Enhancement of nitrite reduction and enrichment of Methylomonas via conductive materials in a nitrite-dependent anaerobic methane oxidation system

Metabolic versatility of aerobic methane-oxidizing bacteria under anoxia in aquatic ecosystems

The potential positive feedback between global aquatic deoxygenation and methane (CH4) emission emphasizes the importance of understanding CH4 cycling under O2-limited conditions. Increasing observations for aerobic CH4-oxidizing bacteria (MOB) under anoxia have updated the prevailing paradigm that MOB are O2-dependent; thus, clarification on the metabolic mechanisms of MOB under anoxia is critical and timely. Here, we mapped the global distribution of MOB under anoxic aquatic zones and summarized four underlying metabolic strategies for MOB under anoxia: (a) forming a consortium with oxygenic microorganisms; (b) self-generation/storage of O2 by MOB; (c) forming a consortium with non-oxygenic heterotrophic bacteria that use other electron acceptors; and (d) utilizing alternative electron acceptors other than O2. Finally, we proposed directions for future research. This study calls for improved understanding of MOB under anoxia, and underscores the importance of this overlooked CH4 sink amidst global aquatic deoxygenation.

Synergistic reduction of pollution and carbon mitigation in constructed wetlands-microbial fuel cell using sludge-derived biochar

Integrating microbial fuel cells (MFC) into constructed wetland systems (CW) has been an efficient wastewater treatment to improve the pollutants removal and regenerate power energy. This study fabricated a sludge biochar material (SBM) to sequestrate the carbon of residual sludge. Thereafter used SBM and modified SBM as the substrate materials to construct three groups of CW-MFC for decreasing the greenhouse gas (GHG) emission. The water quality improvement in removal efficiency achieved (2.59 %, 3.10 %, 5.21 % for COD; 3.31 %, 3.60 %, 6.71 % for TN; 1.80 %, 7.38 %, 4.93 % for TP) by the application of MFC, SBM, and modified SBM in wastewater treatment, respectively. Additionally, the reduction in global warming potential (GWP) realized 17.2 %, 42.2 %, and 64.4 % resulting from these applications. The carbon flow and fate diagrams showed MFC shifted the gas phase‑carbon flow from CH4 to CO2, and SBM promoted this shift trends. Microbial diversity indicated enrichment of electrochemically active bacteria (EAB), denitrifying bacteria, and phosphate accumulating organisms (PAOs) by SBM. Metabolic pathways analysis showed that introduction of MFC and SBM exhibited significant increases of key functional genes in metabolic pathway of anaerobic oxidation of methane (AOM). This study highlights the benefit of CW-MFC in and provides a new strategy for removing pollutants and abating GHG emissions in wastewater treatment.

Biochar modulates intracellular electron transfer for nitrate reduction in denitrifying anaerobic methane oxidizing archaea

Denitrifying anaerobic methane oxidizing (DAMO) archaea plays a significant role in simultaneously nitrogen removal and methane mitigation, yet its limited metabolic activity hinders engineering applications. This study employed biochar to explore its potential for enhancing the metabolic activity and nitrate reduction capacity of DAMO microorganisms. Sawdust biochar (7 g/L) was found to increase the nitrate reduction rate by 2.85 times, although it did not affect the nitrite reduction rate individually. Scanning electron microscopy (SEM) and fluorescence excitation-emission matrix (EEM) analyses revealed that biochar promoted microbial aggregation, and stimulated the secretion of extracellular polymeric substances (EPS). Moreover, biochar bolstered the redox capacity and conductivity of the biofilm, notably enhancing the activity of the electron transfer system by 1.65 times. Key genes involved in intracellular electron transport (Hdr, MHC, Rnf) and membrane transport proteins (BBP, ABC, NDH) of archaea were significantly up-regulated. These findings suggest that biochar regulates electrons generated by reverse methanogenesis to the membrane for nitrate reduction.

