Role and regulation of anaerobic methane oxidation catalyzed by NC10 bacteria and ANME-2d archaea in various ecosystems

Advances in sulfate-reducing bacteria-driven bioelectrolysis: mechanisms and applications in microbial electrolysis cell technology

The discharge of sulfate-rich wastewater from chemical and pharmaceutical and food processing industries results in serious environmental problems that impact both the natural environment and human health. The conventional sulfate removal process using chemical precipitation consumes much energy and results in the production of additional pollutants that decrease its scalability and treatment performance. Microbial electrolysis cells (MECs) using sulfate-reducing bacteria (SRB) is a promising sustainable technology for treating wastewater and recovering resources because the metabolic process of SRB in MECs can convert sulfate to sulfide while the cells also produce bioenergy through electrochemical processes. This review focuses on the processes of sulfate reduction in MECs that have demonstrated potential for sulfate removal and hydrogen production and heavy metal elimination and organic pollutant degradation. This review also systematically discussed machine learning systems that optimize MECs performance and result prediction and efficiency enhancement. The SRB-MECs systems have two advantages by producing clean energy while treating wastewater that makes them suitable for application in industrial processes. The two main challenges for the implementation of these systems are the scalability of the system and its long-term operational reliability. This review highlights the need for more research to enhance system performance and microbial efficiency and accelerate the practical implementation of SRB-MECs technology as a sustainable and energy-efficient solution for treating industrial effluents.

Co-application of digestate and biochar reduced greenhouse gas emissions in paddy soil through enhanced denitrification and anaerobic methane oxidation

Digestate from food waste (FW) has been identified as a promising nutrient resource for agriculture. However, applying digestate directly to soil often produces considerable greenhouse gas (GHG) emissions. As a soil amendment, biochar has demonstrated potential for mitigating GHG emissions. At present, the effect of biochar on GHG emissions and the associated regulatory mechanisms in paddy soils amended with digestate remains unclear. A 45-day soil incubation was conducted with different nitrogen substitution ratios of urea by digestate, coupled with biochar application: CK (100 % urea), D0U100 (100 % urea + biochar), D50U50 (50 % urea, 50 % digestate + biochar), and D100U0 (100 % digestate + biochar). Results indicated that the co-application of biochar and digestate significantly reduced N2O accumulation by 44.99 %-80.39 % compared to CK, primarily due to a decrease in soil NO3--N content and an increase in soil pH, which together significantly improved the distribution of the nosZ gene involved in denitrification. The increase in the abundance of Conexibacter, Symbiobacterium, Anaerolinea, and Candidatus_Solibacter further contributed to N2O reduction. Furthermore, the co-application led to a 21.68 %-38.15 % reduction in CH4 accumulation compared to CK. Biochar increased the abundance of methanotrophic bacteria, such as Methylococcaceae, Methyloligellaceae, and Methylomirabilaceae. Co-application increased the abundance of nitrate-reducing bacteria Symbiobacterium and Anaerolinea, thereafter facilitating nitrite-dependent anaerobic methane oxidation (AOM) dominated by Methylomirabilaceae. Additionally, sulfate-dependent and Iron(III)-dependent AOM likely further contributed to CH4 reduction. Overall, this study proposed a low-carbon management strategy for FW digestate and GHG emissions mitigation of paddy soil.

Effects of drainage and long-term tillage on greenhouse gas fluxes in a natural wetland: insights from microbial mechanisms

BACKGROUND

The conversion of natural wetlands to agricultural land through drainage contributes to 62% of the global wetland loss. Such conversion significantly alters greenhouse gas (GHG) fluxes, yet the underlying mechanisms of GHG fluxes resulting from drainage and long-term tillage practices remain highly uncertain. In this study, we measured GHG fluxes of a natural reed wetland (referred to as "Wetland") and a drained wetland that used as farmland (referred to as "Dryland").

