Denitrifying anaerobic methane oxidation and mechanisms influencing it in Yellow River Delta coastal wetland soil, China

Climate-induced shifts in sulfate dynamics regulate anaerobic methane oxidation in a coastal wetland

Anaerobic methane oxidation (AMO) is a key microbial pathway that mitigates methane emissions in coastal wetlands, but the response of AMO to changing global climate remains poorly understood. Here, we assessed the response of AMO to climate change in a brackish coastal wetland using a 5-year field manipulation of warming and elevated carbon dioxide (eCO2). Sulfate (SO42-)-dependent AMO (S-DAMO) was the predominant AMO process at our study site due to tidal inputs of SO42-. However, SO42- dynamics responded differently to the treatments; warming reduced SO42- concentration by enhancing SO42- reduction, while eCO2 increased SO42- concentration by enhancing SO42- regeneration. S-DAMO rates mirrored these trends, with warming decreasing S-DAMO rates and eCO2 stimulating them. These findings underscore the potential of climate change to alter soil AMO activities through changing SO42- dynamics, highlighting the need to incorporate these processes in predictive models for more accurate representations of coastal wetland methane dynamics.

Methane oxidation driven by multiple electron acceptors in the water level fluctuation zone of the Three Gorges Reservoir area, China

Water level fluctuations in China's Three Gorges Reservoir (TGR) area are typical of many reservoirs and significantly impact water level fluctuation zones (WLFZ), including upstream rivers. Understanding methane oxidation in the TGR-WLFZ is crucial for evaluating the impact of large-scale reservoir construction on global climate change. In this study, we investigated methane oxidation rates in the TGR-WLFZ, focusing on periods of drying and flooding. The highest methane oxidation rates were observed during the drying period, ranging from 35.69 to 56.32 nmol/(g soil)/d, while the lowest rates were recorded during the flooding period, at 11.58 to 11.98 nmol/(g soil)/d, in lab-scale simulated columns. Using 13CH4 labeling experiments, we measured CH4 oxidation potentials for aerobic methane oxidation (AMO) using oxygen and anaerobic oxidation of methane (AOM) using nitrite, nitrate, sulfate, ferric iron, and manganese oxide as electron acceptors at varying concentrations. AMO was the dominant process across all experiments, with potentials ranging from 145.71 to 180.77 nmol 13CO2/(g soil)/d. For AOM, metal-dependent oxidation, particularly with Fe (III) and Mn(IV), was predominant (12.64-17.59 and 3.91-12.69 nmol 13CO2/(g soil)/d, respectively), followed by nitrite and nitrate-dependent pathways (1.49-9.10 nmol 13CO2/(g soil)/d). Sulfate-dependent AOM was limited (1.33-3.27 nmol 13CO2/(g soil)/d). Metagenomic analysis identified key microorganisms responsible for AMO, such as unclassified_f_Methylobacteriaeae and Methylobacterium sp., and for AOM are Ca. Methylomirabilis oxyfera, Ca. Methanoperedens nitroreducens and Ca. Methylomirabilis sp. Complete functional genes and enzymes for the methane oxidation and reverse methanogenesis pathways were obtained in each hydrological period, with the highest content during the drying period and the lowest during flooding. Our study shows that reservoirs, traditionally considered significant sources of methane, may also act as methane sinks. This finding raises new questions: How do different methane oxidation pathways respond to water level fluctuations in reservoirs, and are some pathways more resilient to changes in hydrological conditions?

