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

Denitrifying anaerobic methane oxidation activity and microbial mechanisms in Riparian zone soils of the Yulin River, a tributary of the Three Gorges Reservoir

Riparian zones are recognized as major sources of greenhouse gas emissions, particularly methane (CH4). Denitrifying anaerobic methane oxidation (DAMO) has garnered growing attention due to its significant contribution to mitigating CH4 emissions in wetland environments. Nonetheless, the specific role and microbial mechanisms of DAMO in controlling CH4 release within riparian zones are still not well comprehended. This study employed isotopic labeling experiments to measure the nitrate-dependent anaerobic methane oxidation (NaDAMO) and nitrite-dependent anaerobic methane oxidation (NiDAMO) potential of soil samples from riparian zones that were collected during different hydrological cycles. Moreover, soil physicochemical properties, DAMO activity, and microbial abundance were integrated to analyze the key factors and mechanisms influencing DAMO in riparian zone soils. The isotope tracer results showed that NaDAMO activities (1.41-11.93 nmol 13CO2 g-1day-1) were significantly higher than NiDAMO activities (0.66-9.19 nmol 13CO2 g-1day-1) in the riparian zone (p < 0.05). NiDAMO activities were more strongly influenced by hydrological variations compared to NaDAMO activities, exhibiting higher levels during the discharge period (2.78-9.19 nmol 13CO2 g-1day-1) compared to the impoundment period (0.66-4.10 nmol 13CO2 g-1day-1). The qPCR analysis showed that the gene copies of NaDAMO archaeal mcrA (107 copies g-1) were approximately ten times greater than those of NiDAMO bacterial pmoA (106 copies g-1) in the majority of the sampling sites. Correlation analyses revealed that NiDAMO activity was influenced by soil pH (p < 0.05), while NaDAMO microbes were influenced by temperature, organic carbon, and ammonia nitrogen concentrations (p < 0.05). In summary, this research explored how hydrological changes in the riparian zone influence DAMO activities and their underlying mechanisms, providing a theoretical basis for mitigating CH4 emissions in riparian zones of reservoir regions.

Viral involvement in microbial anaerobic methane oxidation-mediated arsenic mobilization in paddy soil

Anaerobic oxidation of methane (AOM) facilitates arsenic (As) mobilization, posing a significant environmental risk. Soil viruses potentially participate in the microbial AOM process, yet their roles in methane-mediated As mobilization of paddy soil remain elusive. Here, an anaerobic microcosm study was conducted by inoculating microbial suspension with extracellular free virus and mitomycin C (MC)-induced virus, along with 13CH4 injection. The results showed that extracellular free virus enhanced while MC-induced virus suppressed 13CH4-mediated As mobilization. During the AOM process, both viruses inhibited 13CH4 oxidation to 13CO2. However, the extracellular free virus suppressed whereas the MC-induced virus enhanced 13CH4 consumption, likely attributed to the viral influence on the ANME-2d abundance. The methane consumption differences were inferred to influence As reduction, as evidenced by a strong correlation between As(III) and 13CH4 consumption concentrations. Moreover, virus-mediated methane assimilation into microbial biomass carbon influenced the overall microbial population. An increased abundance of Geobacter in the extracellular free virus treatment elevated net As(III) concentrations (up to 260 %) relative to treatment without virus in the presence of 13CH4. In contrast, MC-induced virus led to a net 122 % reduction in As(III) concentration due to decreased Geobacter abundance. These findings provide new insights into soil viruses in microbial AOM-driven As mobilization, highlighting their crucial functions in soil ecosystems.

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.

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.

Nitrite-dependent anaerobic methane oxidation (N-DAMO) in global aquatic environments: A review

The vast majority of processes in the carbon and nitrogen cycles are driven by microorganisms. The nitrite-dependent anaerobic oxidation of methane (N-DAMO) process links carbon and nitrogen cycles, offering a novel approach for the simultaneous reduction of methane emissions and nitrite pollution. However, there is currently no comprehensive summary of the current status of the N-DAMO process in natural aquatic environments. Therefore, our study aims to fill this knowledge gap by conducting a comprehensive review of the global research trends in N-DAMO processes in various aquatic environments (excluding artificial bioreactors). Our review mainly focused on molecular identification, global study sites, and their interactions with other elemental cycling processes. Furthermore, we performed a data integration analysis to unveil the effects of key environmental factors on the abundance of N-DAMO bacteria and the rate of N-DAMO process. By combining the findings from the literature review and data integration analysis, we proposed future research perspectives on N-DAMO processes in global aquatic environments. Our overarching goal is to advance the understanding of the N-DAMO process and its role in synergistically reducing carbon emissions and removing nitrogen. By doing so, we aim to make a significant contribution to the timely achievement of China's carbon peak and carbon neutrality targets.