Anammox (anaerobic ammonium oxidation), which is a reaction that oxidizes ammonium to dinitrogen gas using nitrite as the electron acceptor under anoxic conditions, was an important discovery in the nitrogen cycle. The reaction is mediated by a specialized group of planctomycete-like bacteria that were first discovered in man-made ecosystems. Subsequently, many studies have reported on the ubiquitous distribution of anammox bacteria in various natural habitats, including anoxic marine sediments and water columns, freshwater sediments and water columns, terrestrial ecosystems and some special ecosystems, such as petroleum reservoirs. Previous studies have estimated that the anammox process is responsible for 50% of the marine nitrogen loss. Recently, the anammox process was reported to account for 9–40% and 4–37% of the nitrogen loss in inland lakes and agricultural soils respectively. These findings indicate the great potential for the anammox process to occur in freshwater and terrestrial ecosystems. The distribution of different anammox bacteria and their contribution to nitrogen loss have been described in different natural habitats, demonstrating that the anammox process is strongly influenced by the local environmental conditions. The present mini-review summarizes the current knowledge of the ecological distribution of anammox bacteria, their contribution to nitrogen loss in various natural ecosystems and the effects of major influential factors on the anammox process.
- anaerobic ammonium oxidation (anammox)
- ecological distribution
- environmental conditions
- natural ecosystem
- nitrogen loss
The process of anammox (anaerobic ammonium oxidation), which refers to the oxidation of ammonium coupled with the reduction of nitrite under anoxic conditions, has been predicted to be a more thermodynamically favourable process than aerobic ammonium oxidation , yet the anammox process was not discovered until nearly 20 years later in a wastewater-treatment plant in The Netherlands . The process is mediated by anammox bacteria, a deep-branching monophyletic group of bacteria within the phylum Planctomycetes. To date, anammox bacteria have not been cultured in the laboratory; however, at least five genera and 13 species have been identified using culture-independent molecular techniques. These taxa include the following: Candidatus ‘Brocadia’ (Ca. ‘Brocadia anammoxidans’ , Ca. ‘Brocadia fulgida’  and Ca. ‘Brocadia sinica’ ); Candidatus ‘Kuenenia stuttgartiensis’ ; Candidatus ‘Scalindua’ (Ca. ‘Scalindua brodae’ , Ca. ‘Scalindua wagneri’ , Ca. ‘Scalindua sorokinii’ , Ca. ‘Scalindua arabica’ , Ca. ‘Scalindua sinooilfield’  and Ca. ‘Scalindua zhenghei’ ); Candidatus ‘Anammoxoglobus’ (Ca. ‘Anammoxoglobus propionicus’  and Ca. ‘Anammoxoglobus sulfate’ ) and Candidatus ‘Jettenia asiatica’ . Henceforth, these genera will be referred to simply as Brocadia, Kuenenia, Scalindua, Anammoxoglobus and Jettenia respectively.
Anammox bacteria have been detected in various natural habitats, such as anoxic marine sediments [15–17] and water columns [18–20], freshwater sediments  and water columns , terrestrial ecosystems [23,24] and some special ecosystems, such as petroleum reservoirs . All of the available evidence indicates that the anammox process is critically important in the marine nitrogen cycle, and the relative contribution of the anammox process to the total production of dinitrogen gas (N2) has been estimated to be 50% in the ocean . In addition to marine environments, anammox activity has been detected in natural freshwater and terrestrial environments [22,24], indicating that the anammox process may play an even more significant role in the global nitrogen cycle than previously thought. The ecological distribution of anammox bacteria and their contribution to the nitrogen loss in natural ecosystems are influenced by local environmental conditions: the organic content [26,27], NOx− concentration , environmental stability , salinity [16,17] and temperature  have been described as key influencing factors. The present mini-review summarizes the recent findings concerning the distribution of anammox bacteria, their contribution to N2 production in various natural habitats and the major factors influencing the anammox process.