Promoting agricultural waste-driven denitrification and nitrogen sequestration with nano-enabled strategy

Nanotechnology and biotechnology offer promising avenues for bolstering food security through the facilitation of soil nitrogen (N) sequestration and the reduction of nitrate leaching. Nonetheless, a comprehensive and mechanistic evaluation of their effectiveness and safety remains unclear. In this study, a soil remediation strategy employing nano-Fe3O4 and straw in N-contaminated soil was developed to elucidate N retention mechanisms via diverse metagenomics techniques. The findings revealed that subsoil amended with straw, particularly in conjunction with nano-Fe3O4, significantly increased subsoil N content (53.2%) and decreased nitrate concentration (74.6%) in leachate. Furthermore, the enrichment of functional genes associated with N-cycling, sulfate, nitrate, and iron uptake, along with chemotaxis, and responses to environmental stimuli or microbial collaboration, effectively mitigates nitrate leaching while enhancing soil N sequestration. This study introduces a pioneering approach utilizing nanomaterials in soil remediation, thereby offering the potential for the cultivation of safe vegetables in high N input greenhouse agriculture.

Enhancement of electron transfer via magnetite in nitrite-dependent anaerobic methane oxidation system

Nitrite-dependent anaerobic methane oxidation (n-DAMO) is a novel denitrification process that simultaneously further removes and utilizes methane from anaerobic effluent from wastewater treatment plants. However, the metabolic activity of n-DAMO bacteria is relative low for practical application. In this study, conductive magnetite was added into lab-scale sequencing batch reactor inoculated with n-DAMO bacteria to study the influence on n-DAMO process. With magnetite amendment, the nitrogen removal rate could reach 34.9 mg N·L-1d-1, nearly 2.5 times more than that of control group. Magnetite significantly facilitated the interspecies electron transfer and built electrically connected community with high capacitance. Enzymatic activities of electron transport chain were significantly elevated. Functional gene expression and enzyme activities associated with nitrogen and methane metabolism had been highly up-regulated. These results not only propose a useful strategy in n-DAMO application but also provide insights into the stimulating mechanism of magnetite in n-DAMO process.

Microbially driven Fe-N cycle: Intrinsic mechanisms, enhancement, and perspectives

The iron‑nitrogen (FeN) cycle driven by microbes has great potential for treating wastewater. Fe is a metal that is frequently present in the environment and one of the crucial trace elements needed by microbes. Due to its synergistic role in the microbial N removal process, Fe goes much beyond the essential nutritional needs of microorganisms. Investigating the mechanisms behind the linked Fe-N cycle driven by microbes is crucial. The Fe-N cycle is frequently connected with anaerobic ammonia oxidation (anammox), nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), Feammox, and simultaneous nitrification denitrification (SND), etc. Although the main mechanisms of Fe-mediated biological N removal may vary depending on the valence state of the Fe, their similar transformation pathways may provide information on the study of certain element-microbial interactions. This review offers a thorough analysis of the facilitation effect and influence of Fe on the removal of nitrogenous pollutants in various biological N removal processes and summarizes the ideal Fe dosing. Additionally, the synergistic mechanisms of Fe and microbial synergistic N removal process are elaborated, covering four aspects: enzyme activity, electron transfer, microbial extracellular polymeric substances (EPS) secretion, and microbial community interactions. The methods to improve biological N removal based on the intrinsic mechanism were also discussed, with the aim of thoroughly understanding the biological mechanisms of Fe in the microbial N removal process and providing a reference and thinking for employing Fe to promote microbial N removal in practical applications.

Long-term effect of acetate and biochar addition on enrichment and activity of denitrifying anaerobic methane oxidation microbes

