冻土甲烷循环微生物群落及其对全球变化的响应

Advances in methane-cycling microbial communities of permafrost and their response to global change

冻土是陆地生态系统中最容易受到全球气候变化影响的碳库,既发挥着碳源又起着碳汇的作用。人们非常关注贮存于冻土中有机碳的最终归宿,是因为全球气候变暖会加快冻土的解冻,释放更多的温室气体(二氧化碳和甲烷)到大气中,从而进一步加剧温室效应。据估计每年从北半球冻原陆地生态系统释放进入大气的甲烷约占全球自然界释放甲烷总量的25%。研究证实冻土生物源甲烷的产生和消耗分别由耐(嗜)低温的产甲烷菌(methanogens)和甲烷氧化菌(methanotrophs)介导。鉴于冻土甲烷循环对全球甲烷平衡的显著作用以及在冻土生物地球化学循环中的重要功能,对介导冻土甲烷循环的产甲烷菌和甲烷氧化菌的研究将有助于更好地评估冻土生态系统对全球气候变化的响应和影响,就冻土甲烷循环过程、产甲烷菌、甲烷氧化菌的群落结构、活动、生态功能及其对气候和环境变化的响应机制的最新研究进行综述,以期为我国开展冻土甲烷循环机理研究提供支持。

A better understanding of the global terrestrial carbon cycle has become a policy imperative,both nationally and worldwide.About one third of the global soil carbon is preserved in northern latitudes,mainly in huge layers of frozen ground,which underlie around 24% of the exposed land area of the northern hemisphere.Terrestrial and sub-marine permafrost is one of the most vulnerable carbon pools of the Earth system.Permafrost soils can function as both a source and a sink for carbon dioxide and methane.Under aerobic conditions permafrost soil organic matter（SOM） is respired to CO2,whereas under anaerobic conditions SOM is decomposed to CH4 via a sequence of microbial processes.Thawing of permafrost could release large quantities of greenhouse gases into the atmosphere,thus further increasing global warming and transforming tundra ecosystems from a carbon sink to a carbon source.The atmospheric input of methane from permafrost soils in high latitudes of the northern hemisphere has been estimated to correspond to about 25% of methane emissions from natural sources.It is well known that methane fluxes in permafrost habitats are influenced by a number of biotic and abiotic parameters,including water regime,temperature,type of substrate,and vegetation as well as the availability of organic carbon.The biological formation and consumption of methane are carried out by very specialized microorganisms;methanogens and methanotrophs.Microbial methane production（methanogenesis） is a prominent process during the anaerobic decomposition of organic matter.Methanogenesis is solely driven by a small group of strictly anaerobic organisms called methanogenic archaea,which belong to the kingdom Euryarchaeota.In permafrost environments two main pathways of energy-metabolism by methanogens dominate：（i） the reduction of CO2 to CH4 using H2 as a reductant（hydrogenotrophic methanogenesis） and（ii） the fermentation of acetate to CH4 and CO2（acetoclastic methanogenesis）.Methane transport from anaerobic soil horizons to the atmosphere is carried out via three major pathways： diffusion（slow）,ebullition（fast）,and plant-mediated transport.In contrast,methane-oxidizing bacteria,which belong to the γ-（type Ⅰ methanotrophs） and α-（type Ⅱ methanotrophs） Proteobacteria,are using methane as their sole carbon source,with consequent energy production by the oxidation of CH4 to CO2.Microbial methane oxidation in the oxic zones of the permafrost active layer is of great importance to the control of methane releases from permafrost environments.Most of the methane produced in the soil is oxidized by aerobic methane oxidizing bacteria before reaching the atmosphere.Hence,the biological oxidation of methane by methane-oxidizing bacteria is the major sink for methane in permafrost habitats.Currently,methanogenic archaea and methane-oxidizing bacteria have received a great deal of attention in permafrost studies,because of their pronounced distribution in permafrost soils and their consequent significance for the global methane budget.In this review,we examine the processes of methane cycling in permafrost soils.We also describe the methane-cycling microorganisms,including the possible impacts of global warming on their structure and function,and possible responses of the microbial communities to a changing environment.