Effect of CO2 on Microbial Denitrification via Inhibiting Electron Transport and Consumption

Self-Regulating pH Pyrite-Construction waste Biofilter: Denitrification Performance, Metabolic Pathways, and Clogging Alleviation

Waste-based denitrification filters face challenges like alkalinity accumulation, low efficiency, and clogging. This study proposes a novel denitrification filter using construction waste and pyrite (WPDF) to address these issues. WPDF's performance, safety, and mechanisms were evaluated by measuring effluent, filler characteristics and metagenomics. Results demonstrated a high total nitrogen removal load (88.65 g N m-3d-1) with minimal biofilm (13 %) and filler accumulation (39 %), effectively mitigating clogging. Phosphorus removal relied on chemical precipitation in construction waste. WPDF was pH self-regulating and promoted the formation and release of fulvic acid. Pyrite promotes bio-metabolism, making WPDF enriched in energy metabolism (6 %) and transporter capacity (6 %). Functional prediction indicated that WPDF was more abundant in genes related to denitrification, glycolysis, and electron transport, which promoted the heterotrophic denitrification process. This study provides a novel, efficient, and eco-friendly possible solution for wastewater and offers new insights into the molecular mechanisms of carbon and nitrogen metabolism.

Atrazine disrupts nitrogen removal performance and greenhouse gas abatement in bioretention systems: Unraveling microbiotas and macrophytes responses

The potential impact of stormwater runoff-induced loss of triazine herbicides, like atrazine (ATZ), on soil nitrogen cycling remains poorly understood. Bioretention systems (BRS) represent effective stormwater control measures (SCM) now understood to serve as important ATZ accumulation zones. However, the effects of ATZ exposure on nitrogen removal and greenhouse gas (GHG) abatement within BRS remain unclear. In the present study, bioretention columns were established and exposed to ATZ (0-25 mg kg-1) for 200 days. The results demonstrated that the accumulation of ATZ led to a reduction in total nitrogen removal efficiency (by 7.7-49.3 %) while simultaneously causing an increase in GHG emission fluxes (by 11.2 %-25.1 %). Moreover, ATZ significantly altered microbial activities, including nitrogen metabolism enzymes (hydroxylamine oxidoreductase, nitrate reductase, and nitrite reductase) and the electron transport system (ETSA). Microbial community analysis showed that ATZ reduced the relative abundance of nitrifying bacteria (Nitrospira and Nitrosomonas), along with certain denitrifying bacteria (Thauera, Terrimonas, and Dechloromona). Besides, the compromised function of leaves and roots diminished plant nitrogen uptake, and the application of structural equation modeling (SEM) revealed an increased contribution of plants to nitrogen removal. These findings collectively suggest that the widespread presence of triazine herbicides in urban areas could potentially impact the performance of SCMs.

Microbial denitrification responses to elevated CO2 in lake-shore sediments under different flooding conditions

The lake-shore zone is a critical site for recognizing the ecological impacts of global climate change, such as persistent elevated CO2 (eCO2). However, the direct effect of eCO2 on functional microorganisms in lake-shore sediments is neglected or confused at previous studies. In this study, the short-term direct effects of eCO2 on microbial denitrification in lake-shore sediments were investigated by simulating four flooding conditions (non-flooding (NF), intermittent flooding (IF), alternating high and low water-level flooding (HLF), and constant water-level flooding (WF)) through incubation experiments in the laboratory. This work demonstrated eCO2 directly increased denitrification in lake-shore sediments under different water-level conditions by different regulatory mechanism within the short term of 24 days. The microbial pentose phosphate pathway (PPP) and glycolysis in HLF and WF sediments were enhanced, while glycolysis in NF and IF sediments were obviously suppressed. Additionally, eCO2 strengthened negative associations of nosZ-type and nirK-type denitrifiers communities in NF, while increased their positive associations in IF, HLF, and WF. The regulation of metabolic pathways in denitrifiers would become more important than their microbial community when denitrifiers adapting eCO2. And the rising CO2 levels shifted sediments hotspots for potential N2O emissions from dry-wet alternation areas in lake-shore zone to the near-lake regions and lake basins. Based on these results, we recommend adjusting water-level management strategies in the lake-shore zone to address the greenhouse effect. For example, increasing the size of intermittently flooded areas and decreasing the proportion of continuously flooded areas in the lake-shore zone to mitigate N2O emissions and optimize sediment denitrification.

Electron flow dynamics in sulfur-based autotrophic bioreduction of Cr(VI) mediated by inorganic carbon species: Insights for environmental remediation

The deployment of sulfur-based autotrophic bioremediation for in situ groundwater remediation faces hurdles due to electron competition among electron acceptors, impacting contaminant removal efficiency and causing pH instability. Notably, the sulfur-based bioreduction of Cr(VI) [Cr(VI)-SAR] exemplifies gaps in our comprehension of electron competition dynamics with inorganic carbon (IC), and its subsequent influence on pH. Herein, we established a Cr(VI)-SAR system interfaced with diverse IC species, providing definitive insights into electron transfer mechanisms through rigorous multi-biocycle analysis and thermodynamically consistent half-reaction calculations. Through quantification of electron transfer pathways, we derived reaction equations for Cr(VI) reduction in conjunction with various IC species. Furthermore, metagenomics were used to quantify functional enzymes and identify diverse electron transport patterns alongside IC fixation pathways. Notably, the enrichment of genes associated with electron shuttles and conductive pili expands the paradigm of extracellular electron transfer, while the Wood-Ljungdahl pathway streamlines microbial metabolic proliferation with reduced energy expenditure. Quantitative analysis of these functional genes offers a plausible mechanism underlying the observed shifts in electron competition between IC and Cr(VI). This research marks an advancement in the Cr(VI)-SAR foundational theory, with a particular focus on the dynamics of electron competition, contributing to a deeper understanding of this environmentally significant process.

Mechanistic insights and performance of Mn redox cycling in a dual-bacteria bioreactor for ammonium and Cr(VI) removal

The co-contamination of hexavalent chromium (Cr(VI)) and ammonium (NH4+-N) in industrial wastewater has attracted considerable attention due to its serious threats to both ecological systems and public health. Manganese(IV) (Mn(IV))-driven NH4+-N oxidation (Mnammox) coupled with Mn(II)-mediated denitrification (MnOD), built on the Mn redox cycle, is a promising nitrogen removal process, where Mn(II) and NOx--N generated during Mnammox were effectively controlled by MnOD. Herein, a bioreactor integrating Mnammox and MnOD for NH4+-N and Cr(VI) removal was constructed utilizing core-shell gel beads embedded with two core strains and δ-MnO2. When the C/N was 1.5, pH was 6.5, and HRT was 20 h, the removal efficiencies for Cr(VI) and NH4+-N reached 96.3 and 91.3 %, respectively. Cr(VI) can be bioreduced to Cr(III) in bioreactors. Additionally, the microbial activity and electron transfer properties in the Mn redox system were studied under varying Cr(VI) concentrations. High-throughput data revealed that high Cr(VI) concentrations significantly impacted microbial community diversity, while Aromatoleu and Zoogloea consistently remaining the dominant species in the bioreactor. KEGG database analysis showed that appropriately increasing C/N promoted the expression of genes related to nitrification and Mn redox cycling. This study provides novel perspectives on the application of the Mnammox coupled MnOD process driven by the Mn redox cycle for treating NH4+-N and Cr(VI) co-contaminated industrial wastewater.

Recyclable cation exchange resin-driven fermentation of waste activated sludge in sequential batch-parallel pattern: long-term resin/regenerant recycle stability and triple driving mechanisms

Anaerobic acidogenic fermentation has been posited as a preferable technology for waste activated sludge management, whereas the inefficient hydrolysis, inadequate metabolism and disordered microbiota still existed as three fermentability limitations. The existing solutions addressed single limitation while consuming substantial chemicals/energy, thereby restraining technological dissemination. Innovatively, the recyclable cation exchange resin (CER) is a promising approach for synchronously overcoming these fermentability limitations and reducing chemicals/energy costs from sludge-native metal removal perspective; however, it has been rarely reported. This study pioneered a recyclable CER-driven sludge fermentation in continuous CER and regenerant reuse scene, taking comprehensive insights into long-term performance and multiple mechanisms. The CER induced speciation conversion and stepwise removal of structural metals from sludge, especially organic-binding and residual Ca&Mg, which played triple driving contributions: (1) breaking metal-bridging sites and hydrogen bonds disentangled protein molecules for raising electronegative repulsion and flocculation energy barrier, causing synergic extracellular and intracellular hydrolysis (up to 30.05 %); (2) liberating endogenous redox mediators from metal-complexations for assisting electron shuttle and extracellular respiration, which metabolic electron transfer activity by 1.58 times; (3) triggering "bacteria screening" through sensitive methanogen inhibition and tolerant acidogens growth towards maximum acidogenic eco-functions. Such fermentability breakthroughs greatly promoted short-chain fatty acids (superior carbon sources) accumulation by average 2.52 folds while declining sludge solid by 55.87 %. The NaCl regeneration thoroughly restored CER active sites and eluted pollutant blockages, with negligible capability loss ≤ 3.53 % in 18-cycle operations, which stabilized acidogenic performances during 72-day fermentation (RSD ≤ 7.88 %). The fermentative products presented as high-quality carbon sources with abundant carbon and absent nitrogen, owing to CER-mediated NH4+ exchange. Innovative batch-parallel operation was established in engineering strategy, offering 409.49 CNY/ton SS income. The findings provided mechanism framework linking sludge fermentability with native metal functions.