RESULTS

The results demonstrated that annual cumulative N2O and CO2 fluxes in Dryland were 282.77% and 53.79% higher than those in Wetland, respectively. However, CH4 annual cumulative fluxes decreased from 12,669.45 ± 564.69 kg·ha- 1 to 8,238.40 ± 207.72 kg·ha- 1 in Dryland compared to Wetland. The global warming potential (GWP) showed no significant difference between Dryland and Wetland, with comparable average rates of 427.50 ± 48.83 and 422.21 ± 73.59 mg·CO2-eq·m- 2·h- 1, respectively. Metagenomic analysis showed a decrease in the abundance of acetoclastic methanogens and their functional genes responsible for CH4 production. Functional genes related to CH4 oxidation (pmoA) and gene related to N2O reduction (nosZ) exhibited a substantial sensitivity to variations in TOC concentration (p < 0.05). Candidatus Methylomirabilis, belonging to the NC10 phylum, was identified as the dominant methanotroph and accounted for 49.26% of the methanotrophs. Its relative abundance was significantly higher in Dryland than in Wetland, as the nitrogenous fertilizer applied in Dryland acted as an electron acceptor, with the nearby Wetland produced CH4 serving as an electron donor. This suggests that Dryland may act as a CH4 sink, despite the significant enhancement in CO2 and N2O fluxes.

CONCLUSIONS

In conclusion, this study provides insights into the influence of drainage and long-term tillage on GHG fluxes in wetlands and their contribution to global warming.

Methane cycle in subsurface environment: A review of microbial processes

Methane is a pivotal component of the global carbon cycle. It acts both as a potent greenhouse gas and a vital energy source. While the microbial cycling of methane in subsurface environments is crucial, its impact on geological settings and related engineering projects is often underestimated. This review uniquely integrates the latest findings on methane production, oxidation, and migration processes in strata, revealing novel microbial mechanisms and their implications for environmental sustainability. We address critical issues of methane leakage and engineering safety during resource extraction, underscoring the urgent need for effective methane management strategies. This work clarifies geological factors affecting methane budgets and emissions, deepening our understanding of methane dynamics. It offers practical insights for geological engineering and sustainable natural gas hydrate exploration, paving the way for future research and applications.

Response of denitrifying anaerobic methane oxidation processes in freshwater and marine sediments to polyvinyl chloride microplastics

Nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) plays a crucial role in mitigating methane (CH4) in natural environments. The increasing presence of microplastics (MPs) in these environments due to human activities is a growing concern. However, the impact of MPs on n-DAMO microorganisms and their role in greenhouse gas regulation, particularly CH4 reduction, remains unclear. This study investigates the effects of polyvinyl chloride (PVC) MPs on n-DAMO activity and the associated microbial communities in freshwater and marine sediments at varying concentrations of (R0/M0-no addition, R1/M1-0.5 %, R2/M2-2%). The results showed that the presence of MPs significantly increased the n-DAMO rate (2.89-3.58 nmol 13CO2 g-1 d-1) compared to the control groups (R0: 1.29 nmol 13CO2 g-1 d-1, M0: 0.11 nmol 13CO2 g-1 d-1), with marine sediments showing a more pronounced response. Additionally, the proportional contribution of nitrate-DAMO processes increased following MP exposure. The presence of PVC MPs also altered the microbial diversity of n-DAMO. Upon the addition of MPs, the microbial community composition of n-DAMO in marine sediments changed more significantly. This study provides the first evidence of a positive impact of PVC MPs on n-DAMO processes, suggesting that the presence of PVC MPs in sediments could potentially contribute to the reduction of CH4 emissions.

Vertical distribution of Candidatus Methylomirabilis and Methanoperedens in agricultural soils

