自然生态系统中的厌氧氨氧化

Anaerobic ammonium oxidation in natural ecosystems

厌氧氨氧化(anaerobic ammonium oxidation,anammox)是由anammox菌在缺氧条件下以氨为电子供体、以亚硝酸为电子受体的生物反应,反应产物为氮气,该反应的发现为全球氮素循环增添了新的内容。参与anammox反应的微生物是anammox菌,anammox菌是一群分支很深的浮霉状菌,目前已发现的anammox菌有5个属8个种。催化anammox反应的是一特殊的细胞结构-厌氧氨氧化体,每种已发现的anammox菌中都存在该特殊结构。有关anammox反应的生化机理目前普遍认为,NO和联氨(N2H4)是anammox反应的重要中间体,NO可将NH4+直接氧化,形成N2H4,N2H4在联氨氧化酶的作用下最终转化为氮气。Anammox最初发现于人工脱氮系统,已发现的8种anammox菌中7种来自于人工系统。但越来越多的证据表明,anammox菌广泛分布于自然界的海洋、淡水和陆地生态系统中,在区域氮素循环中起着不同程度的作用。影响自然生态系统中anammox反应的主要环境因子包括有机质含量、NO3-浓度和盐度等,但在不同的生态系统,anammox反应的主导影响因子存在较明显差异。综述了anammox菌的类群和生化反应机理,总结了anammox菌在各种自然生态系统中的分布与生态多样性,论述了anammox反应在全球氮素循环中的重要性以及影响此过程发挥的主要环境因子。

Anaerobic ammonium oxidation（anammox）,a reaction oxidizing ammonium to dinitrogen gas with nitrite as the electron acceptor under anoxic conditions,is one of the most recent discoveries in the biogeochemical nitrogen cycle.The anammox reaction is mediated by anammox bacteria,a deep-branching monophyletic group of bacteria within the phylum Planctomycetes.Presently,five genera and eight species of anammox bacteria have been identified,including Candidatus ＇Brocadia＇（C.＇Brocadia anammoxidans＇ and ＇Brocadia fulgida＇）,C.＇Kuenenia stuttgartiensis＇,C.＇Scalindua＇（C.＇Scalindua brodae＇,＇Scalindua wagneri＇ and ＇Scalindua sorokinii＇）,C.＇Anammoxoglobus propionicus＇ and C.＇Jettenia asiatica＇.The anammox reaction takes place inside the anammoxosome,an intracytoplasmic compartment bounded by a single ladderane lipid-containing membrane.This unique cell structure has been found in all identified anammox bacteria.Although,there is no agreement on the biochemical mechanism of the anammox reaction,nitric oxide（NO） and hydrazine（N2H4） are recognized as important intermediates in the anammox reaction.The NO can oxidize ammonium to N2H4,and the N2H4 generated can be oxidized to dinitrogen gas by a hydrazine-oxidizing enzyme.The anammox process was first observed in a wastewater nitrogen-removal system,and seven of the eight anammox species were found in different wastewater treatment systems.Subsequently,many studies have reported the ubiquitous distribution of anammox bacteria in a variety of natural ecosystems,including anoxic marine sediments and water columns,marine oxygen-minimum zones,estuarine sediments,freshwater sediments and soil ecosystems.Anammox bacteria and their activities were also detected in several special ecosystems,such as high-temperature and low-temperature ecosystems.All available evidence indicates that the anammox process is critically important in natural ecosystems,and particularly in marine environments.In anoxic marine sediments and water columns,the anammox bacteria can account for 20%—79% and 20%—50% of total dinitrogen gas production,respectively.Anammox bacteria are also mainly responsible for nitrogen loss in the marine oxygen-minimum zones that are the most productive regions of the world′s oceans.Anammox activities have been detected in freshwater and soil ecosystems but the specific contribution of anammox to the nitrogen cycle is poorly known and needs further investigation.Although anammox bacteria are broadly distributed in a variety of natural ecosystems,the overall diversity of anammox bacteria is low.The anammox bacteria detected in most marine and freshwater ecosystems are all affiliated with the genera C.＇Scalindua＇ or ＇Brocadia＇.Recently,different anammox bacterial communities composed of C.＇Brocadia＇,＇Kuenenia＇,＇Scalindua＇ and ＇Jettenia＇ species were found simultaneously in some estuarine and soil ecosystems,which expanded knowledge of the diversity of anammox bacteria in natural ecosystems.The different levels of anammox bacterial diversity and contribution of anammox to the nitrogen cycle observed in different natural ecosystems indicate that the anammox process is influenced by local environmental conditions.The organic content,availability of water column NO-3 or NO-2,salinity and temperature are regarded as the most important environmental factors influencing the anammox process in different natural ecosystems.This review summarizes the classification,biochemical mechanism,distribution and diversity of anammox bacteria in natural ecosystems,the importance of anammox in the global nitrogen cycle and the main factors influencing the anammox process in natural habitats.