Quorum sensing-enhanced electron transfer in anammox consortia: A mechanism for improved resistance to variable-valence heavy metals

Quorum sensing (QS) is recognized for enhancing bacterial resistance against heavy metals by regulating the production of extracellular substances that hinder metal penetration into the intracellular environment. However, it remains unclear whether QS contributes to resistance by regulating electron transfer, thereby transforming metals from more toxic to less toxic forms. This study investigated the regulatory mechanism of acyl-homoserine lactone (AHL)-mediated QS on electron transfer under As(III) and Cr(VI) stress. Metagenomic binning results revealed that Candidatus Brocadia sinica serves as a major contributor to AHL production for regulating heavy metal resistance, while other symbiotic bacteria offer complementary resistance pathways. In these bacteria, the AHL synthesis gene htdS plays a pivotal role in QS regulation of electron transfer and heavy metal resistance. Experimental findings demonstrated that AHL increased the electron transport system activity by 19.8 %, and upregulated electron transfer gene expression by 1.1- to 6.9-fold. The enhanced electron transfer facilitated a 28.7 % increase in the transformation of As(III) to less toxic As(V) and monomethylarsonic acid, ultimately achieving efficient nitrogen removal under As(III) stress. This study expands our understanding of how QS strengthens bacterial resistance to heavy metals, offering novel strategies for enhancing nitrogen removal of anammox in heavy metal-contaminated environments.

In-situ synthesis of FeS nanoparticles enhances Sulfamethoxazole degradation via accelerated electron transfer in anaerobic bacterial communities

The impact of nanominerals on microbial electron transfer and energy metabolism strategies during pollutant degradation remains uncertain. This study used in situ synthesized FeS nanoparticles (FeS NPs) to increase the degradation efficiency of SMX by anaerobic bacterial communities from 25.80 % to 47.60 %. The proportion of intracellular degradation by bacteria in the community significantly increased by 23.25 times, which mainly facilitated by NADH-dependent reductases and iron-sulfur proteins. Microbial network analysis and electrochemical analysis indicated that the in-situ synthesis of FeS NPs altered the interactions among different microbial species, enabling Petrimonas to transfer electrons directly to Lysinibacillus more effectively. This adjustment led to an increase in the activity of the electron transport system by 1.2 times, an increase in the electron supply capacity by 2.8 times, and a decrease in the electrochemical impedance (EIS) to 3.21 Ω. Moreover, the coupling of electron transfer pathways and protease transport channels significantly increased Na+/K+-ATPase by 14.72 times. Inhibitor experiments and molecular dynamics (MD) results showed that FeS NPs interact with Nqo1 in the cell membrane via electrostatic force at -28.573 kcal/mol, forming a unique electron conduit with ubiquinone (CoQ). This study provides new insights into the role of in situ nanominerals in electron transfer between different microorganisms, aim to enhance the antibiotic wastewater treatment efficiency in actual anaerobic processes.

Insight into enhanced adaptability of iron-carbon biofilter in treating low-carbon nitrogen mariculture wastewater for nitrogen removal and carbon reduction

Iron-carbon (Fe-C) based biofilters have shown significant advantages in treating mariculture wastewater by facilitating the mixotrophic heterotrophic nitrification-aerobic denitrification (HNAD) process. However, the effects of Fe-C materials and varying carbon-to-nitrogen (C/N) ratios on N removal and C reduction performance remain insufficiently explored. This study demonstrated that the Fe-C biofilter (R-Fe) achieved significantly higher NO3--N removal efficiency (65.1-96.0 %) compared to the control filter (-12.1-76.9 %) across all tested C/N ratios. Furthermore, the N2O emission proportion in R-Fe was reduced by 37.4-42.4 % compared to the control. Increasing the influent C/N ratio enhanced N removal efficiency while reducing the proportion of N2O emissions. This improvement correlated with enhanced electron transfer activity and an increased abundance of heterotrophic nitrifying-aerobic denitrifying bacteria (HNADB) and heterotrophic denitrifying bacteria (DNB), while the abundance of autotrophic denitrifying bacteria declined. Strong correlations were observed among microbial electron transfer activity, denitrifying microbial communities, Fe transport genes, denitrification-related functional genes, N removal efficiency, and N2O emission proportion, highlighting the critical role of electron transfer activity in microbial N removal processes.

Autotrophic ammonium nitrogen removal process mediated by manganese oxides: Bioreactors performance optimization and potential mechanisms

Manganese(IV) (Mn(IV)) reduction coupled with ammonium (NH4+-N) oxidation (Mnammox) has been found to play a significant role in the nitrogen (N) cycle within natural ecosystems. However, research and application of the autotrophic NH4+-N removal process mediated by manganese oxides (MnOx) in wastewater treatment are currently limited. This study established autotrophic NH4+-N removal sludge reactors mediated by various MnOx types, including δ-MnO2 (δ-MSR), β-MnO2 (β-MSR), α-MnO2 (α-MSR), and natural Mn ore (MOSR), investigating their NH4+-N removal performances and mechanisms under different initial N loading and pH conditions. During the 330 d operation, the reactors exhibited NH4+-N removal efficiencies in the order of δ-MSR > α-MSR > β-MSR > MOSR. Notably, metal-reducing bacteria (Candidatus Brocadia, Dechloromonas, and Rhodocyclaceae) and Mn(II) oxidizing bacteria (Pseudomonas and Zoogloea) were enriched in the reactors, especially in the δ-MSR. The presence of these microorganisms facilitated the reduction of Mn(IV) and utilized the generated Mn(II) to drive autotrophic denitrification (MnOAD), thereby completing the Mn(IV)/Mn(II) cycle and enhancing N removal in the system. An active Mn cycle displayed in δ-MSR, which could be demonstrated by the formation of petal-shaped biogenic MnOx and the increased abundance of Mn cycling genes (MtrCDE, MtrA, MtrB, and CotA, etc.). Meanwhile, genes involved in N metabolism were enriched, particularly functional genes associated with nitrification and denitrification. In this study, the coupling of Mnammox and MnOAD was realized via the Mn cycle, providing a new perspective on the application of autotrophic N removal technologies in wastewater treatment.

New Insight Into the Mechanism of Nitrite Enhancement on Heterotrophic Nitrification and Aerobic Denitrification Bacterium in Gene Expression

The growth and nitrogen metabolism of heterotrophic nitrification-aerobic denitrification (HN-AD) bacteria are affected by nitrite, but the mechanisms underlying this for strain Acinetobacter johnsonii EN-J1 are unclear. In this study, the addition of 10 mg/L nitrite increased the reduction rate of ammonium by 1.0 mg/L/h, and 20 mg/L nitrite increased the reduction rate of nitrate by 3.9 mg/L/h. Compared with the control, the nitrate reductase activity, electron transfer activity, and adenosine triphosphate content of EN-J1 were enhanced by 142.0%, 278.0% and 279.0%, respectively, in the nitrate removal process after the addition of 20 mg/L nitrite. The whole genome was annotated with nitrogen removal genes such as narGHI, narK, nsrR, nirBD, nasA, glnA, gltB, gdhA and amt. Transcriptome analysis showed that nitrite triggered significant upregulation of several key pathways, including nitrogen metabolism, the tricarboxylic acid cycle, and amino acid metabolism for enhancing denitrification. The expression of key denitrification genes (narG, narK, hmp, nirBD, glnA and nasA) was detected by real-time quantitative polymerase chain reaction. These results suggested that nitrite enhances denitrification by increasing the expression of denitrification genes, electron transfer and adenosine triphosphate levels, which is important for elucidating the mechanism of nitrite promotion of biological nitrogen removal efficiency.

Metagenomic insight into the diffusion signal factor mediated social traits of anammox consortia after starvation

Biomass starvation is common in biological wastewater treatment. As a social trait of microbial community, how quorum sensing (QS) regulated bacterial trade-off through interactions after starvation remains unclear. This study deciphered the mechanism of anaerobic ammonium oxidation (anammox) consortia in response to starvation, including reducing extracellular electron transfer (EET), adenosine 5'-triphosphate (ATP) content and amino acid metabolism. Metagenomic analysis has shown that the addition of the diffusion signal factor (DSF) resulted in a high abundance of antioxidant genes, which contributed to achieving redox balance in anammox bacteria. There was an enrichment of Geobacter and Methanosarcina, which were QS-responsive direct interspecific electron transfer participants. Furthermore, DSF stimulated the nitrogen and carbon metabolism of Ca. Kuenenia_stuttgartiensis, promoting syntrophy of metabolic intermediates within microbial community. This study highlighted the effect of DSF on the microbial interaction patterns and deciphered the QS-based social traits of anammox consortia after starvation, facilitating the stable operation of the anammox process.