Candidatus Methylomirabilis-related bacteria conduct anaerobic oxidation of methane (AOM) coupling with NO2- reduction, and Candidatus Methanoperedens-related archaea perform AOM coupling with reduction of diverse electron acceptors, including NO3-, Fe (III), Mn (IV) and SO42-. Application of nitrogen fertilization favors the growth of these methanotrophs in agricultural fields. Here, we explored the vertical variations in community structure and abundance of the two groups of methanotrophs in a nitrogen-rich vegetable field via using illumina MiSeq sequencing and quantitative PCR. The retrieved Methylomirabilis-related sequences had 91.12%-97.32% identity to the genomes of known Methylomirabilis species, and Methanoperedens-related sequences showed 85.49%-97.48% identity to the genomes of known Methanoperedens species which are capable of conducting AOM coupling with reduction of NO3- or Fe (III). The Methanoperedens-related archaeal diversity was significantly higher than Methylomirabilis-related bacteria, with totally 74 and 16 operational taxonomic units, respectively. In contrast, no significant difference in abundance between the bacteria (9.19 × 103-3.83 × 105 copies g-1 dry soil) and the archaea (1.55 × 104-3.24 × 105 copies g-1 dry soil) was observed. Furthermore, the abundance of both groups of methanotrophs exhibited a strong vertical variation, which peaked at 30-40 and 20-30 cm layers, respectively. Soil water content and pH were the key factors influencing Methylomirabilis-related bacterial diversity and abundance, respectively. For the Methanoperedens-related archaea, both soil pH and ammonium content contributed significantly to the changes of these archaeal diversity and abundance. Overall, we provide the first insights into the vertical distribution and regulation of Methylomirabilis-related bacteria and Methanoperedens-related archaea in vegetable soils. KEY POINTS: • The archaeal diversity was significantly higher than bacterial. • There was no significant difference in the abundance between bacteria and archaea. • The abundance of bacteria and archaea peaked at 30-40 and 20-30 cm, respectively.

Role and environmental regulation of iron-driven anaerobic methane oxidation in riverine sediment

Iron is an abundant element in the environment and acts as a thermodynamically favorable electron acceptor driving the anaerobic oxidation of methane (AOM). Presently, the role and environmental regulation of iron-driven AOM in rivers, an important source of methane emission, are nearly unknown. Here, we provided direct evidence for iron-driven AOM activity in sediment of a mountainous river (Wuxijiang River, China) through 13C-labeled isotopic experiment. The potential rate of iron-driven AOM ranged between 0.40 and 1.84 nmol 13CO2 g (sediment) d-1, which contributed 36% on average to total AOM activity when combined the potential nitrate- and nitrite-driven AOM rates measured previously. There were significant variations in iron-driven AOM rates among different reaches (upper, middle, and lower) and between seasons (summer and winter). Sediment temperature, pH, and nitrate content were closely associated with the dynamic of AOM activity. Our results indicate that iron-driven AOM has great potential for reducing methane emissions from riverine ecosystems, and suggest the necessity of taking both spatial and temporal scales into account to evaluate the quantitative role of this AOM process.

The Effects of Model Insoluble Copper Compounds in a Sedimentary Environment on Denitrifying Anaerobic Methane Oxidation (DAMO) Enrichment

The contribution of denitrifying anaerobic methane oxidation (DAMO) as a methane sink across different habitats, especially those affected by anthropogenic activities, remains unclear. Mining and industrial and domestic use of metals/metal-containing compounds can all cause metal contamination in freshwater ecosystems. Precipitation of metal ions often limits their toxicity to local microorganisms, yet microbial activity may also cause the redissolution of various precipitates. In contrast to most other studies that apply soluble metal compounds, this study investigated the responses of enriched DAMO culture to model insoluble copper compounds, malachite and covellite, in simulated sedimentary environments. Copper ≤ 0.22 µm from covellite appeared to cause immediate inhibition in 10 h. Long-term tests (54 days) showed that apparent methane consumption was less impacted by various levels of malachite and covellite than soluble copper. However, the medium-/high-level malachite and covellite caused a 46.6-77.4% decline in denitrification and also induced significant death of the representative DAMO microorganisms. Some enriched species, such as Methylobacter tundripaludum, may have conducted DAMO or they may have oxidized methane aerobically using oxygen released by DAMO bacteria. Quantitative polymerase chain reaction analysis suggests that Candidatus Methanoperedens spp. were less affected by covellite as compared to malachite while Candidatus Methylomirabilis spp. responded similarly to the two compounds. Under the stress induced by copper, DAMO archaea, Planctomycetes spp. or Phenylobacterium spp. synthesized PHA/PHB-like compounds, rendering incomplete methane oxidation. Overall, the findings suggest that while DAMO activity may persist in ecosystems previously exposed to copper pollution, long-term methane abatement capability may be impaired due to a shift of the microbial community or the inhibition of representative DAMO microorganisms.