Anammox in marine ecosystems
Dalsgaard and Thamdrup  first detected anammox activities in the sediments of two continental shelf sites of the Skagerrak in the Danish Belt seaway. Subsequently, anammox activities were detected in various marine sediments (Table 1). Although organisms belonging to the Brocadia and Kuenenia genera were found in some coastal and estuarine sediments [17,30,31], the majority of anammox bacteria in marine sediments were affiliated with the Scalindua genus and showed surprisingly low diversity [8,9,15,18,20]. The contribution of the anammox process to regional nitrogen loss is highly variable in marine sediments (Table 1); indeed, the reported amounts of N2 production by the anammox process have ranged from 20 to 80% in shelf and deep sediments [26,32,33]. However, it has also been reported that the anammox process contributed to less than 20% of the N2 production in shallow coastal and estuarine sediments [16,17,27]. Therefore the relative contribution of anammox to nitrogen loss is positively correlated with water depth. Furthermore, the water depth and organic content are negatively correlated because a larger fraction of the organic matter is mineralized during transport to the sediment within a deep water column. Thus the organic content of deep sediment is low . The higher electron donor (organic matter) availability in organic-rich shallow sediments leads to increased competition for NO2− between denitrifiers and anammox bacteria. Yet anammox bacteria are slow-growing organisms  and are less competitive for NO2− than the denitrifiers in organic-rich shallow sediments. In electron-donor-limited deep sediments, nitrate-reducing organisms produce more NO2− for the anammox process owing to the shortage of sufficient organic matter. In addition, the relative importance of the anammox process is directly related to the availability of NO3− [16,28]: a higher NO3− concentration leads to a higher nitrate reduction rate and a greater release of NO2− for anammox. The stability of the environment may also be important for regulating the relative importance of the anammox process , an effect that is due to the low growth rate of the anammox bacteria. The anammox process is only significant in stable environments that allow a prolonged period for the bacterial population to develop . Recent studies have shown that the distribution of anammox bacteria and their contribution to nitrogen loss is correlated with the salinity [17,31]. Scalindua is the most abundant genus in environments with higher salt concentrations and the most halotolerant of the anammox genera . In addition, previous studies showed that higher contributions of the anammox process to nitrogen loss were found in environments with higher salinity  and that the abundances of Brocadia and Kuenenia species were negatively correlated with the salinity [17,31].
Anoxic water columns
In 2003, two parallel studies demonstrated that anammox bacteria were responsible for a substantial portion of the nitrogen loss in the anoxic water columns of the Black Sea and Golfo Dulce, in which 10–35% of the total nitrogen loss was attributable to the anammox process [8,35]. Recent studies indicated that the anammox process was responsible for a greater percentage (more than 50%) of the nitrogen loss in marine water columns, especially in OMZs (oxygen minimum zones) [18–20], and anammox bacteria were reported to be the dominant N2 producer in the OMZs of Namibia, Chile and Peru [18–20]. Denitrification was initially reported as the dominant driver of nitrogen loss in the OMZ of the Arabian Sea ; however, a direct link between the DNRA (dissimilatory nitrite reduction to ammonium) and the anammox process through the 15NO2− signal-mediated production of 15N15N was easily mistaken as a signature for denitrification . This finding indicates that anammox–DNRA coupling, rather than denitrification, was responsible for the massive nitrogen loss in the OMZ of the Arabian Sea. As observed in marine sediments, the dominant anammox species in marine water columns is closely related to the Scalindua genus (Table 1). However, the factors influencing the anammox process in marine water columns are not well known. Recent findings have indicated that aerobic ammonium oxidizers provided NO2− and created anoxic microenvironments for the anammox bacteria through the consumption of oxygen in the OMZs of the Black Sea and Namibian Sea [38,39]. Therefore anammox bacteria may be dependent on the activity of aerobic ammonium oxidizers under oxygen-limited conditions. The NH4+ in the water column is released through the mineralization of organic matter by both denitrifiers and DNRA organisms [20,37]. The availability of organic matter also acts as an important factor controlling the release of NO2− from NO3− for the anammox process. Lam et al.  showed that, in the OMZ of Peru, anammox bacteria obtained at least 67% of their NO2− from nitrate reduction using organic matter as the electron donors, whereas less than 33% of the NO2− was derived from aerobic ammonium oxidation. Therefore the availability of organic matter and its mineralization rate are important factors influencing the anammox process in marine water columns.