Metagenomics and metaproteomics reveal the effects of sludge types and inoculation modes on N,N-dimethylformamide degradation pathways and the microbial community involved

This study demonstrated the effects of the sludge type and inoculation method on the N,N-dimethylformamide degradation pathway and associated microbial communities. The sludge type is critical for DMF metabolism, with acclimatized aerobic sludge having a significant advantage in terms of DMF metabolism performance, whereas acclimatized anaerobic sludge has a reduced DMF metabolism capacity. Metagenomic revealed increased abundances of Methanosarcina, Pelomona and Xanthobacter in the adapted anaerobic sludge, suggesting that anaerobic sludge can utilize the methyl products produced by DMF metabolism for growth. Adapted aerobic sludge had high Mycobacterium abundance, significantly boosting DMF hydrolysis. In addition, a large number of dmfA2 genes were found in aerobic sludge, more so in acclimatized sludge, indicating stronger DMF metabolism. Conversely, acclimatized anaerobic sludge showed lower abundance of dmd-tmd and mauA/B, qhpA genes, implying long-term DMF toxicity reduced anaerobic microbial activity. Metaproteomic analysis showed that Methanosarcina and Methanomethylovorans enzymes in anaerobic sludge metabolized dimethylamine and methylamine to methane, aiding DMF degradation. In the aerobic sludge, aminohydrolase proteins, which hydrolyze DMF, were significantly upregulated. These findings provide insights into DMF wastewater treatment.

Seawater warming rather than acidification profoundly affects coastal geochemical cycling mediated by marine microbiome

The most concerning consequences of climate change include ocean acidification and warming, which can affect microbial communities and thus the biogeochemical cycling they mediate. Therefore, it is urgent to study the impact of ocean acidification and warming on microbial communities. In the current study, metagenomics was utilized to reveal how the structure and function of marine microorganisms respond to ocean warming and acidification. In terms of community structure, Non-metric Multidimensional Scaling analysis visualized the similarity or difference between the control and the warming or acidification treatments, but the inter-group differences were not significant. In terms of gene functionality, warming treatments showed greater effects on microbial communities than acidification. After treatment with warming, the relative abundance of genes associated with denitrification increased, suggesting that ocean nitrogen loss can increase with increased temperature. Conversely, acidification treatments apparently inhibited denitrification. Warming treatment also greatly affected sulfur-related microorganisms, increasing the relative abundance of certain sulfate-reducing prokaryote, and enriched microbial carbon-fixation pathways. These results provide information on the response strategies of coastal microorganisms in the changing marine environments.

Distinct effects of flow intermittency on the benthic microbial diversity and their denitrification on different substrates

Global climate change has significantly increased the duration of droughts in intermittent rivers, impacting benthic microbial-mediated biogeochemical processes. However, the response mechanisms of biofilms on different substrate types to alternating dry and wet conditions and their related ecosystem functions remain poorly understood. This study uses high-throughput sequencing and enzyme assays to investigate the impact of gradient drought stress on microbial diversity and functional changes of biofilm communities inhabiting on gravel, cobblestone, and sediment. Results showed that the duration of drought significantly affects microbial diversity, with algal and bacterial α-diversity declining under extended drought across gravel, cobblestone, and sediment substrates. At the same time, fungal diversity was less affected, likely due to their distinct ecological niches and reproductive strategies. β-diversity analysis revealed significant changes in community heterogeneity, with algae and bacteria showing increased Bray-Curtis dissimilarities, indicating distinct adaptation strategies that may affect ecosystem functioning. Fungal communities, however, were less impacted by drought-induced heterogeneity changes. Network analysis showed that drought altered microbial network connectivity, with algal networks displaying decreased path distances, while bacterial networks remained stable, suggesting greater resilience to drought stress. Functional enzyme assays revealed significantly reduced denitrification rates across all substrates post-drought, with distinct denitrifying enzyme activity responses depending on substrate type. Partial least squares path modeling revealed that algal biodiversity were closely linked to the maintaining of enzyme activities, particularly denitrification rates of biofilms on cobblestone and gravel. These findings indicated the critical role of substrate types in shaping microbial responses to drought stress, with distinct microbial groups and diversity indices playing key roles in maintaining ecosystem functions. This study highlights the importance of understanding the interactions between microbial community dynamics and ecosystem functions under varying environmental stressors in river ecosystems.

Insight into the responses of the anammox granular sludge system to tetramethylammonium hydroxide (TMAH) during chip wastewater treatment

Tetramethylammonium hydroxide (TMAH), an extensively utilized photoresist developer, is frequently present in ammonium-rich wastewater from semiconductor manufacturing, and its substantial ecotoxicity should not be underestimated. This study systematically investigated the effects of TMAH on the anammox granular sludge (AnGS) system and elucidated its inhibitory mechanisms. The results demonstrated that the median inhibitory concentration of TMAH for anammox was 84.85 mg/L. The nitrogen removal performance of the system was significantly decreased after long-term exposure to TMAH (0-200 mg/L) for 30 days (p < 0.05), but it showed adaptability to certain concentrations (≤50 mg/L). Concurrently, the stability of the granules decreased dramatically, resulting in the breakdown of AnGS. Further investigations indicated that TMAH exposure increased the secretion of extracellular polymeric substances but weakened their defense function. The increase in reactive oxygen species resulted in damage to the cell membrane. Reduced activity of anammox bacteria, impeded electron transfer, and changes in enzyme activity suggested that TMAH affected the metabolic activity. Microbiological analysis revealed that TMAH caused a decrease in the abundance of anammox bacteria and a weakening of symbiotic interactions within the microbial community. These results provide valuable guidance for the AnGS system application in chip wastewater treatment.

Will the removal of carbon, nitrogen and mixed disinfectants occur simultaneously: The key role of heterotrophic nitrification-aerobic denitrification strain

The capacity and mechanism of heterotrophic nitrification-aerobic denitrification (HNAD) strain (H1) to remove carbon, nitrogen, disinfectants chloroxylenol (PCMX) and benzethonium chloride (BEC) were investigated in this study. PCMX was removed via metabolism and chemical oxygen demand co-metabolism process. BEC was eliminated through bacterial adsorption, which greatly inhibited the removal of other pollutants. Carbon source optimization tests revealed that glucose was the optimal carbon source for co-removal of pollutants under mixed disinfectants circumstances (PCMX + BEC). Comparing the groups without (G1) and with disinfectants (G2), the content of extracellular polymeric substances was higher, and hydrophobicity was enhanced under the hazardous conditions of G2. All the nitrogen metabolism functional genes in G2 were up-regulated, and the electron transport system activity was also improved. At the same time, G2 had lower reactive oxygen species content, which reduced the probability of resistance genes dissemination, but the abundance of most quantified resistance genes was elevated in G2. Toxicity assessment assays found a dramatic reduction in the virulence of G2's effluent compared with the mixed disinfectants. The results confirmed that H1 strain could be used to treat the disinfectant-containing wastewater, which may aid in the application of HNAD process.

Impact of elevated CO2 on microbial communities and functions in riparian sediments: Role of pollution levels in modulating effects

The impact of elevated CO2 levels on microorganisms is a focal point in studying the environmental effects of global climate change. A growing number of studies have demonstrated the importance of the direct effects of elevated CO2 on microorganisms, which are confounded by indirect effects that are not easily identified. Riparian zones have become key factor in identifying the environmental effects of global climate change because of their special location. However, the direct effects of elevated CO2 levels on microbial activity and function in riparian zone sediments remain unclear. In this study, three riparian sediments with different pollution risk levels of heavy metals and nutrients were selected to explore the direct response of microbial communities and functions to elevated CO2 excluding plants. The results showed that the short-term effects of elevated CO2 did not change the diversity of the bacterial and fungal communities, but altered the composition of their communities. Additionally, differences were observed in the responses of microbial functions to elevated CO2 levels among the three regions. Elevated CO2 promoted the activities of nitrification and denitrification enzymes and led to significant increases in N2O release in the three sediments, with the greatest increase of 76.09 % observed in the Yuyangshan Bay (YYS). Microbial carbon metabolism was promoted by elevated CO2 in YYS but was significantly inhibited by elevated CO2 in Gonghu Bay and Meiliang Bay. Moreover, TOC, TN, and Pb contents were identified as key factors contributing to the different microbial responses to elevated CO2 in sediments with different heavy metal and nutrient pollution. In conclusion, this study provides in-depth insights into the responses of bacteria and fungi in polluted riparian sediments to elevated CO2, which helps elucidate the complex interactions between microbial activity and environmental stressors.