Anaerobic oxidation of methane driven by different electron acceptors: A review

Methane, the most significant reduced form of carbon on Earth, acts as a crucial fuel and greenhouse gas. Globally, microbial methane sinks encompass both aerobic oxidation of methane (AeOM), conducted by oxygen-utilizing methanotrophs, and anaerobic oxidation of methane (AOM), performed by anaerobic methanotrophs employing various alternative electron acceptors. These electron acceptors involved in AOM include sulfate, nitrate/nitrite, humic substances, and diverse metal oxides. The known anaerobic methanotrophic pathways comprise the internal aerobic oxidation pathway found in NC10 bacteria and the reverse methanogenesis pathway utilized by anaerobic methanotrophic archaea (ANME). Diverse anaerobic methanotrophs can perform AOM independently or in cooperation with symbiotic partners through several extracellular electron transfer (EET) pathways. AOM has been documented in various environments, including seafloor methane seepages, coastal wetlands, freshwater lakes, soils, and even extreme environments like hydrothermal vents. The environmental activities of AOM processes, driven by different electron acceptors, primarily depend on the energy yields, availability of electron acceptors, and environmental adaptability of methanotrophs. It has been suggested that different electron acceptors driving AOM may occur across a wider range of habitats than previously recognized. Additionally, it is proposed that methanotrophs have evolved flexible metabolic strategies to adapt to complex environmental conditions. This review primarily focuses on AOM, driven by different electron acceptors, discussing the associated reaction mechanisms and the habitats where these processes are active. Furthermore, it emphasizes the pivotal role of AOM in mitigating methane emissions.

Regulation of nitrite-dependent anaerobic methane oxidation bacteria by available phosphorus and microbial communities in lake sediments of cold and arid regions

As global anthropogenic nitrogen inputs continue to rise, nitrite-dependent anaerobic methane oxidation (N-DAMO) plays an increasingly significant role in CH4 consumption in lake sediments. However, there is a dearth of knowledge regarding the effects of anthropogenic activities on N-DAMO bacteria in lakes in the cold and arid regions. Sediment samples were collected from five sampling areas in Lake Ulansuhai at varying depth ranges (0-20, 20-40, and 40-60 cm). The ecological characterization and niche differentiation of N-DAMO bacteria were investigated using bioinformatics and molecular biology techniques. Quantitative PCR confirmed the presence of N-DAMO bacteria in Lake Ulansuhai sediments, with 16S rRNA gene abundances ranging from 1.72 × 104 to 5.75 × 105 copies·g-1 dry sediment. The highest abundance was observed at the farmland drainage outlet with high available phosphorus (AP). Anthropogenic disturbances led to a significant increase in the abundance of N-DAMO bacteria, though their diversity remained unaffected. The heterogeneous community of N-DAMO bacteria was affected by interactions among various environmental characteristics, with AP and oxidation-reduction potential identified as the key drivers in this study. The Mantel test indicated that the N-DAMO bacterial abundance was more readily influenced by the presence of the denitrification genes (nirS and nirK). Network analysis revealed that the community structure of N-DAMO bacteria generated numerous links (especially positive links) with microbial taxa involved in carbon and nitrogen cycles, such as methanogens and nitrifying bacteria. In summary, N-DAMO bacteria exhibited sensitivity to both environmental and microbial factors under various human disturbances. This study provides valuable insights into the distribution patterns of N-DAMO bacteria and their roles in nitrogen and carbon cycling within lake ecosystems.

Methane-dependent complete denitrification by a single Methylomirabilis bacterium

Methane-dependent nitrate and nitrite removal in anoxic environments is thought to rely on syntrophy between ANME-2d archaea and bacteria in the genus 'Candidatus Methylomirabilis'. Here we enriched and purified a single Methylomirabilis from paddy soil fed with nitrate and methane, which is capable of coupling methane oxidation to nitrate reduction via nitrite to dinitrogen independently. Isotope labelling showed that this bacterium we name 'Ca. Methylomirabilis sinica' stoichiometrically performed methane-dependent complete nitrate reduction to dinitrogen gas. Multi-omics analyses collectively demonstrated that 'M. sinica' actively expressed a well-established pathway for this process, especially including nitrate reductase Nap. Furthermore, 'M. sinica' exhibited a higher nitrate affinity than most denitrifiers, implying its competitive fitness under oligotrophic nitrogen-limited conditions. Our findings revise the paradigm of methane-dependent denitrification performed by two organisms, and the widespread presence of 'M. sinica' in public databases suggests that the coupling of methane oxidation and complete denitrification in single cells substantially contributes to global methane and nitrogen budgets.