Managing the Global Wetland Methane-Climate Feedback: A Review of Potential Options

Methane emissions by global wetlands are anticipated to increase due to climate warming. The increase in methane represents a sizable emissions source (32-68 Tg CH4 year-1 greater in 2099 than 2010, for RCP2.6-4.5) that threatens long-term climate stability and poses a significant positive feedback that magnifies climate warming. However, management of this feedback, which is ultimately driven by human-caused warming and thus "indirectly" anthropogenic, has been largely unexplored. Here, we review the known range of options for direct management of rising wetland methane emissions, outline contexts for their application, and explore a global scale thought experiment to gauge their potential impact. Among potential management options for methane emissions from wetlands, substrate amendments, particularly sulfate, are the most well studied, although the majority have only been tested in laboratory settings and without considering potential environmental externalities. Using published models, we find that the bulk (64%-80%) of additional wetland methane will arise from hotspots making up only about 8% of global wetland extent, primarily occurring in the tropics and subtropics. If applied to these hotspots, sulfate might suppress 10%-21% of the total additional wetland methane emissions, but this treatment comes with considerable negative consequences for the environment. This thought experiment leverages results from experimental simulations of sulfate from acid rain, as there is essentially no research on the use of sulfate for intentional suppression of additional wetland methane emissions. Given the magnitude of the potential climate forcing feedback of methane from wetlands, it is critical to explore management options and their impacts to ensure that decisions made to directly manage-or not manage-this process be made with the best available science.

Enhanced Extracellular Electron Transfer in Magnetite-Mediated Anaerobic Oxidation of Methane Coupled to Humic Substances Reduction: The Pivotal Role of Membrane-Bound Electron Transfer Proteins

Humic substances are organic substances prevalent in various natural environments, such as wetlands, which are globally important sources of methane (CH4) emissions. Extracellular electron transfer (EET)-mediated anaerobic oxidation of methane (AOM)-coupled with humic substances reduction plays an important role in the reduction of methane emissions from wetlands, where magnetite is prevalent. However, little is known about the magnetite-mediated EET mechanisms in AOM-coupled humic substances reduction. This study shows that magnetite promotes the reduction of the AOM-coupled humic substances model compound, anthraquinone-2,6-disulfonate (AQDS). 13CH4 labeling experiments further indicated that AOM-coupled AQDS reduction occurred, and acetate was an intermediate product of AOM. Moreover, 13CH313COONa labeling experiments showed that AOM-generated acetate can be continuously reduced to methane in a state of dynamic equilibrium. In the presence of magnetite, the EET capacity of the microbial community increased, and Methanosarcina played a key role in the AOM-coupled AQDS reduction. Pure culture experiments showed that Methanosarcina barkeri can independently perform AOM-coupled AQDS reduction and that magnetite increased its surface protein redox activity. The metatranscriptomic results indicated that magnetite increased the expression of membrane-bound proteins involved in energy metabolism and electron transfer in M. barkeri, thereby increasing the EET capacity. This phenomenon potentially elucidates the rationale as to why magnetite promoted AOM-coupled AQDS reduction.

Impact of operating conditions on N2O accumulation in Nitrate-DAMO system: Kinetics and microbiological analysis

Nitrate-dependent anaerobic methane oxidation (Nitrate-DAMO) is a novel and sustainable process that removes both nitrogen and methane. Previously, the metabolic pathway of Nitrate-DAMO has been intensively studied with some results. However, the production and consumption of nitrous oxide (N2O) in the Nitrate-DAMO system were widely disregarded. In this study, a Nitrate-DAMO system was used to investigate the effect of operational parameters (C/N ratio, pH, and temperature) on N2O accumulation, and the optimal operating conditions were determined (C/N = 3, pH = 6.5, and temperature = 20 °C). In this study, an enzyme kinetic model was used to fit the nitrate nitrogen degradation and the nitrous oxide production and elimination under different operating conditions. The thermodynamic model of N2O production and elimination in the system also has been constructed. Multiple linear regression analysis found that pH was the most important factor influencing N2O accumulation. The Metagenomics sequencing results showed that alkaline pH promoted the abundance of Nor genes and denitrifying bacteria, which were significantly and positively correlated with N2O emissions. And alkaline pH also promoted the production of Mdo genes related to the N2O-driven AOM reaction, indicating that part of the N2O was consumed by denitrifying bacteria and the other part was consumed by the N2O-driven AOM reaction. These findings reveal the mechanism of N2O production and consumption in DAMO systems and provide a theoretical basis for reducing N2O production and greenhouse gas emissions in actual operation.