Enhancing nitrogen removal in low C/N wastewater with recycled sludge-derived biochar: A sustainable solution

Denitrification is an important biological process in wastewater treatment plants (WWTPs). However, a low carbon-to-nitrogen (C/N) ratio limits the availability of organic carbon, potentially reducing denitrification efficiency. This study investigates the impact of sludge-derived biochar on the nitrogen removal of activated sludge for low C/N ratio municipal wastewater. Sludge-based biochar was characterized by its physicochemical properties, revealing that biochar prepared at 400 °C exhibited the highest specific surface area and the most favorable surface functional groups for electron transfer. The results from batch tests showed that adding 4 g/L of biochar dosage enhanced denitrification rates and total nitrogen (TN) removal efficiency the most. Sequencing batch reactors (SBRs) experiments further confirmed that biochar dosgae improved the removal efficiencies of COD, NH4+-N, and TN, achieving stable values of 97.2 ± 1.2 %, 99.2 ± 0.6 %, and 83.8 ± 2.4 %, respectively. Metabolic and electrochemical analyses revealed that biochar addition enhanced the activity of denitrification enzymes, increasing the ammonia oxidation rate by 12.9 ± 0.7 %, nitrite oxidation rate by 14.7 ± 1.2 %, nitrate reduction rate by 36.9 ± 1.5 %, and nitrite reduction rate by 16.4 ± 0.8 %. The relative abundance of denitrification functional genes (amoA, nirS, nirK, narG, nosZ) increased, and the activities of the corresponding enzymes (AMO, NXR, NAP, NIR) rose by 23±6 %, 53±5 %, 260±15 %, and 55±7 %, respectively. This increase in enzyme activity suggested enhanced denitrification processes, which was further supported by the 60.1 ± 3.7 % increase in electron transfer system activity (ETSA), indicating that biochar acted as an electron shuttle. This study proposes a potential sustainable approach for sludge recycling and enhanced wastewater nitrogen removal under low C/N conditions.

Antagonistic Effect of Microplastic Polyvinyl Chloride and Nitrification Inhibitor on Soil Nitrous Oxide Emission: An Overlooked Risk of Microplastic to the Agrochemical Effectiveness

Microplastics are widely persistent in agricultural ecosystems and may affect soil nitrous oxide (N2O) emissions. Nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) is applied to decelerate nitrification and reduce soil N2O emission. Nevertheless, the interactive effects of nitrification inhibitors and microplastics on soil N2O emissions have not been investigated. Sole DMPP, polyvinyl chloride (PVC), and polystyrene (PS) substantially reduced agricultural soil N2O emission rates by 25.93%, 69.04%, and 73.89%, respectively. Nevertheless, PVC and DMPP had antagonistic effects on the N2O emission rates. The observed reductions in N2O emissions could be attributed to variations in soil oxygen availability, electron transport system activities, and Firmicutes, nap, and GDH genes. Moreover, the DMPP, PVC, and PS alone or copresences significantly enhanced the soil ecosystem multifunctionality (EMF). The findings shed light on the role of microplastics in soil N2O emission, EMF, and the microbial community, expanding the understanding of microplastics' effects on agrochemical effectiveness.

Metagenomic analysis reveals enhanced sludge dewaterability through acidified sludge inoculation: Regulation of Fe (II) oxidation electron transport pathway

The bioleaching utilizing indigenous microbial inoculation can continuously improve the dewaterability of sludge. In this study, metagenomic analysis was innovative employed to identify the key microorganisms and functional genes that affect the dewatering performance of sludge in the bioleaching conditioning process. The results demonstrated that long-term repeated inoculation of acidified sludge resulted in increased abundance of many functional genes associated with the transport of carbohydrate and amino acid. Additionally, genes encoding key iron transport proteins (such as afuA, fhuC, and fhuD) and genes related to electron transfer carriers in ferrous iron oxidation process (such as rus and cyc2) were significantly enriched, thereby promoting the improvement of sludge dewatering performance through enhanced iron oxidation. Notably, Acidithiobacillus, Betaproteobacteria, and Hyphomicrobium were the major sources of functional genes. This study reveals the microscopic mechanisms underlying the improvement of sludge dewaterability through bioleaching based on mixed culture from a novel perspective of gene metabolism.

Effect of magnetic powder (Fe3O4) on heterotrophic-sulfur autotrophic denitrification efficiency and electron transport system activity for marine recirculating aquacultural wastewater treatment

As an efficient nitrogen removal process, heterotrophic-sulfur autotrophic denitrification (HSAD) has attracted extensive attention in wastewater treatment. However, the effects of magnetic powder (Fe3O4) on the electron transport activity in HSAD process remain unclear. Therefore, in this study, a heterotrophic-sulfur autotrophic denitrification system was established to remove nitrogen from marine recirculating aquacultural wastewater for evaluating the effects of Fe3O4. At the optimal Fe3O4 concentration of 50 mg/L, the nitrogen removal efficiency reached 100% with lower sulfate accumulation, and the start-up time was shortened. The assays of denitrifying enzymes and electron transport system activity showed that Fe3O4 improved the activities of nitrate and nitrite reductases, and increased the efficiency of electron transport. Microbial community analysis revealed that Fe3O4 enriched heterotrophic denitrifier Thauera and sulfur autotrophic denitrifier Canditatus Thiobios, and thus enhanced denitrification efficiencies. This study demonstrated that Fe3O4 is an efficient denitrification accelerator in HSAD for treating marine recirculating aquacultural wastewater.

Electric stimulation mitigated the mixed microplastic inhibition to anaerobic digestion during wastewater treatment

The presence of mixed microplastics (MPs) in anaerobic wastewater treatment processes has been shown to impede fermentation performance by suppressing microbial activity. Microbial electrosynthesis (MES), with its extensive potential, offers a promising solution for refractory substances management and methane recovery, achieved through the enhancement of microbial metabolism and interspecies electron transfer. This study, therefore, delves into the functional impacts and the microbial response to MES in the remediation of wastewater contaminated with mixed-MPs. Results indicated that mixed-MPs could inhibit methane production (-52.38%) and substance removal (-26.59%), and MES could effectively mitigate this inhibitory effect (-22.86%, -19.01%). Concurrently, MES also boosts enzymatic activities pivotal for electron transfer, such as cytochrome c and nicotinamide adenine dinucleotide (NADH), as well as those linked to energy metabolism like adenosine triphosphate (ATP). Furthermore, MES bolsters microbial resistance to mixed-MPs, as evidenced by an increase in extracellular polymeric substances (EPS), albeit with a minor rise in reactive oxygen species (ROS) production and lactate dehydrogenase (LDH) release. Correspondingly, electric stimulation promoted the enrichment of functional microorganisms associated with fermentation, acetate production, electrogenesis, and methanogenesis, and stimulated elevated expression levels of genes related to methane metabolism. Notably, the Methanothrix-mediated acetoclastic pathway emerges as the predominant methanogenic route, succeeded by the Methanobacterium-driven hydrogenotrophic pathway. Lastly, the study underscores the supportive role of applied voltage and carriers in energy metabolism and substance transport, which are associated with methanogenesis. Overall, MES demonstrates efficacy in mitigating the biotoxicity induced by mixed-MPs exposure and in enhancing anaerobic wastewater treatment and methane recovery.

Manganese(IV) reduction coupled with ammonium oxidation mediated by a single strain Aromatoleum evansii MAY27: Performance, metabolomics, and mechanism

Manganese(IV) (Mn(IV)) reduction coupled to anaerobic ammonium (NH4+-N) oxidation (Mnammox) is a recently identified metal oxide-mediated nitrogen (N) loss pathway, holding potential value for the efficient removal of NH4+-N from wastewater. However, little is known about the application of Mnammox in wastewater treatment. Here, a novel Mnammox bacterium Aromatoleum evansii (strain MAY27) was screened. Strain MAY27 can utilize MnO2 as an electron acceptor to achieve NH4+-N removal under a low C/N condition (C/N = 0.5). The influencing factors in the Mnammox process and the Mn(IV) reduction driving effect on NH4+-N oxidation were investigated. The physiological characteristics of strain MAY27 and differential metabolic pathways were identified through whole-genome sequencing and metabolomic analyses. A significant up-regulation of several key pathways upon the addition of MnO2, including glycolysis/gluconeogenesis, transmembrane transporter activity, and oxidoreductase activity. This study contributes to the advancement of biotechnological approaches for treating N-containing wastewater.

High nitrite accumulation in hydrogenotrophic denitrification at low temperature: Transcriptional regulation and microbial community succession

High Pressure Hydrogenotrophic Denitrification (HPHD) provided a promising alternative for efficient and clean nitrate removal. In particular, the denitrification rates at low temperature could be compensated by elevated H2 partial pressure. However, nitrite reduction was strongly inhibited while nitrate reduction was barely affected at low temperature. In this study, the nitrate reduction gradually recovered under long-term low temperature stress, while nitrite accumulation increased from 0.1 to 41.0 mg N/L. The activities of the electron transport system (ETS), nitrate reductase (NAR), and nitrite reductase (NIR) decreased by 45.8 %, 27.3 %, and 39.3 %, respectively, as the temperature dropped from 30 °C to 15 °C. Real time quantitative PCR analysis revealed that the denitrifying gene expression rather than gene abundance regulated nitrogen biotransformation. The substantial nitrite accumulation was attributed to the significant up-regulation by 54.7 % of narG gene expression and down-regulation by 73.7 % of nirS gene expression in hydrogenotrophic denitrifiers. In addition, the nirS-gene-bearing denitrifiers were more sensitive to low temperature compared to those bearing nirK gene. The dominant populations shifted from the genera Paracoccus to Hydrogenophaga under long-term low temperature stress. Overall, this study revealed the microbial mechanism of high nitrite accumulation in hydrogenotrophic denitrification at low temperature.