Characteristics of nitrogen flow and its environmental effects in the Yellow River Basin, China

In large river basins, the nitrogen (N) cycle is largely regulated by complicated interactions between human and natural elements. Nevertheless, the origins, fate and driving forces of reactive nitrogen (Nr) flows in the basins are still not clear. An estimation model of N flux was established for evaluating the Nr flow and its environmental effects in the Yellow River Basin of Henan Province (HYRB) by means of material flow analysis. Total N input and output of HYRB were calculated at 4090.9 and 3948.8 Gg N yr-1, causing 142.0 Gg yr-1 of N stored in the basin. Industry, cropland and residential activities, respectively occupied for 42.2%, 23.2% and 12.3% of the whole input, as well as 43.6%, 25.1% and 12.3% of the whole output, which were central to HYRB's N cycle. Anthropogenic activities regulated above 95.0% of total inputs, and 49.5% of N outputs was emitted into the air and 4.5% into hydrosphere. High N input, energy intensive, imperfect sewage disposal facilities and low N utilization efficiency were the main reasons of contaminate. How to effectively regulate the input of high-strength N pollutant associated with insufficient N cycling and Nr elimination is the main issue in the Yellow River Basin. In present research also put forward corresponding control measures according to the evaluation of N flow and Nr release of HYRB. The research can supply science foundation for coordinating the human-environment relationship in large basins, and also has important reference significance for the formulation of Nr emission reduction strategies.

Anaerobic oxidation of methane in terrestrial wetlands: The rate, identity and metabolism

The recent discovery of anaerobic oxidation of methane (AOM) in freshwater ecosystems has caused a great interest in "cryptic methane cycle" in terrestrial ecosystems. Anaerobic methanotrophs appears widespread in wetland ecosystems, yet, the scope and mechanism of AOM in natural wetlands remain poorly understood. In this paper, we review the recent progress regarding the potential of AOM, the diversity and distribution, and the metabolism of anaerobic methanotrophs in wetland ecosystems. The potential of AOM determined through laboratory incubation or in situ isotopic labeling ranges from 1.4 to 704.0 nmol CH4·g-1 dry soil·d-1. It appears that the availability of electron acceptors is critical in driving different AOM in wetland soils. The environmental temperature and salinity exert a significant influence on AOM activity. Reversal methanogenesis and extracellular electron transfer are likely involved in the AOM process. In addition to anaerobic methanotrophic archaea, the direct involvement of methanogens in AOM is also probable. This review presented an overview of the rate, identity, and metabolisms to unravel the biogeochemical puzzle of AOM in wetland soils.

Enhanced anaerobic oxidation of methane with the coexistence of iron oxides and sulfate fertilizer in paddy soil

Iron oxides and sulfate are usually abundant in paddy soil, but their role in reducing methane emissions is little known. In this work, paddy soil was anaerobically cultivated with ferrihydrite and sulfate for 380 days. An activity assay, inhibition experiment, and microbial analysis were conducted to evaluate the microbial activity, possible pathways, and community structure, respectively. The results showed that anaerobic oxidation of methane (AOM) was active in the paddy soil. The AOM activity was much higher with ferrihydrite than sulfate, and an extra 10% of AOM activity was stimulated when ferrihydrite and sulfate coexisted. The microbial community was highly similar to the duplicates but totally different with different electron acceptors. The microbial abundance and diversity decreased due to the oligotrophic condition, but mcrA-carrying archaea increased 2-3 times after 380 days. Both the microbial community and the inhibition experiment implied that there was an intersection between iron and sulfur cycles. A "cryptic sulfur cycle" might link the two cycles, in which sulfate was quickly regenerated by iron oxides, and it might contribute 33% of AOM in the tested paddy soil. Complex links between methane, iron, and sulfur geochemical cycles occur in paddy soil, which may be significant in reducing methane emissions from rice fields.