Anammox in freshwater ecosystems
Although there is growing evidence for the widespread occurrence of anammox bacteria in marine environments, evidence for the existence of anammox bacteria in natural freshwater habitats is limited (Table 1). The first direct evidence of anammox bacteria in freshwater ecosystems was provided by Schubert et al. , who have reported that the anammox process contributed to 9–13% of the N2 production and was responsible for 0.2 Tg of the fixed inorganic nitrogen loss per year in Lake Tanganyika, the second largest freshwater body in the world. Temperature has been identified as an important factor influencing anammox activity in this freshwater ecosystem . A higher relative contribution of the anammox process to nitrogen loss was reported in a eutrophic freshwater lake, Lake Kitaura, in which up to 40% of the N2 production was correlated to anammox activity . A positive correlation between the NO3− concentration and the relative importance of the anammox process was found in this lake, suggesting that the NO3− concentration was the key factor for the development of anammox bacterial populations and their activities in this freshwater habitat . Anammox activities were also recently detected in ammonium-contaminated groundwater sites in Canada, where anammox activity was responsible for 18–36% of the nitrogen loss . Because the highest relative contribution of the anammox process to nitrogen loss was found in sites with high concentrations of dissolved organic matter, NH4+ and NO3−, researchers hypothesized that these factors influenced anammox activity . In addition, anammox bacteria have also been detected in the sediments of the Xinyi River  and Lake Wintergreen . Unlike marine environments, in which the anammox communities were exclusively dominated by the Scalindua genus, different dominant anammox species have been observed in freshwater ecosystems. In Lake Tanganyika and Lake Wintergreen, the Scalindua genus was the dominant anammox species [22,42], whereas in the sediments of the Xinyi River, Lake Kitaura and Canada's groundwater environments, Brocadia organisms were the most common representatives [21,40,41]. These findings suggest that freshwater environments are more favourable for the growth of different groups of anammox bacteria than marine environments.
Anammox in terrestrial ecosystems
Because anammox depends on the concomitant presence of oxidized and reduced inorganic nitrogen compounds under anoxic conditions, the oxic/anoxic interfaces in terrestrial areas may provide suitable habitats for anammox bacteria . However, little is known about the presence of anammox bacteria in terrestrial ecosystems (Table 1). Humbert et al.  first reported the distribution of diverse anammox bacteria in agricultural and permafrost soils, which contained Brocadia, Kuenenia, Scalindua and Jettenia. Four different anammox genera were also simultaneously detected in fertilized paddy soil in Southern China , and three distinct anammox genera were found together in nitrogen-loaded peat soil . These results suggest a higher diversity of anammox bacteria in terrestrial ecosystems than usually observed in aquatic habitats and may be mediated by the larger variety of suitable niches for anammox bacteria in terrestrial habitats . The dominant anammox species in the reported terrestrial environments were affiliated with the Brocadia and Kuenenia genera [23,24,44], demonstrating that Brocadia and Kuenenia organisms exhibited a better adaptation capacity in these ecosystems . Brocadia and Kuenenia organisms possess a more mixotrophic metabolism than previously thought [4,12,44]. These microbes use ferrous iron and a variety of organic compounds, such as formate, acetate, propionate and methylamines, as electron donors [4,12,44] and employ ferric iron and manganese oxides as electron acceptors during their metabolic activities . Therefore the versatile metabolism of Brocadia and Kuenenia organisms may be the main reason for their better adaptation in heterogeneous terrestrial environments. Anammox activity was reported to account for 4–37% of the soil N2 production in a fertilized paddy soil, and the substantial contribution of anammox to nitrogen loss in the paddy field was related to the high concentrations of NH4+ that were introduced by the fertilization . Moreover, Hu et al.  obtained an enriched anammox culture from a nitrogen-loaded peat soil and showed a significant amount of anammox activity. In controlled environments, the presence of slowly released organic matter (e.g. humic acids) was identified as an important factor that influenced the distribution of anammox bacteria (Brocadia and Jettenia) in peat soil .