Impacts of Elevated CO2 and a Nitrogen Supply on the Growth of Faba Beans (Vicia faba L.) and the Nitrogen-Related Soil Bacterial Community

Ecosystems that experience elevated CO2 (eCO2) are crucial interfaces where intricate interactions between plants and microbes occur. This study addressed the impact of eCO2 and a N supply on faba bean (Vicia faba L.) growth and the soil microbial community in auto-controlled growth chambers. In doing so, two ambient CO2 concentrations (aCO2, daytime/nighttime = 410/460 ppm; eCO2, 550/610 ppm) and two N supplement levels (without a N supply-N0-and 100 mg N as urea per kg of soil-N100) were applied. The results indicated that eCO2 mitigated the inhibitory effects of a N deficiency on legume photosynthesis and affected the CO2 assimilation efficiency, in addition to causing reduced nodulation. While the N addition counteracted the reductions in the N concentrations across the faba beans' aboveground and belowground plant tissues under eCO2, the CO2 concentrations did not significantly alter the soil NH4+-N or NO3--N responses to a N supply. Notably, under both aCO2 and eCO2, a N supply significantly increased the relative abundance of Nitrososphaeraceae and Nitrosomonadaceae, while eCO2 specifically reduced the Rhizobiaceae abundance with no significant changes under aCO2. A redundancy analysis (RDA) highlighted that the soil pH (p < 0.01) had the most important influence on the soil microbial community. Co-occurrence networks indicated that the eCO2 conditions mitigated the impact of a N supply on the reduced structural complexity of the soil microbial communities. These findings suggest that a combination of eCO2 and a N supply to crops can provide potential benefits for managing future climate change impacts on crop production.

Elevated CO2 and Nitrogen Supply Boost N Use Efficiency and Wheat (T. aestivum cv. Yunmai) Growth and Differentiate Soil Microbial Communities Related to Ammonia Oxidization

Elevated CO2 levels (eCO2) pose challenges to wheat (Triticum aestivum L.) growth, potentially leading to a decline in quality and productivity. This study addresses the effects of two ambient CO2 concentrations (aCO2, daytime/nighttime = 410/450 ± 30 ppm and eCO2, 550/600 ± 30 ppm) and two nitrogen (N) supplements (without N supply-N0 and with 100 mg N supply as urea per kg soil-N100) on wheat (T. aestivum cv. Yunmai) growth, N accumulation, and soil microbial communities related to ammonia oxidization. The data showed that the N supply effectively mitigated the negative impacts of eCO2 on wheat growth by reducing intercellular CO2 concentrations while enhancing photosynthesis parameters. Notably, the N supply significantly increased N concentrations in wheat tissues and biomass production, thereby boosting N accumulation in seeds, shoots, and roots. eCO2 increased the agronomic efficiency of applied N (AEN) and the physiological efficiency of applied N (PEN) under N supply. Plant tissue N concentrations and accumulations are positively related to plant biomass production and soil NO3--N. Additionally, the N supply increased the richness and evenness of the soil microbial community, particularly Nitrososphaeraceae, Nitrosospira, and Nitrosomonas, which responded differently to N availability under both aCO2 and eCO2. These results underscore the importance and complexity of optimizing N supply and eCO2 for enhancing crop tissue N accumulation and yield production as well as activating nitrification-related microbial activities for soil inorganic N availability under future global environment change scenarios.

Exploring modes of microbial interactions with implications for methane cycling

Methanotrophs are the sole biological sink of methane. Volatile organic compounds (VOCs) produced by heterotrophic bacteria have been demonstrated to be a potential modulating factor of methane consumption. Here, we identify and disentangle the impact of the volatolome of heterotrophic bacteria on the methanotroph activity and proteome, using Methylomonas as model organism. Our study unambiguously shows how methanotrophy can be influenced by other organisms without direct physical contact. This influence is mediated by VOCs (e.g. dimethyl-polysulphides) or/and CO2 emitted during respiration, which can inhibit growth and methane uptake of the methanotroph, while other VOCs had a stimulating effect on methanotroph activity. Depending on whether the methanotroph was exposed to the volatolome of the heterotroph or to CO2, proteomics revealed differential protein expression patterns with the soluble methane monooxygenase being the most affected enzyme. The interaction between methanotrophs and heterotrophs can have strong positive or negative effects on methane consumption, depending on the species interacting with the methanotroph. We identified potential VOCs involved in the inhibition while positive effects may be triggered by CO2 released by heterotrophic respiration. Our experimental proof of methanotroph-heterotroph interactions clearly calls for detailed research into strategies on how to mitigate methane emissions.

How denitrifiers defense ciprofloxacin: Insights from intracellular and extracellular stress response

Overuse of antibiotics has led to their existence in nitrogen-containing water. The impacts of antibiotics on bio-denitrification and the metabolic response of denitrifiers to antibiotics are unclear. We systematically analyzed the effect of ciprofloxacin (CIP) on bio-denitrification and found that 5 mg/L CIP greatly inhibited denitrification with a model denitrifier (Paracoccus denitrificans). Nitrate reduction decreased by 32.89 % and nitrous oxide emission increased by 75.53 %. The balance analysis of carbon and nitrogen metabolism during denitrification showed that CIP exposure blocked electron transfer and reduced the flow of substrate metabolism used for denitrification. Proteomics results showed that CIP exposure induced denitrifiers to use the pentose phosphate pathway more for substrate metabolism. This caused a substrate preference to generate NADPH to prevent cellular damage rather than NADH for denitrification. Notably, despite denitrifiers having antioxidant defenses, they could not completely prevent oxidative damage caused by CIP exposure. The effect of CIP exposure on denitrifiers after removal of extracellular polymeric substances (EPS) demonstrated that EPS around denitrifiers formed a barrier against CIP. Fluorescence and infrared spectroscopy revealed that the binding effect of proteins in EPS to CIP prevented damage. This study shows that denitrifiers resist antibiotic stress through different intracellular and extracellular defense strategies.

Non-native Pathway Engineering with CRISPRi for Carbon Dioxide Assimilation and Valued 5-Aminolevulinic Acid Synthesis in Escherichia coli Nissle

Carbon dioxide emission and acidification during chemical biosynthesis are critical challenges toward microbial cell factories' sustainability and efficiency. Due to its acidophilic traits among workhorse lineages, the probiotic Escherichia coli Nissle (EcN) has emerged as a promising chemical bioproducer. However, EcN lacks a CO2-fixing system. Herein, EcN was equipped with a simultaneous CO2 fixation system and subsequently utilized to produce low-emission 5-aminolevulinic acid (5-ALA). Two different artificial CO2-assimilating pathways were reconstructed: the novel ribose-1,5-bisphosphate (R15P) route and the conventional ribulose-5-phosphate (Ru5P) route. CRISPRi was employed to target the pfkAB and zwf genes in order to redirect the carbon flux. As expected, the CRISPRi design successfully strengthened the CO2 fixation. The CO2-fixing route via R15P resulted in high biomass, while the engineered Ru5P route acquired the highest 5-ALA and suppressed the CO2 release by 77%. CO2 fixation during 5-ALA production in EcN was successfully synchronized through fine-tuning the non-native pathways with CRISPRi.

Sulfur-carbon loop enhanced efficient nitrogen removal mechanism from iron sulfide-mediated mixotrophic partial denitrification/anammox systems

This study successfully established Iron Sulfide-Mediated mixotrophic Partial Denitrification/Anammox system, achieving nitrogen and phosphorus removal efficiency of 97.26% and 78.12%, respectively, with COD/NO3--N of 1.00. Isotopic experiments and X-ray Photoelectron Spectroscopy analysis confirmed that iron sulfide enhanced autotrophic Partial Denitrification performance. Meanwhile, various sulfur valence states functioned as electron buffers, reinforcing nitrogen and sulfur cycles. Microbial community analysis indicated reduced heterotrophic denitrifiers (OLB8, OLB13) under lower COD/NO3--N, creating more niche space for autotrophic bacteria and other heterotrophic denitrifiers. The prediction of functional genes illustrated that iron Sulfide upregulated genes related to carbon metabolism, denitrification, anammox and sulfur oxidation-reduction, facilitating the establishment of carbon-nitrogen-sulfur cycle. Furthermore, this cycle primarily produced electrons via nicotinamide adenine dinucleotide and sulfur oxidation-reduction processes, subsequently utilized within the electron transfer chain. In summary, the Partial Denitrification/Anammox system under the influence of iron sulfide achieved effient nitrogen removal by expediting electron transfer through the carbon-nitrogen-sulfur cycle.

