO3−-N removal and TCA cycle were up-regulated, offering a rationale for the improved NO3−-N removal in CW with iron-scrap. This study offers comprehensive insights into the NO3−-N removal and its mechanism through iron mediated CWs in controlling groundwater pollution.

Hydrological and biogeochemical processes controlling riparian groundwater quantity and quality during riverbank filtration

Groundwater exploitation in a riparian zone causes water infiltration from the river into the aquifer. Owing to adsorption and redox reactions along the flow path, the quality of water flowing from the river to groundwater wells is variably altered. The riverbed composition often involves spatiotemporal differences due to frequent changes in hydrological conditions. These changes create uncertainties in the transport and removal of solutes in the river water. In this study, the hydrodynamic field associated with riparian groundwater, changes in the structure of riverbed sediments caused by erosion and deposition, fluctuations in surface water and groundwater levels, and the removal efficiency of pollutants from groundwater through pumping were investigated. This involved in situ monitoring and sample testing of the composition of the river water, riverbed sediments, riverbed pore water, and groundwater during dry and wet seasons. Implementation of field in situ column experiments and molecular biology evidences were conducive to identifying the main biogeochemical processes occurring in the riverbed. The findings indicated that riparian groundwater exploitation alters the natural groundwater flow field, while fine sand deposition and microbial adsorption can reduce river recharge to aquifers by diminishing riverbed hydraulic conductivity. Shallow sediments within 1 m depth mainly involve NO3− reduction and E. coli adsorption. Reductive dissolution of Mn dominates in the deeper sediments. Additionally, reductive dissolution of Fe and dissimilatory nitrate reduction to ammonium (DNRA) drive high Fe2+ and NH4+ concentrations in groundwater. The findings can improve the management of riparian groundwater and aid in the optimization of a plan for its exploitation.

Competitive partitioning of denitrification pathways during arrested methanogenesis: Implications in ammonium recovery, N2O emission, and volatile fatty acid production

The complex interaction between nitrate (NO3−) reduction and fermentation is poorly understood when high levels of NO3− are introduced into anaerobic systems. This study investigated the competitive distribution between conventional denitrification (DEN) and dissimilatory nitrate reduction to ammonium (DNRA) during simultaneous denitrification and fermentation in arrested methanogenesis. Up to 62% of initial NO3− (200 mg-N/L) was retained as ammonium through DNRA at a chemical oxygen demand (COD)/N ratio of 25. Significant N2O emission occurred (1.7 – 8.0% of the initial NO3−) with limited carbon supply (≤1600 mg COD/L) and sludge concentration (≤3000 mg COD/L). VFA composition shifted predominantly towards acetic acid (>50%) in the presence of nitrate. A novel kinetic model was developed to predict DNRA vs. DEN partitioning and NO2− accumulation. Overall, NO3− input, organic loading, and carbon source characteristics independently and collectively controlled competitive DNRA vs. DEN partitioning.

Nitrogen and sulfur cycling and their coupling mechanisms in eutrophic lake sediment microbiomes

Microorganisms play important roles in the biogeochemical cycles of lake sediment. However, the integrated metabolic mechanisms governing nitrogen (N) and sulfur (S) cycling in eutrophic lakes remain poorly understood. Here, metagenomic analysis of field and bioreactor enriched sediment samples from a typical eutrophic lake were applied to elucidate the metabolic coupling of N and S cycling. Our results showed significant diverse genes involved in the pathways of dissimilatory sulfur metabolism, denitrification and dissimilatory nitrate reduction to ammonium (DNRA). The N and S associated functional genes and microbial groups generally showed significant correlation with the concentrations of NH4+, NO2− and SO42, while with relatively low effects from other environmental factors. The gene-based co-occurrence network indicated clear cooperative interactions between N and S cycling in the sediment. Additionally, our analysis identified key metabolic processes, including the coupled dissimilatory sulfur oxidation (DSO) and DNRA as well as the association of thiosulfate oxidation complex (SOX systems) with denitrification pathway. However, the enriched N removal microorganisms in the bioreactor ecosystem demonstrated an additional electron donor, incorporating both the SOX systems and DSO processes. Metagenome-assembled genomes-based ecological model indicated that carbohydrate metabolism is the key linking factor for the coupling of N and S cycling. Our findings uncover the coupling mechanisms of microbial N and S metabolism, involving both inorganic and organic respiration pathways in lake sediment. This study will enhance our understanding of coupled biogeochemical cycles in lake ecosystems.