The denitrifying anaerobic methane oxidation (DAMO) process plays a crucial role in the global carbon/nitrogen cycles and methane emission control, and also has application potential in biological wastewater treatment. However, given that DAMO microbes are susceptible to external conditions such as additional carbon source in the system, it is essential to evaluate the effect of alternative carbon substance on the enrichment efficiency and metabolic activity of DAMO microbes. To this end, this study investigated the effect of acetate (0.1 mmol/L-R2, 0.5 mmol/L-R3) and biochar addition (R4) on the enrichment and activity of DAMO microbes. The long-term operation showed that the NO2--N and CH4 consumption rates in the reactors almost presented the sequence of R4>R2>R3>R1. However, the short-term activity test with isotope labelling showed the sequence of R2>R4>R1>R3. Furthermore, the addition of acetate and biochar improved the electrochemical activity and extracellular polymeric substance (EPS) secretion in the systems. In R4 reactor, the proportion of DAMO bacteria was the highest (7.20%), indicating that the addition of biochar could promote the enrichment of DAMO bacteria, and Thauera was co-enriched with the proportion increasing from 0.26% to 6.73%. While in R1, R2 and R3 reactors, DAMO bacteria were enriched with relatively low abundances (0.10%, 0.23%, 0.15%, respectively), together with methanogens and denitrifiers. This study showed that biochar and acetate with appropriate concentration could enhance the enrichment and activity of DAMO bacteria, the results can provide reference for the enrichment of DAMO microbes and its application in the biological nitrogen removal of wastewater.

How efficiently does a metabolically enhanced system with denitrifying anaerobic methane oxidizing microorganisms remove antibiotics?

In this work, the novel N-damo (Nitrite dependent anaerobic methane oxidation) process was investigated at high biomass activities for its potential to remove simultaneously nitrite and methane, as well as selected antibiotics commonly found in sewage in trace amounts. For this purpose, two MBRs were operated at three high nitrite loading rates (NLRs), namely 76 ± 9.9, 161.5 ± 11.4 and 215.2 ± 24.2 mg N-NO⁻2 L-1 d-1, at long-term operation. The MBRs performance achieved a significantly high nitrite removal activity for an N-damo process (specific denitrifying activity of up to 540 mg N-NO⁻2 g-1 VSS d-1), even comparable to heterotrophic denitrification values. In this study, we have implemented a novel operational strategy that sets our work apart from previous studies with similar bioreactors. Specifically, we have introduced Cerium as a trace element in the feeding medium, which serves as a key differentiating factor. It allowed maintaining a stable reactor operation at high NLRs. Microbial community composition evidenced that both MBRs were dominated with N-damo bacteria (67-87% relative abundance in period III and I, respectively). However, a decrease in functional N-damo bacteria (Candidatus Methylomirabilis) abundance was observed during the increase in biomass activity and concentration, concomitantly with an increase of the other minor families (Hypomicrobiaceae and Xanthobacteraceae). Most of the selected antibiotics showed high biotransformation such as sulfamethoxazole, trimethoprim, cefalexin and azithromycin, whereas others such as roxithromycin and clarithromycin were only partially degraded (20-35%). On the contrary, ciprofloxacin showed almost no removal. Despite the metabolic enhancement, no apparent increase on the antibiotic removal was observed throughout the operation, suggesting that microbiological composition was of greater influence than its primary metabolic activity on the removal of antibiotics.

Combining biological denitrification and electricity generation in methane-powered microbial fuel cells

Methane has been demonstrated to be a feasible substrate for electricity generation in microbial fuel cells (MFCs) and denitrifying anaerobic methane oxidation (DAMO). However, these two processes were evaluated separately in previous studies and it has remained unknown whether methane is able to simultaneously drive these processes. Here we investigated the co-occurrence and performance of these two processes in the anodic chamber of MFCs. The results showed that methane successfully fueled both electrogenesis and denitrification. Importantly, the maximum nitrate removal rate was significantly enhanced from (1.4 ± 0.8) to (18.4 ± 1.2) mg N/(L·day) by an electrogenic process. In the presence of DAMO, the MFCs achieved a maximum voltage of 610 mV and a maximum power density of 143 ± 12 mW/m². Electrochemical analyses demonstrated that some redox substances (e.g. riboflavin) were likely involved in electrogenesis and also in the denitrification process. High-throughput sequencing indicated that the methanogen Methanobacterium, a close relative of Methanobacterium espanolae, catalyzed methane oxidation and cooperated with both exoelectrogens and denitrifiers (e.g., Azoarcus). This work provides an effective strategy for improving DAMO in methane-powered MFCs, and suggests that methanogens and denitrifiers may jointly be able to provide an alternative to archaeal DAMO for methane-dependent denitrification.