Multiple pathways of vanadate reduction and denitrification mediated by denitrifying bacterium Acidovorax sp. strain BoFeN1

Contamination of aquifers by a combination of vanadate [V(V)] and nitrate (NO3-) is widespread nowadays. Although bioremediation of V(V)- and nitrate-contaminated environments is possible, only a limited number of functional species have been identified to date. The present study demonstrates the effectiveness of V(V) reduction and denitrification by a denitrifying bacterium Acidovorax sp. strain BoFeN1. The V(V) removal efficiency was 76.5 ± 5.41 % during 120 h incubation, with complete removal of NO3- within 48 h. Inhibitor experiments confirmed the involvement of electron transport substances and denitrifying enzymes in the bioreduction of V(V) and NO3-. Cyt c and riboflavin were important for extracellular V(V) reduction, with quinone and EPS more significant for NO3- removal. Intracellular reductive compounds including glutathione and NADH directly reduce V(V) and NO3-. Reverse transcription quantitative PCR confirmed the important roles of nirK and napA genes in regulating V(V) reduction and denitrification. Bioaugmentation by strain BoFeN1 increased V(V) and NO3- removal efficiency by 55.3 % ± 2.78 % and 42.1 % ± 1.04 % for samples from a contaminated aquifer. This study proposes new microbial resources for the bioremediation of V(V) and NO3-contaminated aquifers, and contributes to our understanding of coupled vanadium, nitrogen, and carbon biogeochemical processes.

Identification of extracellular polymeric substances layer barrier in chloroquine phosphate-disturbed anammox consortia and mechanism dissection on cytotoxic behavior by computational chemistry

The over-dosing use of chloroquine phosphate (CQ) poses severe threats to human beings and ecosystem due to the high persistence and biotoxicity. The discharge of CQ into wastewater would affect the biomass activity and process stability during the biological processes, e.g., anammox. However, the response mechanism of anammox consortia to CQ remain unknown. In this study, the accurate role of extracellular polymeric substances barrier in attenuating the negative effects of CQ, and the mechanism on cytotoxic behavior were dissected by molecular spectroscopy and computational chemistry. Low concentrations (≤6.0 mg/L) of CQ hardly affected the nitrogen removal performance due to the adaptive evolution of EPS barrier and anammox bacteria. Compact protein of EPS barrier can bind more CQ (0.24 mg) by hydrogen bond and van der Waals force, among which O-H and amide II region respond CQ binding preferentially. Importantly, EPS contributes to the microbiota reshape with selectively enriching Candidatus_Kuenenia for self-protection. Furthermore, the macroscopical cytotoxic behavior was dissected at a molecular level by CQ fate/distribution and computational chemistry, suggesting that the toxicity was ascribed to attack of CQ on functional proteins of anammox bacteria with atom N17 (f-=0.1209) and C2 (f+=0.1034) as the most active electrophilic and nucleophilic sites. This work would shed the light on the fate and risk of non-antibiotics in anammox process.

Effects of nitrate reduction on the biotransformation of 1H-1,2,4-triazole: Mechanism and community evolution

Due to the refractory of 1 H-1,2,4-triazole (TZ), conventional anaerobic biological treatment technology is usually restricted by low removal efficiency and poor system stability. In this study, TZ biodegradation and nitrate reduction was coupled to improve the removal efficiency of TZ from polluted wastewater. Batch assay was performed with pure culture strain Raoultella sp. NJUST42, which was reported to have the capability to degrade TZ in our previous study. Based on batch assay result, complete removal of TZ could be achieved in the presence of nitrate, whereas only 50% of TZ could be removed in the control system. Long-term stability experiment indicated that the relative abundance of microorganisms (Bacteroidetes_vadinHA17, Georgenia, Anaerolinea, etc) was obviously enhanced under nitrate reduction condition. During long-term period, major intermediates for TZ biodegradation such as [1,2,4]Triazolidine-3,5-diol, hydrazine dibasic carboxylic acid and carbamic acid were detected. A novel TZ biotransformation approach via hydration, TZ-ring cleavage, deamination and oxidation was speculated. PICRUSt1 and KEGG pathway analyses indicated that hydration (dch), oxidation (adhD, oah, pucG, fdhA) of TZ and nitrate reduction (Nar, napA, nrfA, nirBK, norB, nosZ) were significantly enhanced in the presence of nitrate. Moreover, the significant enrichment of TCA cycle (gab, sdh, fum, etc.) indicated that carbon and energy metabolism were facilitated with the addition of nitrate, thus improved TZ catabolism. The proposed mechanism demonstrated that TZ biodegradation coupled with nitrate reduction would be a promising approach for efficient treatment of wastewater contaminated by TZ.

New insights into substrates shaped nutrients removal, species interactions and community assembly mechanisms in tidal flow constructed wetlands treating low carbon-to-nitrogen rural wastewater

A limited understanding of microbial interactions and community assembly mechanisms in constructed wetlands (CWs), particularly with different substrates, has hampered the establishment of ecological connections between micro-level interactions and macro-level wetland performance. In this study, CWs with distinct substrates (zeolite, CW_A; manganese ore, CW_B) were constructed to investigate the nutrient removal efficiency, microbial interactions, metabolic mechanisms, and ecological assembly for treating rural sewage with a low carbon-to-nitrogen ratio. CW_B showed higher removal of ammonia nitrogen and total nitrogen by about 1.75-6.75 % and 3.42-5.18 %, respectively, compared to CW_A. Candidatus_Competibacter (denitrifying glycogen-accumulating bacteria) was the dominant microbial genus in CW_A, whereas unclassified_f_Blastocatellaceae (involved in carbon and nitrogen transformation) dominated in CW_B. The null model revealed that stochastic processes (drift) dominated community assembly in both CWs; however, deterministic selection accounted for a higher proportion in CW_B. Compared to those in CW_A, the interactions between microbes in CW_B were more complex, with more key microbes involved in carbon, nitrogen, and phosphorus conversion; the synergistic cooperation of functional bacteria facilitated simultaneous nitrification-denitrification. Manganese ores favour biofilm formation, increase the activity of the electron transport system, and enhance ammonia oxidation and nitrate reduction. These results elucidated the ecological patterns exhibited by microbes under different substrate conditions thereby contributing to our understanding of how substrates shape distinct microcosms in CW systems. This study provides valuable insights for guiding the future construction and management of CWs.

Amorphous Fe substrate enhances nitrogen and phosphorus removal in sulfur autotrophic process

The autotrophic denitrification of coupled sulfur and natural iron ore can remove nitrogen and phosphorus from wastewater with low C/N ratios. However, the low solubility of crystalline Fe limits its bioavailability and P absorption capacity. This study investigated the effects of amorphous Fe in drinking water treatment residue (DWTR) and crystalline Fe in red mud (RM) on nitrogen and phosphorus removal during sulfur autotrophic processes. Two types of S-Fe cross-linked filler particles with three-dimensional mesh structures were obtained by combining sulfur with the DWTR/RM using the hydrogel encapsulation method. Two fixed-bed reactors, sulfur-DWTR autotrophic denitrification (SDAD) and sulfur-RM autotrophic denitrification (SRAD), were constructed and stably operated for 236 d Under a 5-8-h hydraulic retention time, the average NO3--N, TN, and phosphate removal rates of SDAD and SRAD were 99.04 %, 96.29 %, 94.03 % (SDAD) and 97.33 %, 69.97 %, 82.26 % (SRAD), respectively. It is important to note that fermentative iron-reducing bacteria, specifically Clostridium_sensu_stricto_1, were present in SDAD at an abundance of 58.17 %, but were absent from SRAD. The presence of these bacteria facilitated the reduction of Fe (III) to Fe (II), which led to the complete denitrification of the S-Fe (II) co-electron donor to produce Fe (III), completing the iron cycle in the system. This study proposes an enhancement method for sulfur autotrophic denitrification using an amorphous Fe substrate, providing a new option for the efficient treatment of low-C/N wastewater.

Metagenomics revealing biomolecular insights into the enhanced toluene removal and electricity generation in PANI@CNT bioanode

Microbial fuel cells (MFCs) have significant potential for environmental remediation and energy recycling directly from refractory aromatic hydrocarbons. To boost the capacities of toluene removal and the electricity production in MFCs, this study constructed a polyaniline@carbon nanotube (PANI@CNT) bioanode with a three-dimensional framework structure. Compared with the control bioanode based on graphite sheet, the PANI@CNT bioanode increased the output voltage and toluene degradation kinetics by 2.27-fold and 1.40-fold to 0.399 V and 0.60 h-1, respectively. Metagenomic analysis revealed that the PANI@CNT bioanode promoted the selective enrichment of Pseudomonas, with the dual functions of degrading toluene and generating exogenous electrons. Additionally, compelling genomic evidence elucidating the relationship between functional genes and microorganisms was found. It was interesting that the genes derived from Pseudomonas related to extracellular electron transfer, tricarboxylic acid cycle, and toluene degradation were upregulated due to the existence of PANI@CNT. This study provided biomolecular insights into key genes and related microorganisms that effectively facilitated the organic pollutant degradation and energy recovery in MFCs, offering a novel alternative for high-performance bioanode.