Deciphering linkages between DON and the microbial community for nitrogen removal using two green sorption media in a surface water filtration system

The presence of dissolved organic nitrogen (DON) in stormwater treatment processes is a continuous challenge because of the intertwined nature of its decomposition, bioavailability, and biodegradability and its unclear molecular characteristics. In this paper, 21 T Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) in combination with quantitative polymerase chain reaction was applied to elucidate the molecular change of DON and microbial population dynamics in a field-scale water filtration system filled with two specialty adsorbents for comparison in South Florida where the dry and wet seasons are distinctive annually. The adsorbents included CPS (clay-perlite and sand sorption media) and ZIPGEM (zero-valent iron and perlite-based green environmental media). Our study revealed that seasonal effects can significantly influence the dynamic characteristics and biodegradability of DON. The microbial population density in the filter beds indicated that three microbial species in the nitrogen cycle were particularly thrived for denitrification, dissimilatory nitrate reduction to ammonium, and anaerobic ammonium oxidation via competition and commensalism relationships during the wet season. Also, there was a decrease in the compositional complexity and molecular weight of the DON groups (CnHmOpN1, CnHmOpN2, CnHmOpN3, and CnHmOpN4), revealed by the 21 T FT-ICR MS bioassay, driven by a microbial population quantified by polymerase chain reaction from the dry to the wet season. These findings indirectly corroborate the assumption that the metabolism of microorganisms is much more vigorous in the wet season. The results affirm that the sustainable materials (CPS and ZIPGEM) can sustain nitrogen removal intermittently by providing a suitable living environment in which the metabolism of microbial species can be cultivated and enhanced to facilitate physico-chemical nitrogen removal across the two types of green sorption media.

Enhancing nitrogen removal performance using immobilized aerobic denitrifying bacteria by modified polyvinyl alcohol/sodium alginate (PVA/SA)

Aerobic denitrification has emerged as a promising and efficient method for nitrogen removal from wastewater. However, the direct application of aerobic denitrifying bacteria has faced challenges such as low nitrogen removal efficiency, bacterial loss, and poor stability. To address these issues, this study developed a novel microbial particle carrier using NaHCO3-modified polyvinyl alcohol (PVA)/sodium alginate (SA) gel (NaHCO3-PVA/SA). This carrier exhibits several advantageous properties, including excellent mass transfer efficiency, favorable biocompatibility, convenient film formation, abundant biomass, and exceptional pollutant treatment capacity. The carrier was modified with 0.3% NaHCO3, 8.0% PVA, and 1.0% SA, resulting in a remarkable 3.4-fold increase in the average pore diameter and a 12.8% improvement in mass transfer efficiency. This carrier was utilized to immobilize the aerobic denitrifying bacterium Stutzerimonas stutzeri W-2 to enhance nitrogen removal (NaHCO3-PVA/SA@W-2), resulting in a NO3−-N removal efficiency of 99.06%, which was 21.39% higher than that without modification. Compared with the non-immobilized W-2, the degradation efficiency was improved by 43.70%. After five reuses, the NO3−-N and TN removal rates remained at 99% and 93.01%, respectively. These results provide a solid foundation for the industrial application of the modified carrier as an effective tool for nitrogen removal in large-scale wastewater treatment processes.

Effects of soil water content at freezing, thaw temperature, and snowmelt infiltration on N2O emissions and denitrifier gene and transcript abundance during a single freeze-thaw event