Simultaneous methane mitigation and nitrogen removal by denitrifying anaerobic methane oxidation in lake sediments

Methane (CH4) is a potent greenhouse gas, with lake ecosystems significantly contributing to its global emissions. Denitrifying anaerobic methane oxidation (DAMO) process, mediated by NC10 bacteria and ANME-2d archaea, links global carbon and nitrogen cycles. However, their potential roles in mitigating methane emissions and removing nitrogen from lake ecosystems remain unclear. This study explored the spatial variations in activities of nitrite- and nitrate-DAMO and their functional microbes in Changdanghu Lake sediments (Jiangsu Province, China). The results showed that although the average abundance of ANME-2d archaea (5.0 × 106 copies g-1) was significantly higher than that of NC10 bacteria (2.1 × 106 copies g-1), the average potential rates of nitrite-DAMO (4.59 nmol 13CO2 g-1 d-1) and nitrate-DAMO (5.01 nmol 13CO2 g-1 d-1) showed no significant difference across all sampling sites. It is estimated that nitrite- and nitrate-DAMO consumed approximately 6.46 and 7.05 mg CH4 m-2 d-1, respectively, which accordingly achieved 15.07-24.95 mg m-2 d-1 nitrogen removal from the studied lake sediments. Statistical analyses found that nitrite- and nitrate-DAMO activities were both significantly related to sediment nitrate contents and ANME-2d archaeal abundance. In addition, NC10 bacterial and ANME-2d archaeal community compositions showed significant correlations with sediment organic carbon content and water depth. Overall, this study underscores the dual roles of nitrite- and nitrate-DAMO processes in CH4 mitigation and nitrogen elimination and their key environmental impact factors (sediment organic carbon and inorganic nitrogen contents, and water depth) in shallow lake, enhancing the understanding of carbon and nitrogen cycles in freshwater aquatic ecosystems.

Equal importance of humic acids and nitrate in driving anaerobic oxidation of methane in paddy soils

Methane (CH4) is both generated and consumed in paddy soils, where anaerobic oxidation of methane (AOM) serves as a crucial process for mitigating CH4 emissions. Although the participation of humic acids (HA) and nitrate in AOM has been recognized, their relative roles and significance in paddy soils remain insufficiently investigated. In this study, we explored the potential activity of AOM driven by HA and nitrate, as well as the composition of archaeal communities in paddy soils across different rice growth periods and fertilization treatments. AOM activity ranged from 0.81 to 1.33 and 1.26 to 2.38 nmol of 13CO2 g-1 (dry soil) day-1 with HA and nitrate, respectively. No significant differences (p < 0.05) were observed between the AOM activity driven by HA and nitrate across the three fertilization treatments. According to AOM activity, the annual consumption of CH4 was estimated at approximately 0.49 ± 0.06 and 0.83 ± 0.19 Tg for AOM processes driven by HA and nitrate in Chinese paddy soils. Nitrate-driven AOM activity exhibited a positive (p < 0.05) correlation with the abundance of the ANME-2d mcrA gene but a negative (p < 0.05) correlation with the content of dissolved organic carbon. Intriguingly, HA-driven AOM activity was only correlated positively with the nitrate-driven AOM activity. Soil water content, soil organic carbon, nitrate and nitrite contents were significantly correlated with the relative abundance of methanogenic and methanotrophic archaea. These results identified the potential importance of HA and nitrate in driving AOM processes within paddy soils, providing a comprehensive understanding of the complex microbial processes regulating greenhouse gas emissions from paddy soils.