Efficient nitrate and Cr(VI) removal by denitrifier: The mechanism of S. oneidensis MR-1 promoting electron production, transportation and consumption

When Cr(VI) and nitrate coexist, the efficiency of both bio-denitrification and Cr(VI) bio-reduction is poor because chromate hinders bacterial normal functions (i.e., electron production, transportation and consumption). Moreover, under anaerobic condition, the method about efficient nitrate and Cr(VI) removal remained unclear. In this paper, the addition of Shewanella oneidensis MR-1 to promote the electron production, transportation and consumption of denitrifier and cause an increase in the removal of nitrate and Cr(VI). The efficiency of nitrate and Cr(VI) removal accomplished by P. denitrificans as a used model denitrifier increased respectively from 51.3% to 96.1% and 34.3% to 99.8% after S. oneidensis MR-1 addition. The mechanism investigations revealed that P. denitrificans provided S. oneidensis MR-1 with lactate, which was utilized to secreted riboflavin and phenazine by S. oneidensis MR-1. The riboflavin served as coenzymes of cellular reductants (i.e., thioredoxin and glutathione) in P. denitrificans, which created favorable intracellular microenvironment conditions for electron generation. Meanwhile, phenazine promoted biofilm formation, which increased the adsorption of Cr(VI) on the cell surface and accelerated the Cr(VI) reduction by membrane bound chromate reductases thereby reducing damage to other enzymes respectively. Overall, this strategy reduced the negative effect of chromate, thus improved the generation, transportation, and consumption of electrons. SYNOPSIS: The presence of S. oneidensis MR-1 facilitated nitrate and Cr(VI) removal by P. denitrificans through decreasing the negative effect of chromate due to the metabolites' secretion.

Mechanisms of endogenous and exogenous partial denitrification in response to different carbon/nitrogen ratios: Transcript levels, nitrous oxide production, electron transport

Nitrite as an important substrate for Anammox can be provided by partial denitrification (PD). In this study, endogenous partial denitrification (EdPD) and exogenous partial denitrification (ExPD) sludge were domesticated and their nitrite transformation rate reached 74.4% and 83.4%, respectively. The impact of four carbon/nitrogen (C/N) ratios (1.5, 3.0, 5.0 and 6.0) on nitrous oxide (N2O) emission and denitrification functional genes expression in both PD systems were investigated. Results showed that elevated C/N ratios enhanced most denitrification genes expression, but in EdPD, high nitrite levels suppressed nosZ genes expression (from 9.4% to 1.4%), leading to increased N2O emission (0 to 3.4%). EdPD also exhibited lower electron transfer system activity, resulting in slower nitrogen oxide conversion efficiency and more stable nitrite accumulation compared to ExPD. These findings offer insights for optimizing PD systems under varying water quality conditions.

Acidification Offset Warming-Induced Increase in N2O Production in Estuarine and Coastal Sediments

Global warming and acidification, induced by a substantial increase in anthropogenic CO2 emissions, are expected to have profound impacts on biogeochemical cycles. However, underlying mechanisms of nitrous oxide (N2O) production in estuarine and coastal sediments remain rarely constrained under warming and acidification. Here, the responses of sediment N2O production pathways to warming and acidification were examined using a series of anoxic incubation experiments. Denitrification and N2O production were largely stimulated by the warming, while N2O production decreased under the acidification as well as the denitrification rate and electron transfer efficiency. Compared to warming alone, the combination of warming and acidification decreased N2O production by 26 ± 4%, which was mainly attributed to the decline of the N2O yield by fungal denitrification. Fungal denitrification was mainly responsible for N2O production under the warming condition, while bacterial denitrification predominated N2O production under the acidification condition. The reduced site preference of N2O under acidification reflects that the dominant pathways of N2O production were likely shifted from fungal to bacterial denitrification. In addition, acidification decreased the diversity and abundance of nirS-type denitrifiers, which were the keystone taxa mediating the low N2O production. Collectively, acidification can decrease sediment N2O yield through shifting the responsible production pathways, partly counteracting the warming-induced increase in N2O emissions, further reducing the positive climate warming feedback loop.

Deciphering the potential role of quorum quenching in efficient aerobic denitrification driven by a synthetic microbial community

Low efficiency is one of the main challenges for the application of aerobic denitrification technology in wastewater treatment. To improve denitrification efficiency, a synthetic microbial community (SMC) composed of denitrifiers Acinetobacter baumannii N1 (AC), Pseudomonas aeruginosa N2 (PA) and Aeromonas hydrophila (AH) were constructed. The nitrate (NO3--N) reduction efficiency of the SMC reached 97 % with little nitrite (NO2--N) accumulation, compared to the single-culture systems and co-culture systems. In the SMC, AH proved to mainly contribute to NO3--N reduction with the assistance of AC, while PA exerted NO2--N reduction. AC and AH secreted N-hexanoyl-DL-homoserine lactone (C6-HSL) to promote the electron transfer from the quinone pool to nitrate reductase. The declined N-(3-oxododecanoyl)-L-homoserine lactone (3OC12-HSL), resulting from quorum quenching (QQ) by AH, stimulated the excretion of pyocyanin, which could improve the electron transfer from complex III to downstream denitrifying enzymes for NO2--N reduction. In addition, C6-HSL mainly secreted by PA led to the up-regulation of TCA cycle-related genes and provided sufficient energy (such as NADH and ATP) for aerobic denitrification. In conclusion, members of the SMC achieved efficient denitrification through the interactions between QQ, electron transfer, and energy metabolism induced by N-acyl-homoserine lactones (AHLs). This study provided a theoretical basis for the engineering application of synthetic microbiome to remove nitrate wastewater.

Nitrate reduction coupling with As(III) oxidation in neutral As-contaminated paddy soil preserves nitrogen, reduces N2O emissions and alleviates As toxicity

In arsenic (As)-contaminated paddy soil, microbial-driven nitrate (NO3-) reduction coupled with arsenite (As(III)) oxidation can reduce As toxicity, but the whereabouts of NO3- remain unclear. In this study, the experiments were established using selective streptomycin (STP) and cyclohexylamine to inhibit bacterial and fungal functional responses, respectively, and metagenomic sequencing techniques were used to explain the biological mechanisms of NO3- reduction coupled with As(III) oxidation in neutral As-contaminated paddy soil. The results indicated that fungal denitrification resulted in stronger nitrous oxide (N2O) emissions (321.6 μg kg-1) than bacterial denitrification (175.9 μg kg-1) in neutral As-contaminated paddy soil, but NO3- reduction coupled with As(III) oxidation reduced the N2O emissions. Only adding STP led to ammonium (NH4+) generation (17.7 mg kg-1), and simultaneously more NH4+ appeared in NO3- reduction coupled with As(III) oxidation; this may be because it improved the electron transfer efficiency by 18.2 %. Achromobacter was involved in denitrification coupled with As(III) oxidation. Burkholderiales was responsible for NO3- reduction to NH4+ coupled with As(III) oxidation. This study provided a theoretical basis for NO3- reduction coupled with As(III) oxidation reducing N2O emissions, promoting the reduction of NO3- to NH4+, and reducing As toxicity in paddy soil.

Insights into the Acceleration Mechanism of Intracellular N and Fe Co-doped Carbon Dots on Anaerobic Denitrification Using Proteomics and Metabolomics Techniques

Bulk carbon-based materials can enhance anaerobic biodenitrification when they are present in extracellular matrices. However, little information is available on the effect of nitrogen and iron co-doped carbon dots (N, Fe-CDs) with sizes below 10 nm on this process. This work demonstrated that Fe-NX formed in N, Fe-CDs and their low surface potentials facilitated electron transfer. N, Fe-CDs exhibited good biocompatibility and were effectively absorbed by Pseudomonas stutzeri ATCC 17588. Intracellular N, Fe-CDs played a dominant role in enhancing anaerobic denitrification. During this process, the nitrate removal rate was significantly increased by 40.60% at 11 h with little nitrite and N2O accumulation, which was attributed to the enhanced activities of the electron transport system and various denitrifying reductases. Based on proteomics and metabolomic analysis, N, Fe-CDs effectively regulated carbon/nitrogen/sulfur metabolism to induce more electron generation, less nitrite/N2O accumulation, and higher levels of nitrogen removal. This work reveals the mechanism by which N, Fe-CDs enhance anaerobic denitrification and broaden their potential application in nitrogen removal.

Optimizing waste molasses utilization to enhance electron transfer via micromagnetic carriers: Mechanisms and high-nitrate wastewater denitrification performance

The biological denitrification of high-nitrate wastewater (HNW) is primarily hindered by insufficient carbon sources and excessive nitrite accumulation. In this study, micromagnetic carriers with varying micromagnetic field (MMF) strengths (0.0, 0.3, 0.6, 0.9 mT) were employed to enhance the denitrification of HNW using waste molasses (WMs) as a carbon source. The results revealed that 0.6 mT MMF significantly improved the total nitrogen removal (TN) efficiency at 96.3%. A high nitrate (NO3--N) removal efficiency at 99.3% with a low nitrite (NO2--N) accumulation at 25.5 mg/L was achieved at 0.6 mT MMF. The application of MMF facilitated the synthesis of adenosine triphosphate (ATP) and stimulated denitrifying enzymes (e.g., nitrate reductase (NAR), nitrite reductase (NIR), and nitric oxide reductase (NOR)), which thereby promoting denitrification. Moreover, the effluent chemical oxygen demand (COD), tryptophan and fulvic-like substances exhibited their lowest levels at 0.6 mT MMF. Analysis through 16S ribosomal ribonucleic acid gene sequencing indicated a significant enrichment of denitrifying bacteria including Castellaniella Klebsiella under the influence of MMF. Besides, the proliferation of Acholeplasma, Klebsiella and Proteiniphilum at 0.6 mT MMF promoted the hydrolysis and acidification of WMs. This study offers new insights into the enhanced utilization of WMs and the denitrification of HNW through the application of MMF.