Climate change-related warming and increased precipitation may alter winter snow cover and thawing events, and therefore, may carry significant consequences for nitrous oxide (N2O) production pathways such as denitrification, and the abundance and expression of denitrifying microorganisms. We used a soil microcosm study to investigate the combined effect of soil thaw temperature, initial water filled pore space (WFPS) prior to soil freezing, and snowmelt infiltration simulated by the addition of water on N2O emission and denitrification rates, soil respiration rate, and the abundance and transcription of denitrifying (nirK, nirS, and nosZ) bacteria during a single freeze-thaw event. Soil respiration rate was primarily controlled by an increase in soil thaw temperature, whereas soil N2O emission and denitrification rates were generally greater in soils with a higher initial WFPS and soil thaw temperature. In contrast, snowmelt infiltration generally had a negligible effect on these rates, which may be related to pre-existing soil conditions that were already conducive to denitrification. Unexpectedly, the nosZ transcript/nosZ gene abundance ratio was lower in soils thawed at 8.0 °C compared to 1.5 °C; however, this may have resulted in a lower N2O reduction, thus explaining the greater levels of N2O emitted from soils thawed at 8.0 °C. Overall, this study demonstrated that increased N2O production during a single freeze-thaw event was primarily linked to antecedent conditions of high initial WFPS, soil thaw temperature, and a synergistic interplay between these two environmental parameters, and provides evidence that an increase in annual temperature and precipitation, along with the timing of precipitation, may further stimulate N2O production pathways.

Impact of bioelectricity on DNRA process and microbial community composition within cathodic biofilms in dual-chambered bioelectrode microbial fuel cell (MFC)

The synchronous bioelectricity generation and dissimilatory nitrate reduction to ammonium (DNRA) pathway in Klebsiella variicola C1 was investigated. The presence of bioelectricity facilitated cell growth on the anodic biofilms, consequently enhancing the nitrate removal efficiency decreasing total nitrogen levels and causing a negligible accumulation of NO2– in the supernatant. Genomic analysis revealed that K. variicola C1 possessed a complete DNRA pathway and largely annotated electron shuttles. The up-regulated expression of genes narG and nirB, encoding nitrite oxidoreductase and nitrite reductase respectively, was closely associated with increased extracellular electron transfer (EET). High-throughput sequencing analysis was employed to investigate the impact of bioelectricity on microbial community composition within cathodic biofilms. Results indicated that Halomonas, Marinobacter and Prolixibacteraceae were enriched at the cathode electrodes. In conclusion, the integration of a DNRA strain with MFC facilitated the efficient removal of wastewater containing high concentrations of NO3– and enabled the environmentally friendly recovery of NH4+.

Deep insights into the roles and microbial ecological mechanisms behind waste activated sludge digestion triggered by persulfate oxidation activated through multiple modes

Persulfate oxidation (PS) is widely employed as a promising alternative for waste activated sludge pretreatment due to the capability of generating free radicals. The product differences and microbiological mechanisms by which PS activation triggers WAS digestion through multiple modes need to be further investigated. This study comprehensively investigated the effects of persulfate oxidation activated through multiple modes, i.e., ferrous, zero-valent iron (ZVI), ultraviolet (UV) and heat, on the performance of sludge digestion. Results showed that PS_ZVI significantly accelerated the methane production rate to 12.02 mL/g VSS. By contrast, PS_Heat promoted the sludge acidification and gained the maximum short-chain fatty acids (SCFAs) yield (277.11 ± 7.81 mg COD/g VSS), which was 3.41-fold compared to that in PS_ZVI. Moreover, ferrous and ZVI activated PS achieved the oriented conversion of acetate, the proportions of which took 73% and 78%, respectively. MiSeq sequencing results revealed that PS_Heat and PS_UV evidently enriched anaerobic fermentation bacteria (AFB) (i.e., Macellibacteroides and Clostridium XlVa). However, PS_Ferrous and PS_ZVI facilitated the enrichment of Woesearchaeota and methanogens. Furthermore, molecular ecological network and mantel test revealed the intrinsic interactions among the multiple functional microbes and environmental variables. The homo-acetogens and sulfate-reducing bacterial had potential cooperative and symbiotic relationships with AFB, while the nitrate-reducing bacteria displayed distinguishing ecological niches. Suitable activation modes for PS pretreatments resulted in an upregulation of genes expression responsible for digestion. This study established a scientific foundation for the application of sulfate radical-based oxidation on energy or high value-added chemicals recovery from waste residues.

