Multidimensional Reveal of Nitrogen Regulation on Comammox

2020 ◽  
Author(s):  
Yuxiang Zhao ◽  
Jiajie Hu ◽  
Weiling Yang ◽  
Jiaqi Wang ◽  
Zhongjun Jia ◽  
...  
Keyword(s):  
Nitrogen Source ◽  
Nitrate Reduction ◽  
Ammonia Oxidation ◽  
Enrichment Culture ◽  
Ammonia Oxidizer ◽  
Nitrification Rate ◽  
Ammonia Oxidizing Bacteria ◽  
Metagenomic Sequencing ◽  
Sole Nitrogen Source ◽  
Total Bacteria

Abstract Background The discovery of complete ammonia oxidizer (comammox) was groundbreaking. Comammox can use ammonia as the sole nitrogen source and turn it to nitrate. Moreover, genomic data indicated that comammox contained genes which can metabolize urea and nitrite. However, the feasibility of enriching comammox with urea and nitrite in long term has not been proved. This study enriched comammox’s culture by using nitrite in reactor SA and urea in reactor SB. Results The nitrification rate of reactor SB (1.29 mg N·g -1 biofilm · d -1 ) was higher than that in reactor SA (0.6 mg N · g -1 biofilm · d -1 ) at the 390 th day. Comammox outnumbered ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in both reactor SA (9.04 × 10 9 copies / g biofilm) and reactor SB (5.34×10 10 copies/ g biofilm). In reactor SA, comammox’s amoA accounted for 92% of the total amoA, which was higher than that in reactor SB (85%). However, the percentage of comammox (4%) in total bacteria was much lower than reactor SB (14%). The results of metagenomic sequencing showed that all the pathways of nitrogen cycle including nitrification, nitrogen fixation, denitrification, assimilation nitrate reduction, and dissimilation nitrate reduction can be detected in both reactor SA and reactor SB except the anammox pathway. The genes related to nitrite oxidation and nitrate reduction in reactor SA (TPM = 5099; TPM = 3329) was higher than that of in reactor SB (TPM = 4071; TPM = 2984), presumably due to the demand of turning nitrite to nitrate and turning nitrate to ammonia. While genes related to ammonia oxidation and urea metabolism in reactor SB (TPM = 3915; TPM = 3638) was higher than that in reactor SA (TPM = 2708; TPM = 3002). Conclusion Nitrite and urea can regulate the enrichment culture of comammox by converting its metabolic pathway. Using nitrite as sole nitrogen source can improve the proportion comammox’s amoA in total amoA while using urea as the sole nitrogen source may increase comammox’s proportion in total bacteria. These results can accelerate the enrichment of comammox and facilitate the promotion of comammox’s engineering operation.

scholarly journals Ecophysiological Characterization of Ammonia-Oxidizing Archaea and Bacteria from Freshwater

2012 ◽  
Vol 78 (16) ◽  
pp. 5773-5780 ◽  
Author(s):  
Elizabeth French ◽  
Jessica A. Kozlowski ◽  
Maitreyee Mukherjee ◽  
George Bullerjahn ◽  
Annette Bollmann
Keyword(s):  
Growth Rates ◽  
Ammonia Oxidation ◽  
Light Exposure ◽  
Enrichment Culture ◽  
Ammonia Oxidizing Bacteria ◽  
Content Type ◽  
Ammonia Oxidizing Archaea ◽  
Group I ◽  
Nitrosomonas Oligotropha

ABSTRACTAerobic biological ammonia oxidation is carried out by two groups of microorganisms, ammonia-oxidizing bacteria (AOB) and the recently discovered ammonia-oxidizing archaea (AOA). Here we present a study using cultivation-based methods to investigate the differences in growth of three AOA cultures and one AOB culture enriched from freshwater environments. The strain in the enriched AOA culture belong to thaumarchaeal group I.1a, with the strain in one enrichment culture having the highest identity with “CandidatusNitrosoarchaeum koreensis” and the strains in the other two representing a new genus of AOA. The AOB strain in the enrichment culture was also obtained from freshwater and had the highest identity to AOB from theNitrosomonas oligotrophagroup (Nitrosomonascluster 6a). We investigated the influence of ammonium, oxygen, pH, and light on the growth of AOA and AOB. The growth rates of the AOB increased with increasing ammonium concentrations, while the growth rates of the AOA decreased slightly. Increasing oxygen concentrations led to an increase in the growth rate of the AOB, while the growth rates of AOA were almost oxygen insensitive. Light exposure (white and blue wavelengths) inhibited the growth of AOA completely, and the AOA did not recover when transferred to the dark. AOB were also inhibited by blue light; however, growth recovered immediately after transfer to the dark. Our results show that the tested AOB have a competitive advantage over the tested AOA under most conditions investigated. Further experiments will elucidate the niches of AOA and AOB in more detail.