Anammox in special ecosystems
Anammox bacteria are active at 6–43°C, with an optimal temperature of 35°C in laboratory bioreactors . However, it was recently observed that this process also occurred at 52°C in hot springs , 72°C in petroleum reservoirs  and even at 85°C in hydrothermal vents . The dominant anammox species in these high-temperature habitats were Brocadia or Kuenenia, but not Scalindua (Table 1). This result is likely to be due to the higher optimal growth temperatures of Brocadia and Kuenenia organisms (35°C in bioreactors) compared with that of Scalindua organisms (12–15°C in marine ecosystems) . The production of long-chain fatty acids in anammox bacteria at elevated temperatures mediates their adaption to high-temperature environments . The versatile metabolism of some anammox species (such as Brocadia and Kuenenia) is critical for survival in high-temperature habitats . Byrne et al.  detected the activity of anammox bacteria in a hydrothermal vent, indicating that anammox may play an important role in the nitrogen cycle in thermophilic and mesophilic environments.
Anammox bacteria were also detected in some marine sponges, which are ancient animals [48,49]. The majority of anammox sequences found in marine sponges were affiliated with the Scalindua or Brocadia genus. Hoffmann et al.  detected the activity of anammox bacteria in marine sponges and showed that anammox activity contributed to 1.25% of the N2 production; in addition, the nitrification and denitrification processes were also found in the marine sponges, and the related and complex interactions of these nitrogen-cycling processes were mainly controlled by the metabolic waste products (e.g. NH4+ and organic matter) of the sponge host. Therefore the anammox process in marine sponges was largely influenced by the amount of NH4+ and organic matter produced by the sponge host.
Conclusions and outlook
With the current knowledge of the anammox process in natural ecosystems, it is clear that anammox bacteria have a widespread distribution in various natural habitats. Among the anammox bacteria that have been described to date, Scalindua organisms appear to be the most widespread anammox species in natural ecosystems. However, Brocadia and Kuenenia species appear to be more likely to live in freshwater and terrestrial ecosystems. The published data have indicated that anammox activity was responsible for 50% of the marine nitrogen loss and may also play an important role in the nitrogen cycle of freshwater and terrestrial ecosystems. In contrast with marine ecosystems, the detection of anammox bacteria, the measurement of anammox activity and elucidation of environmental factors influencing the anammox process are still lacking for freshwater and terrestrial environments. Therefore more studies on the anammox process in a broader range of freshwater and terrestrial habitats are required to obtain a better comprehension of the ecological distribution of anammox bacteria and their potential importance in marine ecosystems as natural ecosystems. In addition, further study will enhance our understanding of the influential factors that control anammox activity in natural environments.
Our research was supported by the National Hi-Tech Research and Development Program of China (863) [grant number 2006AA06Z332] and the Fundamental Research Funds for the Central Universities [grant number 2010QNA6017].
ICoN2 and the NCycle16: The 2nd International Conference on Nitrification (ICoN2) and the 16th European Nitrogen Cycle (NCycle16) Linked Independent Meetings held at Hotel Val Monte, Berg en Dal, The Netherlands, 3–7 July 2011. Organized and Edited by Mike Jetten (Radboud University, Nijmegen, The Netherlands) and David Richardson (University of East Anglia, Norwich, U.K.).
Abbreviations: anammox, anaerobic ammonium oxidation; DNRA, dissimilatory nitrite reduction to ammonium; OMZ, oxygen minimum zone
- © The Authors Journal compilation © 2011 Biochemical Society