Soil nitrogen content and key functional microorganisms influence the response of wetland anaerobic oxidation of methane to trivalent iron input

Trivalent iron (Fe³⁺)-dependent anaerobic oxidation of methane (Fe-AOM), which is mediated by metal-reducing bacteria, is widely recognized as a major sink for the greenhouse gas methane (CH₄), and is a key driver of the carbon (C) biogeochemical cycle. However, the effect of Fe³⁺ addition on AOM in the present investigation is still ambiguous, and the mechanism is vague. In this study, we investigated the mechanism of changes in AOM response to Fe³⁺ input at different wetlands by using laboratory incubation methods combined with molecular biology techniques. Results indicated that Fe³⁺ input did not always lead to promoted AOM rates, which may be mediated by complex environmental factors, while lower soil total nitrogen (TN) had a positive effect on the response of AOM subjected to Fe³⁺ input. Notably, the promoted response of AOM was regulated by higher soil microbial diversity, of which the Shannon index was a key indicator leading to variation in the AOM response. Additionally, several biomarkers, including Planctomycetota and Burkholderiaceae, were key microorganisms responsible for alterations in AOM response. Our results suggest that the capacity of Fe³⁺ cycling-mediated AOM may gradually decrease in light of increasing anthropogenic N and Fe inputs to global estuarine wetlands, while its reaction processes will become more complex and more strongly coupled with multiple environmental factors. This finding contributes to the enhanced understanding and prediction of the wetland CH₄-related C with Fe cycles, as well as provides theoretical support for the underlying mechanisms.

Long-term effects of soluble and insoluble ferric irons on anaerobic oxidation of methane in paddy soil

Iron-dependent anaerobic oxidation of methane (Fe-AOM) is an important process to reduce methane emissions into the atmosphere. It is well known that iron bioavailability largely influences microbial iron reduction, but the long-term effects of different ferric irons on soil Fe-AOM remain unknown. In this work, paddy soil in the ferruginous zone was collected and inoculated with insoluble ferrihydrite and soluble EDTA-Fe(III) for 420 days. Stable isotope experiments, activity inhibition tests, and molecular biological techniques were performed to reveal the activity, microbial community, and possible mechanism of paddy soil Fe-AOM. The results showed that ferrihydrite was a better electron acceptor for long-term Fe-AOM cultivation. Although EDTA-Fe(III) is highly bioavailable and could stimulate Fe-AOM activity for a short time, it restricted the activity increase in the long term. The abundances of archaea, iron-reducing bacteria (IRB), and gene mcrA largely increased after cultivation, indicating the important roles of mcrA-carrying archaea and IRB. Remarkably, archaeal communities were similar, but bacteria were totally different with different ferric irons. The results of the microbial community and activity inhibition suggested that Fe-AOM was performed likely by the cooperation between archaea (Methanomassiliicoccaceae or pGrfC26) and IRB in the cultures.

Soil Fungal Community Structure and Its Effect on CO2 Emissions in the Yellow River Delta