scholarly journals Identifying Potential Mechanisms Enabling Acidophily in the Ammonia-Oxidizing Archaeon “Candidatus Nitrosotalea devanaterra”

2016 ◽  
Vol 82 (9) ◽  
pp. 2608-2619 ◽  
Author(s):  
Laura E. Lehtovirta-Morley ◽  
Luis A. Sayavedra-Soto ◽  
Nicolas Gallois ◽  
Stefan Schouten ◽  
Lisa Y. Stein ◽  
...  
Keyword(s):  
Biochemical Characterization ◽  
Ammonia Oxidation ◽  
Membrane Lipids ◽  
Acid Soils ◽  
Low Ph ◽  
Ammonia Oxidizer ◽  
Ammonia Oxidizing Bacteria ◽  
Ph Homeostasis ◽  
A Genome ◽  
Genes Encoding

ABSTRACTAmmonia oxidation is the first and rate-limiting step in nitrification and is dominated by two distinct groups of microorganisms in soil: ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). AOA are often more abundant than AOB and dominate activity in acid soils. The mechanism of ammonia oxidation under acidic conditions has been a long-standing paradox. While high rates of ammonia oxidation are frequently measured in acid soils, cultivated ammonia oxidizers grew only at near-neutral pH when grown in standard laboratory culture. Although a number of mechanisms have been demonstrated to enable neutrophilic AOB growth at low pH in the laboratory, these have not been demonstrated in soil, and the recent cultivation of the obligately acidophilic ammonia oxidizer “CandidatusNitrosotalea devanaterra” provides a more parsimonious explanation for the observed high rates of activity. Analysis of the sequenced genome, transcriptional activity, and lipid content of “Ca. Nitrosotalea devanaterra” reveals that previously proposed mechanisms used by AOB for growth at low pH are not essential for archaeal ammonia oxidation in acidic environments. Instead, the genome indicates that “Ca. Nitrosotalea devanaterra” contains genes encoding both a predicted high-affinity substrate acquisition system and potential pH homeostasis mechanisms absent in neutrophilic AOA. Analysis of mRNA revealed that candidate genes encoding the proposed homeostasis mechanisms were all expressed during acidophilic growth, and lipid profiling by high-performance liquid chromatography–mass spectrometry (HPLC-MS) demonstrated that the membrane lipids of “Ca. Nitrosotalea devanaterra” were not dominated by crenarchaeol, as found in neutrophilic AOA. This study for the first time describes a genome of an obligately acidophilic ammonia oxidizer and identifies potential mechanisms enabling this unique phenotype for future biochemical characterization.

scholarly journals Metabolism of Thioamides by Ralstonia pickettii TA

2006 ◽  
Vol 72 (12) ◽  
pp. 7468-7476 ◽  
Author(s):  
Anthony G. Dodge ◽  
Jack E. Richman ◽  
Gilbert Johnson ◽  
Lawrence P. Wackett
Keyword(s):  
Nitrogen Source ◽  
Enrichment Culture ◽  
Carbon Sources ◽  
Resting Cells ◽  
Sole Nitrogen Source ◽  
Antituberculosis Drug ◽  
Ralstonia Pickettii ◽  
Dead End ◽  
Spontaneous Reaction ◽  
Related Compounds

ABSTRACT Information on bacterial thioamide metabolism has focused on transformation of the antituberculosis drug ethionamide and related compounds by Mycobacterium tuberculosis. To study this metabolism more generally, a bacterium that grew using thioacetamide as the sole nitrogen source was isolated via enrichment culture. The bacterium was identified as Ralstonia pickettii and designated strain TA. Cells grown on thioacetamide also transformed other thioamide compounds. Transformation of the thioamides tested was dependent on oxygen. During thioamide degradation, sulfur was detected in the medium at the oxidation level of sulfite, further suggesting an oxygenase mechanism. R. pickettii TA did not grow on thiobenzamide as a nitrogen source, but resting cells converted thiobenzamide to benzamide, with thiobenzamide S-oxide and benzonitrile detected as intermediates. Thioacetamide S-oxide was detected as an intermediate during thioacetamide degradation, but the only accumulating metabolite of thioacetamide was identified as 3,5-dimethyl-1,2,4-thiadiazole, a compound shown to derive from spontaneous reaction of thioacetamide and oxygenated thioacetamide species. This dead-end metabolite accounted for only ca. 12% of the metabolized thioacetamide. Neither acetonitrile nor acetamide was detected during thioacetamide degradation, but R. pickettii grew on both compounds as nitrogen and carbon sources. It is proposed that R. pickettii TA degrades thioamides via a mechanism involving consecutive oxygenations of the thioamide sulfur atom.