Uncovering interactions among ternary electron donors of organic carbon source, thiosulfate and Fe0 in mixotrophic advanced denitrification: Proof of concept from simulated to authentic secondary effluent

To offset the imperfections of higher cost and emission of CO2 greenhouse gas in heterotrophic denitrification (HDN) as well as longer start-up time in autotrophic denitrification (ADN), we synergized the potential ternary electron donors of organic carbon source, thiosulfate and zero-valent iron (Fe0) to achieve efficient mixotrophic denitrification (MDN) of oligotrophic secondary effluent. When the influent chemical oxygen demand to nitrogen (COD/N) ratio ascended gradually in the batch operation with sufficient sulfur to nitrogen (S/N) ratio, the MDN with thiosulfate and Fe0 added achieved the highest TN removal for treating simulated and authentic secondary effluents. The external carbon is imperative for initiating MDN, while thiosulfate is indispensable for promoting TN removal efficiency. Although Fe0 hardly donated electrons for denitrification, the suitable circumneutral environment for denitrification was implemented by OH- released from Fe0 corrosion, which neutralized H+generated during thiosulfate-driven ADN. Meanwhile, Fe0 corrosion consumed the dissolved oxygen (DO) and created the low DO environment suitable for anoxic denitrification. This process was further confirmed by the continuous flow operation for treating authentic secondary effluent. The TN removal efficiency achieved its maximum under the combination condition of influent COD/N ratio of 3.1-3.5 and S/N ratio of 2.0-2.1. Whether in batch or continuous flow operation, the coordination of thiosulfate and Fe0 maintained the dominance of Thiobacillus for ADN, with the dominant heterotrophic denitrifiers (e.g., Plasticicumulans, Terrimonas, Rhodanobacter and KD4-96) coexisting in MDN system. The interaction insights of ternary electron donors in MDN established a pathway for realizing high-efficiency nitrogen removal of secondary effluent.

Efficient photocatalytic reduction of nitrate byproducts during anammox process by novel extracellular polymeric substances-embedded NH2-MIL-101(Fe) photocatalysts

Anaerobic ammonium oxidation (anammox) is an efficient nitrogen removal process; however, nitrate byproducts hampered its development. In this study, extracellular polymeric substances (EPS) were embedded into NH2-MIL-101(Fe), creating NH2-MIL-101(Fe)@EPS to reduce nitrate. Results revealed that chemical nitrate reduction efficiency of NH2-MIL-101(Fe)@EPS surpassed that of NH2-MIL-101(Fe) by 17.3 %. After adding 0.5 g/L NH2-MIL-101(Fe)@EPS within the anammox process, nitrate removal efficiency reached63.9 %, consequently elevating the total nitrogen removal efficiency to 92.4 %. 16S rRNA sequencing results elucidated the predominant role of Candidatus Brocadia within NH2-MIL-101(Fe)@EPS-anammox system. Concurrently, sufficient photogenerated electrons were transferred to microorganisms, promoting the growth of Desnitratisoma and OLB17. Additionally, photogenerated electrons activated flavin and Complex III, thereby up-regulating crucial genes involved in intra/extracellular electron transfer. Subsequently, denitrification and dissimilatory nitrate reduction to ammonium were activated to reduce nitrate. In summary, this study achieved a notable rate of photocatalytic nitrate reduction within anammox process through the NH2-MIL-101(Fe)@EPS photocatalysts.

Phosphomolybdic acid enhancing hexavalent chromium bio-reduction in long-term operation: Optimal dosage and mechanism analysis

The bio-reduction of Cr(VI) is regarded as a feasible and safe strategy to treat Cr pollution. The optimal concentration of phosphomolybdic acid (PMo12) for Cr(VI) reduction and the catalytic mechanism of electron behavior (electron production, electron transport and electron consumption) were revealed in denitrifying biofilm systems. The results showed that 0.1 mM PMo12 could achieve 92.5 % removal efficiency of 90 mg/L Cr(VI), which was 47.7 % higher than that of PMo12-free system, and improve the extracellular fixation capacity of Cr(III). The activity of peroxidase (POD) was significantly promoted by PMo12 to repair oxidative stress damage caused by Cr(VI) reduction. Additionally, analysis of electron behavior demonstrated that PMo12 could enhance key indicators of electron production, transport and consumption. This led to rapid activation of the electron pathway inhibited by Cr(VI), enabling simultaneous efficient nitrogen removal and Cr(VI) reduction in the biofilm system. This discovery will provide an efficient technique for Cr-containing wastewater treatment.

Responses of soil bacterial and fungal denitrification and associated N2O emissions to organochloride pesticide

The extensive application of organochloride pesticides in agriculture has raised concerns about their potential negative impacts on soil microbial denitrification and associated N2O emissions. However, most studies have primarily focused on bacteria, and the contribution of fungi to N2O emissions and their response to organochloride pesticides have often been overlooked. In this study, 15N tracing combined with the respiration inhibition method was applied to examine the impacts of chlorothalonil on both fungal and bacterial denitrification. The results demonstrated that fungal N2O emissions dominated in the absence of chlorothalonil, accounting for 73 % of total N2O emissions. Chlorothalonil inhibited fungal and bacterial denitrification via different mechanisms and altered the main pathways of soil N2O emissions. Amplicon sequencing analyses indicated that chlorothalonil significantly reduced the abundances of N2O-producing fungi owing to its fungicidal effect and fungal N2O emissions significantly dropped. Molecular biological analyses revealed that chlorothalonil induced lower electron generation, transport, and consumption efficiencies, which led to the inhibition of denitrifying enzymes in bacteria. Bacterial N2O emissions dramatically increased and became the dominant source. These findings provide insights into the mechanisms by which N2O emissions from fungal and bacterial denitrification are influenced by chlorothalonil.

Elucidating Multiple Electron-Transfer Pathways for Metavanadate Bioreduction by Actinomycetic Streptomyces microflavus

While microbial reduction has gained widespread recognition for efficiently remediating environments polluted by toxic metavanadate [V(V)], the pool of identified V(V)-reducing strains remains rather limited, with the vast majority belonging to bacteria and fungi. This study is among the first to confirm the V(V) reduction capability of Streptomyces microflavus, a representative member of ubiquitous actinomycetes in environment. A V(V) removal efficiency of 91.0 ± 4.35% was achieved during 12 days of operation, with a maximum specific growth rate of 0.073 d-1. V(V) was bioreduced to insoluble V(IV) precipitates. V(V) reduction took place both intracellularly and extracellularly. Electron transfer was enhanced during V(V) bioreduction with increased electron transporters. The electron-transfer pathways were revealed through transcriptomic, proteomic, and metabolomic analyses. Electrons might flow either through the respiratory chain to reduce intracellular V(V) or to cytochrome c on the outer membrane for extracellular V(V) reduction. Soluble riboflavin and quinone also possibly mediated extracellular V(V) reduction. Glutathione might deliver electrons for intracellular V(V) reduction. Bioaugmentation of the aquifer sediment with S. microflavus accelerated V(V) reduction. The strain could successfully colonize the sediment and foster positive correlations with indigenous microorganisms. This study offers new microbial resources for V(V) bioremediation and improve the understanding of the involved molecular mechanisms.

Comprehensive metagenomic and enzyme activity analysis reveals the inhibitory effects and potential toxic mechanism of tetracycline on denitrification in groundwater

The widespread use of tetracycline (TC) inevitably leads to its increasing emission into groundwater. However, the potential risks of TC to denitrification in groundwater remain unclear. In this study, the effects of TC on denitrification in groundwater were systematically investigated at both the protein and gene levels from the electron behavior aspect for the first time. The results showed that increasing TC from 0 to 10 µg·L-1 decreased the nitrate removal rate from 0.41 to 0.26 mg·L-1·h-1 while enhancing the residual nitrite concentration from 0.52 mg·L-1 to 50.60 mg·L-1 at the end of the experiment. From a macroscopic view, 10 µg·L-1 TC significantly inhibited microbial growth and altered microbial community structure and function in groundwater, which induced the degeneration of denitrification. From the electron behavior aspect (the electron production, electron transport and electron consumption processes), 10 µg·L-1 TC decreased the concentration of electron donors (nicotinamide adenine dinucleotide, NADH), electron transport system activity, and denitrifying enzyme activities at the protein level. At the gene level, 10 µg·L-1 TC restricted the replication of genes related to carbon metabolism, the electron transport system and denitrification. Moreover, discrepant inhibitory effects of TC on individual denitrification steps, which led to the accumulation of nitrite, were observed in this study. These results provide the information that is necessary for evaluating the potential environmental risk of antibiotics on groundwater denitrification and bring more attention to their effects on geochemical nitrogen cycles.