Soil salinization is one of the most compelling environmental problems on a global scale. Fungi play a crucial role in promoting plant growth, enhancing salt tolerance, and inducing disease resistance. Moreover, microorganisms decompose organic matter to release carbon dioxide, and soil fungi also use plant carbon as a nutrient and participate in the soil carbon cycle. Therefore, we used high-throughput sequencing technology to explore the characteristics of the structures of soil fungal communities under different salinity gradients and whether the fungal communities influence CO₂ emissions in the Yellow River Delta; we then combined this with molecular ecological networks to reveal the mechanisms by which fungi adapt to salt stress. In the Yellow River Delta, a total of 192 fungal genera belonging to eight phyla were identified, with Ascomycota dominating the fungal community. Soil salinity was the dominant factor affecting the number of OTUs, Chao1 index, and ACE index of the fungal communities, with correlation coefficients of -0.66, 0.61, and -0.60, respectively (p < 0.05). Moreover, the fungal richness indices (Chao1 and ACE) and OTUs increased with the increase in soil salinity. Chaetomium, Fusarium, Mortierella, Alternaria, and Malassezia were the dominant fungal groups, leading to the differences in the structures of fungal communities under different salinity gradients. Electrical conductivity, temperature, available phosphorus, available nitrogen, total nitrogen, and clay had a significant impact on the fungal community structure (p < 0.05). Electrical conductivity had the greatest influence and was the dominant factor that led to the difference in the distribution patterns of fungal communities under different salinity gradients (p < 0.05). The node quantity, edge quantity, and modularity coefficients of the networks increased with the salinity gradient. The Ascomycota occupied an important position in the saline soil environment and played a key role in maintaining the stability of the fungal community. Soil salinity decreases soil fungal diversity (estimate: -0.58, p < 0.05), and soil environmental factors also affect CO₂ emissions by influencing fungal communities. These results highlight soil salinity as a key environmental factor influencing fungal communities. Furthermore, the significant role of fungi in influencing CO₂ cycling in the Yellow River Delta, especially in the environmental context of salinization, should be further investigated in the future.

Denitrifying anaerobic methane-oxidizing bacteria in river networks of the Taihu Basin: Community dynamics and assembly process

Denitrifying anaerobic methane-oxidizing bacteria (DAMO bacteria) plays an important role in reducing methane emissions from river ecosystems. However, the assembly process of their communities underlying different hydrologic seasons remains unclarified. In this study, the dynamics of DAMO bacterial communities in river networks of the Taihu Basin were investigated by amplicon sequencing across wet, normal, and dry seasons followed by multiple statistical analyses. Phylogenetic analysis showed that Group B was the major subgroup of DAMO bacteria and significant dynamics for their communities were observed across different seasons (constrained principal coordinate analysis, p = 0.001). Furthermore, the neutral community model and normalized stochasticity ratio model were applied to reveal the underlying assembly process. Stochastic process and deterministic process dominated the assembly process in wet season and normal season, respectively and similar contributions of deterministic and stochastic processes were observed in dry season. Meanwhile, abundant (relative abundance >0.1%) and rare (relative abundance <0.01%) DAMO bacterial communities were found to be shaped via distinct assembly processes. Deterministic and stochastic processes played a considerable role in shaping abundant DAMO bacterial communities, while deterministic process mainly shaped rare DAMO bacterial communities. Results of this study revealed the dynamics of DAMO bacterial communities in river networks and provided a theoretical basis for further understanding of the assembly process.

Functional Diversity and CO2 Emission Characteristics of Soil Bacteria during the Succession of Halophyte Vegetation in the Yellow River Delta

Carbon dioxide (CO₂) is the most important greenhouse gas in the atmosphere, which is mainly derived from microbial respiration in soil. Soil bacteria are an important part of the soil ecosystem and play an important role in the process of plant growth, mineralization, and decomposition of organic matter. In this paper, we discuss a laboratory incubation experiment that we conducted to investigate the CO₂ emissions and the underlying bacterial communities under the natural succession of halophyte vegetation in the Yellow River Delta by using high-throughput sequencing technology and PICRUSt functional prediction. The results showed that the bacterial abundance and diversity increased significantly along with the succession of halophyte vegetation. Metabolic function is the dominant function of soil bacteria in the study area. With the succession of halophyte vegetation, the rate of CO₂ emissions gradually increased, and were significantly higher in soil covered with vegetation than that of the bare land without vegetation coverage. These results helped to better understand the relationships of soil bacterial communities under the background of halophyte vegetation succession, which can help to make efficient strategies to mitigate CO₂ emissions and enhance carbon sequestration.