scholarly journals The Response of Estuarine Ammonia-Oxidizing Communities to Constant and Fluctuating Salinity Regimes

2020 ◽  
Vol 11 ◽  
Author(s):  
João Pereira Santos ◽  
António G. G. Sousa ◽  
Hugo Ribeiro ◽  
Catarina Magalhães
Keyword(s):  
Ammonia Oxidation ◽  
Estuarine Sediments ◽  
Ammonia Oxidizer ◽  
Rrna Gene ◽  
Ammonia Oxidizing Bacteria ◽  
Salinity Regime ◽  
Salinity Fluctuation ◽  
Ammonia Oxidizing Archaea ◽  
Negative Effect ◽  
The Impact

Aerobic nitrification is a fundamental nitrogen biogeochemical process that links the oxidation of ammonia to the removal of fixed nitrogen in eutrophicated water bodies. However, in estuarine environments there is an enormous variability of water physicochemical parameters that can affect the ammonia oxidation biological process. For instance, it is known that salinity can affect nitrification performance, yet there is still a lack of information on the ammonia-oxidizing communities behavior facing daily salinity fluctuations. In this work, laboratory experiments using upstream and downstream estuarine sediments were performed to address this missing gap by comparing the effect of daily salinity fluctuations with constant salinity on the activity and diversity of ammonia-oxidizing microorganisms (AOM). Activity and composition of AOM were assessed, respectively by using nitrogen stable isotope technique and 16S rRNA gene metabarcoding analysis. Nitrification activity was negatively affected by daily salinity fluctuations in upstream sediments while no effect was observed in downstream sediments. Constant salinity regime showed clearly higher rates of nitrification in upstream sediments while a similar nitrification performance between the two salinity regimes was registered in the downstream sediments. Results also indicated that daily salinity fluctuation regime had a negative effect on both ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) community’s diversity. Phylogenetically, the estuarine downstream AOM were dominated by AOA (0.92–2.09%) followed by NOB (0.99–2%), and then AOB (0.2–0.32%); whereas NOB dominated estuarine upstream sediment samples (1.4–9.5%), followed by AOA (0.27–0.51%) and AOB (0.01–0.23%). Analysis of variance identified the spatial difference between samples (downstream and upstream) as the main drivers of AOA and AOB diversity. Our study indicates that benthic AOM inhabiting different estuarine sites presented distinct plasticity toward the salinity regimes tested. These findings help to improve our understanding in the dynamics of the nitrogen cycle of estuarine systems by showing the resilience and consequently the impact of different salinity regimes on the diversity and activity of ammonia oxidizer communities.

Numbers, activities, and diversity of autotrophic ammonia-oxidizing bacteria in a freshwater, eutrophic lake sediment

1991 ◽  
Vol 37 (11) ◽  
pp. 828-833 ◽  
Author(s):  
W. T. Smorczewski ◽  
E. L. Schmidt
Keyword(s):  
Lake Sediment ◽  
Environmental Changes ◽  
Ammonia Oxidation ◽  
Eutrophic Lake ◽  
Ammonia Oxidizer ◽  
Most Probable Number ◽  
Ammonia Oxidizing Bacteria ◽  
Ammonia Oxidizers ◽  
Probable Number ◽  
Nitrite Oxidizers

The microbiological and chemical potential for ammonia oxidation in a freshwater, eutrophic lake sediment was examined in relation to environmental changes caused by seasonal, dimictic circulation. Poulations of both ammonia and nitrite oxidizers as estimated by most probable number (MPN) were sustained throughout extended anaerobic summer intervals, with nitrite oxidizers outnumbering ammonia oxidizers by a factor ranging from 3.0 to 8.1. Ammonia oxidation potential on a per cell basis was affected by seasonal changes and was seen to decrease as oxygen was removed from the sediments. Pure-culture isolations from a positive MPN tube inoculated with oxygenated sediment and representing a single point in a seasonal cycle produced ammonia-oxidizing strains belonging to the genus Nitrosospira. These strains did not react with known ammonia-oxidizer serotypes and, therefore, extend the serological diversity of this group of bacteria. An immunofluorescence analysis of MPN tubes from sediment collected during a period of lake stratification revealed progressive changes in the diversity of the ammonia-oxidizer population. The genera Nitrosomonas, Nitrosolobus, and Nitrosospira, including the novel serotype of Nitrosospira isolated from the sediment a year earlier, were found to coexist in well-oxygenated sediment. This diversity was seen to disappear, with Nistrosomonas surviving, as anaerobic conditions persisted. Key words: ammonia oxidizers, lake sediments, nitrifiers, nitrification.