Enhancement resistance to microbiologically influenced stress corrosion of Cu-bearing steel against Bacillus cereus

Microorganisms are notoriously known to cause local corrosion and stress corrosion cracking (SCC), which seriously endangers the materials service safety. Cu can enhance antibacterial function of the material and reduce the vulnerability to hydrogen embrittlement (HE). However, the dilemma of how much Cu content generates the best resistance to microbiological corrosion and SCC arises. Here, we modified the Cu content in pipeline steel to obtain the best antibacterial effect to nitrate reducing bacteria Bacillus cereus and HE resistance. The findings offer a fresh perspective on how to design and prepare a steel that are both resistant to microbiological corrosion and SCC.

Oral nitrate-reducing bacteria as potential probiotics for blood pressure homeostasis.

Hypertension is a leading cause of morbidity and mortality worldwide and poses a major risk factor for cardiovascular diseases and chronic kidney disease. Research has shown that nitric oxide (NO) is a vasodilator that regulates vascular tension and the decrease of NO bioactivity is considered one of the potential pathogenesis of essential hypertension. The L-arginine-nitric oxide synthase (NOS) pathway is the main source of endogenous NO production. However, with aging or the onset of diseases, the function of the NOS system becomes impaired, leading to insufficient NO production. The nitrate-nitrite-NO pathway allows for the generation of biologically active NO independent of the NOS system, by utilizing endogenous or dietary inorganic nitrate and nitrite through a series of reduction cycles. The oral cavity serves as an important interface between the body and the environment, and dysbiosis or disruption of the oral microbiota has negative effects on blood pressure regulation. In this review, we explore the role of oral microbiota in maintaining blood pressure homeostasis, particularly the connection between nitrate-reducing bacteria and the bioavailability of NO in the bloodstream and blood pressure changes. This review aims to elucidate the potential mechanisms by which oral nitrate-reducing bacteria contribute to blood pressure homeostasis and to highlight the use of oral nitrate-reducing bacteria as probiotics for oral microbiota intervention to prevent hypertension.

Nitrate-dependent anaerobic methane oxidation coupled to Fe(III) reduction as a source of ammonium and nitrous oxide

‘Candidatus Methanoperedens nitroreducens’ is an archaeal methanotroph with global importance that links carbon and nitrogen cycles and great potential for sustainable operation of wastewater treatment. It has been reported to mediate the anaerobic oxidation of methane through a reverse methanogenesis pathway while reducing nitrate to nitrite. Here, we demonstrate that ‘Ca. M. nitroreducens’ reduces ferric iron forming ammonium (23.1 %) and nitrous oxide (N2O, 46.5 %) from nitrate. These results are supported with the upregulation of genes coding for proteins responsible for dissimilatory nitrate reduction to ammonium (nrfA), N2O formation (norV, cyt P460), and multiple multiheme c-type cytochromes for ferric iron reduction. Concomitantly, an increase in the N2O-reducing SJA-28 lineage and a decrease in the nitrite-reducing ‘Candidatus Methylomirabilis oxyfera’ are consistent with the changes in ‘Ca. M. nitroreducens’ end products. These findings demonstrate the highly flexible physiology of ‘Ca. M. nitroreducens’ in anaerobic ecosystems with diverse electron acceptor conditions, and further reveals its roles in linking methane oxidation to global biogeochemical cycles. ‘Ca. M. nitroreducens’ could significantly affect the bioavailability of nitrogen sources as well as the emission of greenhouse gas in natural ecosystems and wastewater treatment plants.

Genome-based approach to evaluate the metabolic potentials and exopolysaccharides production of Bacillus paralicheniformis CamBx3 isolated from a Chilean hot spring.

In the present study, a thermophilic strain designated CamBx3 was isolated from the Campanario hot spring, Chile. Based on 16S rRNA gene sequence, phylogenomic, and average nucleotide identity analysis the strain CamBx3 was identified as Bacillus paralicheniformis. Genome analysis of B. paralicheniformis CamBx3 revealed the presence of genes related to heat tolerance, exopolysaccharides (EPS), dissimilatory nitrate reduction, and assimilatory sulfate reduction. The pangenome analysis of strain CamBx3 with eight Bacillus spp. resulted in 26,562 gene clusters, 7,002 shell genes, and 19,484 cloud genes. The EPS produced by B. paralicheniformis CamBx3 was extracted, partially purified, and evaluated for its functional activities. B. paralicheniformis CamBx3 EPS with concentration 5 mg mL-1 showed an optimum 92 mM ferrous equivalent FRAP activity, while the same concentration showed a maximum 91% of Fe2+ chelating activity. B. paralicheniformis CamBx3 EPS (0.2 mg mL-1) demonstrated β-glucosidase inhibition. The EPS formed a viscoelastic gel at 45°C with a maximum instantaneous viscosity of 315 Pa.s at acidic pH 5. The present study suggests that B. paralicheniformis CamBx3 could be a valuable resource for biopolymers and bioactive molecules for industrial applications.

