内陆湿地与水体甲烷厌氧氧化功能微生物研究进展

内陆湿地与水体(如湖泊、河流、水库等)是温室气体甲烷的重要排放源。微生物介导的甲烷厌氧氧化(anaerobic oxidation of methane,AOM)反应在控制内陆湿地与水体甲烷排放中起着不可忽视的作用,对缓解全球温室效应具有重要意义。内陆湿地与水体易形成缺氧环境,且电子受体的种类和数量繁多,是发生AOM反应的理想生境。近年来,不断有研究表明,内陆湿地与水体中存在多种电子受体(NO₂^-、NO₃^-、SO₄^2-、Fe (III)等)驱动的AOM途径。NC10门细菌和甲烷厌氧氧化古菌(anaerobic methanotrophic archaea,ANME)的一新分支ANME-2d主导了湿地和水体环境中的AOM反应,其中ANME-2d具有根据环境条件选择不同电子受体的潜力。研究系统综述了内陆湿地与水体中不同电子受体驱动的AOM途径及其参与的主要功能微生物类群;分析了AOM反应在控制温室气体甲烷排放中的作用及其环境影响因素;总结了相关功能微生物的分子生物学检测方法及甲烷厌氧氧化活性测定的同位素示踪技术。最后,对未来相关研究方向进行了展望。

Research progress on anaerobic methanotrophs in inland wetlands and freshwater aquatic systems

Methane is an important greenhouse gas, and the global warming potential of methane is about 20-30 folds greater than carbon dioxide on a per-molecule basis. Inland wetlands and freshwater aquatic systems (like lakes, rivers and reservoirs) are important sources of methane emissions. It is estimated that the annul flux of methane from inland wetlands is approximately 100-200 Tg, accounting for 30% of the global annual methane emissions. The annul fluxes of methane from lakes, rivers and reservoirs are estimated to be 8-48, 1.5-26.8 and 8.9-22.2 Tg, respectively. Microbial-mediated anaerobic oxidation of methane (AOM) plays an important role in reducing methane emissions form these ecosystems, which can greatly alleviate global warming. The anoxic conditions can develop easily in inland wetlands, lakes, rivers and reservoirs. In the meantime, these environments contain a great variety of electron acceptors. Such conditions provide an ideal environment for AOM. In recent years, there has been an increasing evidence showing the occurrence of AOM driven by different electron acceptors, including NO₂^-, NO₃^-, SO₄^2- and Fe(III) in inland wetlands and freshwater aquatic systems. The nitrite-dependent AOM is performed by the NC10 phylum bacteria, which can produce oxygen intracellularly from two NO molecules for methane oxidation and respiration. Under anoxic conditions, these bacteria can transcribe and express the entire biochemical pathway of aerobic methane oxidation catalyzed by particulate methane monooxygenase. The nitrate-dependent AOM is catalyzed by a new cluster of anaerobic methanotrophic archaea (ANME)-ANME-2d, which is capable of performing AOM through reverse methanogenesis coupling with the reduction of NO₃^- to NO₂^-. But these archaea cannot reduce the produced NO₂^- further to NO, N₂O or N₂. The ANME-2d also has the potential to use Fe(III), Mn(IV), Cr(VI) and SO₄^2- as electron acceptors for methane oxidation. It has been reported that the ANME-2d could oxidize methane solely and transfer electrons directly to metal compound. These archaea may also be involved in metal-dependent AOM together with metal-reducing bacteria. Similarly, the ANME-2d can conduct AOM couple to the conversion of SO₄^2- to S^2-, in collaboration with sulfate-reducing bacteria. Due to the presence of diverse electron acceptors in inland wetlands and freshwater aquatic systems, they may support a greater variety of AOM pathways. This work systematically reviewed the AOM pathways driven by different electron acceptors and the responsible microorganisms, and analyzed the importance and environmental regulation of these AOM pathways in reducing methane emissions from inland wetlands and freshwater aquatic systems. Further, the methods for molecular detection of anaerobic methanotrophs and the stable isotope technology for determination of the activity of anaerobic methane oxidation were summarized. Finally, some future research directions were suggested.