scholarly journals Ammonia-oxidizing archaea possess a wide range of cellular ammonia affinities

2021 ◽  
Author(s):  
Man-Young Jung ◽  
Christopher J. Sedlacek ◽  
K. Dimitri Kits ◽  
Anna J. Mueller ◽  
Sung-Keun Rhee ◽  
...  
Keyword(s):  
Ammonia Oxidation ◽  
Oxidation Kinetic ◽  
Ammonia Oxidizer ◽  
Substrate Affinity ◽  
Kinetic Properties ◽  
Ammonia Oxidizing Bacteria ◽  
Ammonia Oxidizers ◽  
Kinetic Measurements ◽  
Ammonia Oxidizing Archaea ◽  
Wide Range

AbstractNitrification, the oxidation of ammonia to nitrate, is an essential process in the biogeochemical nitrogen cycle. The first step of nitrification, ammonia oxidation, is performed by three, often co- occurring guilds of chemolithoautotrophs: ammonia-oxidizing bacteria (AOB), archaea (AOA), and complete ammonia oxidizers (comammox). Substrate kinetics are considered to be a major niche-differentiating factor between these guilds, but few AOA strains have been kinetically characterized. Here, the ammonia oxidation kinetic properties of 12 AOA representing all major phylogenetic lineages were determined using microrespirometry. Members of the genus Nitrosocosmicus have the lowest substrate affinity of any characterized AOA, which are similar to previously determined affinities of AOB. This contrasts previous assumptions that all AOA possess much higher substrate affinities than their comammox or AOB counterparts. The substrate affinity of ammonia oxidizers correlated with their cell surface area to volume ratios. In addition, kinetic measurements across a range of pH values strongly supports the hypothesis that – like for AOB – ammonia and not ammonium is the substrate for the ammonia monooxygenase enzyme of AOA and comammox. Together, these data will facilitate predictions and interpretation of ammonia oxidizer community structures and provide a robust basis for establishing testable hypotheses on competition between AOB, AOA, and comammox.

scholarly journals Microbial community analysis of biofilters reveals a dominance of either comammox Nitrospira or archaea as ammonia oxidizers in freshwater aquaria

2021 ◽  
Author(s):  
Michelle M McKnight ◽  
Josh D Neufeld
Keyword(s):  
Microbial Communities ◽  
Ammonia Oxidation ◽  
16S Rrna Gene Sequencing ◽  
Community Analysis ◽  
Microbial Community Analysis ◽  
Ammonia Oxidizer ◽  
Future Research ◽  
Rrna Gene ◽  
Ammonia Oxidizing Bacteria ◽  
Ammonia Oxidizers

Nitrification by aquarium biofilters transforms toxic ammonia waste (NH3/NH4+) to less toxic nitrate (NO3-) via nitrite (NO2-). Ammonia oxidation is mediated by ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), and the recently discovered complete ammonia oxidizing (comammox) Nitrospira. Prior to the discovery of comammox Nitrospira, previous research revealed that AOA dominate among ammonia oxidizers in freshwater biofilters. Here, we characterized the composition of aquarium filter microbial communities and quantified the abundance of all three known groups of ammonia oxidizers. Aquarium biofilter and water samples were collected from representative freshwater and saltwater systems in Southwestern Ontario, Canada. Using extracted DNA, we performed 16S rRNA gene sequencing and quantitative PCR (qPCR) to assess community composition and quantify the abundance of amoA genes, respectively. Our results show that aquarium biofilter microbial communities were consistently represented by putative heterotrophs of the Proteobacteria and Bacteroides phyla, with distinct profiles associated with fresh versus saltwater biofilters. Among nitrifiers, comammox Nitrospira amoA genes were detected in all 38 freshwater aquarium biofilter samples and were the most abundant ammonia oxidizer in 30 of these samples, with the remaining biofilters dominated by AOA, based on amoA gene abundances. In saltwater biofilters, AOA or AOB were differentially abundant, with no comammox Nitrospira detected. These results demonstrate that comammox Nitrospira play an important role in biofilter nitrification that has been previously overlooked and such microcosms are useful for exploring the ecology of nitrification for future research.