Metagenomics reveals the variations in functional metabolism associated with greenhouse gas emissions during legume-vegetable rotation process

Legume-based rotation is commonly recognized for its mitigation efficiency of greenhouse gas (GHG) emissions. However, variations in GHG emission-associated metabolic functions during the legume-vegetable rotation process remain largely uncharacterized. Accordingly, a soybean-radish rotation field experiment was designed to clarify the responses of microbial communities and their GHG emission-associated functional metabolism through metagenomics. The results showed that the contents of soil organic carbon and total phosphorus significantly decreased during the soybean-radish process (P < 0.05), while soil total potassium content and bacterial richness and diversity significantly increased (P < 0.05). Moreover, the predominant bacterial phyla varied, with a decrease in the relative abundance of Proteobacteria and an increase in the relative abundance of Acidobacteria, Gemmatimonadetes, and Chloroflexi. Metagenomics clarified that bacterial carbohydrate metabolism substantially increased during the rotation process, whereas formaldehyde assimilation, methanogenesis, nitrification, and dissimilatory nitrate reduction decreased (P < 0.05). Specifically, the expression of phosphate acetyltransferase (functional methanogenesis gene, pta) and nitrate reductase gamma subunit (functional dissimilatory nitrate reduction gene, narI) was inhibited, indicating of low methane production and nitrogen metabolism. Additionally, the partial least squares path model revealed that the Shannon diversity index was negatively correlated with methane and nitrogen metabolism (P < 0.01), further demonstrating that the response of the soil bacterial microbiome responses are closely linked with GHG-associated metabolism during the soybean-radish rotation process. Collectively, our findings shed light on the responses of soil microbial communities to functional metabolism associated with GHG emissions and provide important insights to mitigate GHG emissions during the rotational cropping of legumes and vegetables.

Genome-resolved metagenomics identifies novel active microbes in biogeochemical cycling within methanol-enriched soil.

Metagenome assembled genomes (MAGs), generated from sequenced 13C-labelled DNA from 13C-methanol enriched soils, were binned using an ensemble approach. This method produced a significantly larger number of higher-quality MAGs compared to direct binning approaches. These MAGs represent both the primary methanol utilizers and the secondary utilizers labelled via cross-feeding and predation on the labelled methylotrophs, including numerous uncultivated taxa. Analysis of these MAGs enabled the identification of multiple metabolic pathways within these active taxa that have climatic relevance relating to nitrogen, sulfur and trace gas metabolism. This includes denitrification, dissimilatory nitrate reduction to ammonium, ammonia oxidation and metabolism of organic sulfur species. The binning of viral sequence data also yielded extensive viral MAGs, identifying active viral replication by both lytic and lysogenic phages within the methanol-enriched soils. These MAGs represent a valuable resource for characterizing biogeochemical cycling within terrestrial environments.

Colloidal Filterable Bacteria Enhance Ammonia Nitrogen Enrichment in River Colloids under Different Turbidity Conditions: Bacterial Diversity, Assembly Mechanism, and Nitrogen Transformation

