Summary Anaerobic ammonium oxidation (anammox) is a microbial process in which NH4+ is oxidized to N2 gas with NO2− as an electron acceptor. The anammox process is mediated by bacterial members affiliated with the phylum Planctomycetes, which are ubiquitously detected from anoxic natural and man-made ecosystems and a key player in the global nitrogen cycle. In the past two decades, phylogenetically different anammox bacteria have been recognized in natural and synthetic ecosystems (i.e. 'Candidatus Kuenenia', 'Candidatus Brocadia', 'Candidatus Jettenia', 'Candidatus Anammoxoglobus' and 'Candidatus Scalindua' genera), and the geographic distributions of these anammox bacteria indicate that they have genus-specific or species-specific habitats. Recently, we revealed the physiological characteristics of 'Ca. Jettenia' in addition to 'Ca. Kuenenia', 'Ca. Brocadia' and 'Ca. Scalindua', and, as a result, it is possible to compare the physiological characteristics of the anammox bacteria and discuss their niche partitioning. Therefore, we summarize the current knowledge of anammox bacterial ecology and physiology in this review to assess the potential ecological niche partitioning of anammox bacteria in natural and synthetic ecosystems.

The present study investigated the phylogenetic affiliation and physiological characteristics of bacteria responsible for anaerobic ammonium oxidization (anammox); these bacteria were enriched in an anammox reactor with a nitrogen removal rate of 26.0 kg N m "3 day "1 . The anammox bacteria were identified as representing 'Candidatus Brocadia sinica' on the basis of phylogenetic analysis of rRNA operon sequences. Physiological characteristics examined were growth rate, kinetics of ammonium oxidation and nitrite reduction, temperature, pH and inhibition of anammox. The maximum specific growth rate (m max ) was 0.0041 h "1 , corresponding to a doubling time of 7 days. The half-saturation constants (K s ) for ammonium and nitrite of 'Ca. B. sinica' were 28±4 and 86±4 mM, respectively, higher than those of 'Candidatus Brocadia anammoxidans' and 'Candidatus Kuenenia stuttgartiensis'. The temperature and pH ranges of anammox activity were 25-45 6C and pH 6.5-8.8, respectively. Anammox activity was inhibited in the presence of nitrite (50 % inhibition at 16 mM), ethanol (91 % at 1 mM) and methanol (86 % at 1 mM). Anammox activities were 80 and 70 % of baseline in the presence of 20 mM phosphorus and 3 % salinity, respectively. The yield of biomass and dissolved organic carbon production in the culture supernatant were 0.062 and 0.005 mol C (mol NH þ 4 ) "1 , respectively.This study compared physiological differences between three anammox bacterial enrichment cultures to provide a better understanding of anammox niche specificity in natural and man-made ecosystems.

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To date, six candidate genera of anaerobic ammonium-oxidizing (anammox) bacteria have been identified, and numerous studies have been conducted to understand their ecophysiology. In this study, we examined the physiological characteristics of an anammox bacterium in the genus 'Candidatus Jettenia'. Planctomycete KSU-1 was found to be a mesophilic (20-42.5°C) and neutrophilic (pH 6.5-8.5) bacterium with a maximum growth rate of 0.0020 h(-1) . Planctomycete KSU-1 cells showed typical physiological and structural features of anammox bacteria; i.e. (29) N2 gas production by coupling of (15) NH4 (+) and (14) NO2 (-) , accumulation of hydrazine with the consumption of hydroxylamine and the presence of anammoxosome. In addition, the cells were capable of respiratory ammonification with oxidation of acetate. Notably, the cells contained menaquinone-7 as a dominant respiratory quinone. Proteomic analysis was performed to examine underlying core metabolisms, and high expressions of hydrazine synthase, hydrazine dehydrogenase, hydroxylamine dehydrogenase, nitrite/nitrate oxidoreductase and carbon monoxide dehydrogenase/acetyl-CoA synthase were detected. These proteins require iron or copper as a metal cofactor, and both were dominant in planctomycete KSU-1 cells. On the basis of these experimental results, we proposed the name 'Ca. Jettenia caeni' sp. nov. for the bacterial clade of the planctomycete KSU-1.