Metalloenzymes play major roles to achieve high-rate nitrogen removal in N-damo communities: Lessons from metaproteomics

Nitrite-driven anaerobic methane oxidation (N-damo) is a promising biological process to achieve carbon-neutral wastewater treatment solutions, aligned with the sustainable development goals. Here, the enzymatic activities in a membrane bioreactor highly enriched in N-damo bacteria operated at high nitrogen removal rates were investigated. Metaproteomic analyses, with a special focus on metalloenzymes, revealed the complete enzymatic route of N-damo including their unique nitric oxide dismutases. The relative protein abundance evidenced that "Ca. Methylomirabilis lanthanidiphila" was the predominant N-damo species, attributed to the induction of its lanthanide-binding methanol dehydrogenase in the presence of cerium. Metaproteomics also disclosed the activity of the accompanying taxa in denitrification, methylotrophy and methanotrophy. The most abundant functional metalloenzymes from this community require copper, iron, and cerium as cofactors which was correlated with the metal consumptions in the bioreactor. This study highlights the usefulness of metaproteomics for evaluating the enzymatic activities in engineering systems to optimize microbial management.

Effects of a long-term operation wetland for wastewater treatment on the spatial pattern and function of microbial communities in groundwater

Constructed wetlands have been used globally for wastewater treatment owing to low energy inputs and operating costs. However, the impact of their long-term operation on groundwater microbial communities is still unclear. This study aims to investigate the effects and further reveal the linkage between a large-scale surface flow constructed wetland (in operation for 14 years) and groundwater. Changes in the characteristics of groundwater microbial communities and their potential influencing factors were studied based on hydrochemical analysis, Illumina MiSeq sequencing, and multivariate statistical analysis methods. Results show that the long-term operation wetland significantly elevated groundwater nutrient levels and increased the risk of ammonia nitrogen pollution compared to background values. An apparent heterogeneity of microbial communities exhibited in the vertical direction and a similarity in the horizontal direction. Wetland operations substantially altered the structure of microbial communities at 3, 5, and 12 m depths, particularly a reduced abundance of denitrifying and chemoheterotrophic functional genera. The formation and evolution of groundwater microbial community structure mainly subjected to the contributions of dissolved oxygen (33.70%), total nitrogen (21.40%), dissolved organic carbon (11.09%), and pH (10.60%) variations resulted from the wetland operation and largely differed in depths. A combined effect of these factors on the groundwater should be concerned for such a long-term running wetland system. This study provides a new insight into the responses of groundwater microbial community structure driving by wetland operation and a better understanding of corresponding variation of microbial-based geochemical processes.

Research progress on the magnetite nanoparticles in the fields of water pollution control and detection

Magnetite nanoparticles (MNPs) have shown increasing application in the fields of water pollution control and detection due to their perfect combination of interfacial functionalities and physicochemical properties, such as surface interface adsorption, (synergistic) reduction, catalytic oxidation, and electrical chemistry. This review presents the research advances in the synthesis and modification methods of MNPs in recent years, systematically summarizes the performances of MNPs and their modified materials in terms of three technical systems, including single decontamination system, coupled reaction system, and electrochemical system. In addition, the progress of the key roles played by MNPs in adsorption, reduction, catalytic oxidative degradation and their coupling with zero-valent iron for the reduction of pollutants are described. Moreover, the application prospect of MNPs-based electrochemical working electrodes for detecting micro-pollutants in water were also discussed in detail. This review addresses that the construction of MNPs-based systems for water pollution control and detection should be adapted to the natures of the target pollutants in water. Finally, the following research directions of MNPs and their remaining challenges are outlooked. In general, this review will inspire MNPs researchers in different fields for effective control and detection of a variety of contaminants in water.