Turbidity has been one of the most typical problems in urban rivers, accompanied by eutrophication. Though the colloid is a nonnegligible factor associated with turbidity and nutrient enrichment in urban rivers, the characteristics of nitrogen enrichment and bacterial communities of colloids under different turbidity conditions of urban rivers have not been well understood. In this study, colloids of low and high molecular weights (LMW, 30 kDa–0.2 μm, and HMW, 0.2–1 μm) were separately collected from the bulk water (&lt;1 μm) of several typical urban rivers in China. Since the colloidal concentration presented the significantly highest correlation with turbidity, colloidal characteristics were further explored under three turbidity gradients with two cutoffs of 10 and 30 NTU. Results showed that colloidal organic matter in medium and high turbidity rivers was mainly sourced from the release of endogenous plankton and the proportion of colloidal organic carbon in dissolved organic carbon increased from 33% to 38% with increased turbidity. Colloidal ammonia nitrogen in medium turbidity accounted for the highest proportion (an average of 60%) in bulk water, which could be explained by the significantly positive correlation of colloidal ester groups and ammonia nitrogen (R2 = 0.47). Bulk water, HMW, and LMW colloids presented different dominant bacterial genera and LMW colloids also contained three unique dominant filterable genera: Flavobacterium, Acinetobacter, and Limnohabitans. LMW colloidal filterable bacteria under medium and high turbidities presented the greatest potential for dissimilatory nitrate reduction to ammonium, which might further enhance the enrichment of ammonia nitrogen in colloids. This study provides a primary understanding of the characteristics of colloids and colloidal bacterial communities in urban rivers from the perspective of turbidity and puts a new insight on the remediation of rivers under medium turbidity.

Resting cells of Skeletonema marinoi assimilate organic compounds and respire by dissimilatory nitrate reduction to ammonium in dark, anoxic conditions.

Diatoms can survive long periods in dark, anoxic sediments by forming resting spores or resting cells. These have been considered dormant until recently when resting cells of Skeletonema marinoi were shown to assimilate nitrate and ammonium from the ambient environment in dark, anoxic conditions. Here, we show that resting cells of S. marinoi can also perform dissimilatory nitrate reduction to ammonium (DNRA), in dark, anoxic conditions. Transmission electron microscope analyses showed that chloroplasts were compacted, and few large mitochondria had visible cristae within resting cells. Using secondary ion mass spectrometry and isotope ratio mass spectrometry combined with stable isotopic tracers, we measured assimilatory and dissimilatory processes carried out by resting cells of S. marinoi under dark, anoxic conditions. Nitrate was both respired by DNRA and assimilated into biomass by resting cells. Cells assimilated nitrogen from urea and carbon from acetate, both of which are sources of dissolved organic matter produced in sediments. Carbon and nitrogen assimilation rates corresponded to turnover rates of cellular carbon and nitrogen content ranging between 469 and 10,000 years. Hence, diatom resting cells can sustain their cells in dark, anoxic sediments by slowly assimilating and respiring substrates from the ambient environment.

Prediction of microbial activity and abundance using interpretable machine learning models in the hyporheic zone of effluent-dominated receiving rivers

Serving as a vital linkage between surface water and groundwater, the hyporheic zone (HZ) plays a fundamental role in improving water quality and maintaining ecological security. In arid or semi-arid areas, effluent discharge from wastewater treatment facilities could occupy a predominant proportion of the total base flow of receiving rivers. Nonetheless the relationship between microbial activity, abundance and environmental factors in the HZ of effluent-receiving rivers appear to be rarely addressed. In this study, a spatiotemporal field study was performed in two representative effluent-dominated receiving rivers in Xi'an, China. Land use data, physical and chemical water quality parameters of surface and subsurface water were used as predictive variables, while the microbial respiratory electron transport system activity (ETSA), the Chao1 and Shannon index of total microbial community, as well as the Chao1 and Shannon index of denitrifying bacteria community were used as response variables, while ETSA was used as response variables indicating ecological processes and Shannon and Chao1 were utilized as parameters indicating microbial diversity. Two machine learning models were utilized to provide evidence-based information on how environmental factors interact and drive microbial activity and abundance in the HZ at variable depths. The models with Chao1 and Shannon as response variables exhibited excellent predictive performances (R2: 0.754–0.81 and 0.783–0.839). Dissolved organic nitrogen (DON) was the most important factor affecting the microbial functions, and an obvious threshold value of ∼2 mg/L was observed. Credible predictions of models with Chao1 and Shannon index of denitrifying bacteria community as response variables were detected (R2: 0.484–0.624 and 0.567–0.638), with soluble reactive phosphorus (SRP) being the key influencing factor. Fe (Ⅱ) was favorable in predicting denitrifying bacteria community. The ESTA model highlighted the importance of total nitrogen in the ecological health monitoring in HZ. These findings provide novel insights in predicting microbial activity and abundance in highly-impacted areas such as the HZ of effluent-dominated receiving rivers.