bThe phylogenetic affiliation and physiological characteristics (e.g., K s and maximum specific growth rate [ max ]) of an anaerobic ammonium oxidation (anammox) bacterium, "Candidatus Scalindua sp.," enriched from the marine sediment of Hiroshima Bay, Japan, were investigated. "Candidatus Scalindua sp." exhibits higher affinity for nitrite and a lower growth rate and yield than the known anammox species.

Emission of nitrous oxide (N 2 O) during biological wastewater treatment is of growing concern since N 2 O is a major stratospheric ozone-depleting substance and an important greenhouse gas. The emission of N 2 O from a lab-scale granular sequencing batch reactor (SBR) for partial nitrification (PN) treating synthetic wastewater without organic carbon was therefore determined in this study, because PN process is known to produce more N 2 O than conventional nitrification processes. The average N 2 O emission rate from the SBR was 0.32 ± 0.17 mg-N L -1 h -1 , corresponding to the average emission of N 2 O of 0.8 ± 0.4% of the incoming nitrogen load (1.5 ± 0.8% of the converted NH 4 + ). Analysis of dynamic concentration profiles during one cycle of the SBR operation demonstrated that N 2 O concentration in off-gas was the highest just after starting aeration whereas N 2 O concentration in effluent was gradually increased in the initial 40 min of the aeration period and was decreased thereafter. Isotopomer analysis was conducted to identify the main N 2 O production pathway in the reactor during one cycle. The hydroxylamine (NH 2 OH) oxidation pathway accounted for 65% of the total N 2 O production in the initial phase during one cycle, whereas contribution of the NO 2 -reduction pathway to N 2 O production was comparable with that of the NH 2 OH oxidation pathway in the latter phase. In addition, 3 spatial distributions of bacteria and their activities in single microbial granules taken from the reactor were determined with microsensors and by in situ hybridization. Partial nitrification occurred mainly in the oxic surface layer of the granules and ammonia-oxidizing bacteria were abundant in this layer. N 2 O production was also found mainly in the oxic surface layer. Based on these results, although N 2 O was produced mainly via NH 2 OH oxidation pathway in the autotrophic partial nitrification reactor, N 2 O production mechanisms were complex and could involve multiple N 2 O production pathways.

Summary Although metabolic pathways and associated enzymes of anaerobic ammonium oxidation (anammox) of ‘Ca. Kuenenia stuttgartiensis’ have been studied, those of other anammox bacteria are still poorly understood. NO2− reduction to NO is considered to be the first step in the anammox metabolism of ‘Ca. K. stuttgartiensis’, however, ‘Ca. Brocadia’ lacks the genes that encode canonical NO‐forming nitrite reductases (NirS or NirK) in its genome, which is different from ‘Ca. K. stuttgartiensis’. Here, we studied the anammox metabolism of ‘Ca. Brocadia sinica’. 15N‐tracer experiments demonstrated that ‘Ca. B. sinica’ cells could reduce NO2− to NH2OH, instead of NO, with as yet unidentified nitrite reductase(s). Furthermore, N2H4 synthesis, downstream reaction of NO2− reduction, was investigated using a purified ‘Ca. B. sinica' hydrazine synthase (Hzs) and intact cells. Both the ‘Ca. B. sinica’ Hzs and cells utilized NH2OH and NH4+, but not NO and NH4+, for N2H4 synthesis and further oxidized N2H4 to N2 gas. Taken together, the metabolic pathway of ‘Ca. B. sinica’ is NH2OH‐dependent and different from the one of ‘Ca. K. stuttgartiensis’, indicating metabolic diversity of anammox bacteria.