Enhancement of nitrate-dependent anaerobic methane oxidation via granular activated carbon

Denitrifying anaerobic methane oxidation (DAMO) is a bioprocess utilizing methane as the electron source to remove nitrate or nitrite, but denitrification rate especially for nitrate-dependent DAMO is usually limited due to the low methane mass transfer efficiency. In this research, granular active carbon (GAC) was added to enhance the nitrate-dependent DAMO process. The results showed that the maximum nitrate removal rate of GAC assisted DAMO system reached as high as 61.17 mg L^-1 d^-1, 8 times higher than that of non-amended control SBR. The porous structure of GAC can not only adsorb methane, but also keep the internal DAMO archaea from being washed out, and thus benefits for DAMO archaea enrichment. The relative abundance of DAMO archaea accounted for 96.3% in GAC-SBR, which was significantly higher than that of non-amended control SBR system (29.9%). Furthermore, GAC amendment up-regulated metabolic status of denitrification and methane oxidation based on gene sequence composition. The absolute abundances of function genes (NC10 pmoA and ANME mcrA) in GAC-SBR were almost 20 times higher than that of non-amended control SBR. This study provides a novel technique to stimulate the nitrate-dependent DAMO process.

Response of denitrifying anaerobic methane oxidation enrichment to salinity stress: Process and microbiology

Denitrifying anaerobic methane oxidation (DAMO) is a novel biological process which could decrease nitrogen pollution and methane emission simultaneously in wastewater treatment. Salinity as a key environmental factor has important effects on microbial community and activity, however, it remains unclear for DAMO microorganisms. In this study, response of the enrichment of DAMO archaea and bacteria to different salinity was investigated from the aspect of process and microbiology. The results showed that the increasing salinity from 0.14% to 25% evidently deteriorated DAMO process, with the average removal rate of nitrate and methane decreased from 1.91 mg N/(L·d) to 0.07 mg N/(L·d) and 3.22 μmol/d to 0.59 μmol/d, respectively. The observed IC₅₀ value of salinity on the DAMO culture was 1.73%. Further microbial analyses at the gene level suggested that the relative abundance of DAMO archaea in the enrichment decreased to 46%, 39%, 38% and 33% of the initial value. However, DAMO bacteria suffered less impact with the relative abundance maintaining over 75% of the initial value (except 1% salinity). In functional genes of DAMO bacteria, pmoA, decreased gradually from 100% to 86%, 43%, 15% and 2%, while mcrA (DAMO archaea) maintained at 67%-97%. This difference probably indicated DAMO bacteria appeared functional inhibition prior to community inhibition, which was opposite for the DAMO archaea. Results above-mentioned concluded that, though the process of nitrate-dependent anaerobic methane oxidation was driven by the couple of DAMO archaea and bacteria, they individually featured different response to high salinity stress. These findings could be helpful for the application of DAMO-based process in high salinity wastewater treatment, and also the understanding to DAMO microorganisms.

Influence of iron species on the simultaneous nitrate and sulfate removal in constructed wetlands under low/high COD concentrations

Nitrate and sulfate are crucial factors of eutrophication and black and odorous water in the surface water and thus have raised increasing environmental concerns. Constructed wetlands (CWs) are the last ecological barrier before effluent enters the natural water body. To explore the simultaneous removal of nitrate and sulfate, the CW microcosms of CW-Con (with quartz sand), CW-ZVI (quartz sand and zero-valent iron), CW-Mag (quartz sand and magnetite), CW-ZVI + Mag (quartz sand, ZVI and magnetite) groups were set up under the low (100 mg/L)/high (300 mg/L) chemical oxygen demand (COD) concentration. Under the high COD condition, CW-ZVI group showed the best performance in nitrate (97.1%) and sulfate (96.9%) removal. Under the low COD concentration, the removal content of nitrate and sulfate in CW-ZVI group was better than CW-Mag group. The reason for this result was that zero-valent iron (ZVI) could be the electron donor for nitrate and sulfate reduction. Meanwhile, ZVI promoted chemical denitrification under high COD concentration according to PCA analysis. In addition, the produced sulfides inhibited the relative abundance of denitrifying bacteria, resulting in the lowest nitrate removal rate in CW-Mag group with sufficient electron donors. This study provided an alternative method to enhance simultaneous sulfate and nitrate removal in CWs.