Competition of Ammonia-Oxidizing Archaea and Bacteria from Freshwater Environments

2021 ◽  
Author(s):  
Elizabeth French ◽  
Jessica A. Kozlowski ◽  
Annette Bollmann
Keyword(s):  
Ammonia Oxidation ◽  
Heterotrophic Bacteria ◽  
Ammonia Oxidizer ◽  
Ammonium Concentration ◽  
Natural Environments ◽  
Ammonia Oxidizing Bacteria ◽  
Ammonia Oxidizers ◽  
Batch Cultures ◽  
Freshwater Systems ◽  
Ammonia Oxidizing Archaea

In the environment, nutrients are rarely available in constant supply. Therefore, microorganisms require strategies to compete for limiting nutrients. In freshwater systems, ammonia-oxidizing archaea (AOA) and bacteria (AOB) compete with heterotrophic bacteria, photosynthetic microorganisms, and each other for ammonium, which AOA and AOB utilize as their sole source of energy and nitrogen. We investigated the competition between highly enriched cultures of an AOA (AOA-AC1) and an AOB (AOB-G5-7) for ammonium. Based on the amoA gene, the newly enriched archaeal ammonia oxidizer in AOA-AC1 was closely related to Nitrosotenuis spp. and the bacterial ammonia oxidizer in AOB-G5-7, Nitrosomonas sp. Is79, belonged to the Nitrosomonas oligotropha group ( Nitrosomonas cluster 6a). Growth experiments in batch cultures showed that AOB-G5-7 had higher growth rates than AOA-AC1 at higher ammonium concentrations. During chemostat competition experiments under ammonium-limiting conditions, AOA-AC1 dominated the cultures, while AOB-G5-7 decreased in abundance. In batch cultures, the outcome of the competition between AOA and AOB was determined by the initial ammonium concentrations. AOA-AC1 was the dominant ammonia oxidizer at an initial ammonium concentration of 50 μM and AOB-G5-7 at 500 μM. These findings indicate that, during direct competition, AOA-AC1 was able to use ammonium that was unavailable to AOB-G5-7, while AOB-G5-7 dominated at higher ammonium concentrations. The results are in strong accordance with environmental survey data suggesting that AOA are mainly responsible for ammonia oxidation under more oligotrophic conditions, whereas AOB dominate under eutrophic conditions. Importance Nitrification is an important process in the global nitrogen cycle. The first step - ammonia oxidation to nitrite – can be carried out by Ammonia-oxidizing Archaea (AOA) and Ammonia-oxidizing Bacteria (AOB). In many natural environments, these ammonia oxidizers coexist. Therefore, it is important to understand the population dynamics in response to increasing ammonium concentrations. Here, we study the competition between AOA and AOB enriched from freshwater systems. The results demonstrate that AOA are more abundant in systems with low ammonium availabilities and AOB when the ammonium availability increases. These results will help to predict potential shifts in community composition of ammonia oxidizers in the environment due to changes in ammonium availability.

scholarly journals Filling in the Gaps in Metformin Biodegradation: A New Enzyme and a Metabolic Pathway for Guanylurea

2021 ◽  
Author(s):  
Lambros J. Tassoulas ◽  
Ashley Robinson ◽  
Betsy Martinez-Vaz ◽  
Kelly G. Aukema ◽  
Lawrence P. Wackett
Keyword(s):  
Wastewater Treatment ◽  
Nitrogen Source ◽  
Wastewater Treatment Plant ◽  
Wastewater Treatment Plants ◽  
Treatment Plant ◽  
Metagenomic Sequencing ◽  
Pseudomonas Mendocina ◽  
Sole Nitrogen Source ◽  
Aquatic Life ◽  
Dead End

The widely prescribed pharmaceutical metformin and its main metabolite guanylurea are currently two of the most common contaminants in surface and wastewater. Guanylurea often accumulates and is poorly, if at all, biodegraded in wastewater treatment plants. This study describes Pseudomonas mendocina strain GU isolated from a municipal wastewater treatment plant using guanylurea as its sole nitrogen source. The genome was sequenced with 36-fold coverage and mined to identify guanylurea degradation genes. The gene encoding the enzyme initiating guanylurea metabolism was expressed, the enzyme purified and characterized. Guanylurea hydrolase, a newly described enzyme, was shown to transform guanylurea to one equivalent of ammonia and guanidine. Guanidine also supports growth as a sole nitrogen source. Cell yields from growth on limiting concentrations of guanylurea revealed that metabolism releases all four nitrogen atoms. Genes encoding complete metabolic transformation were identified bioinformatically, defining the pathway as follows: guanylurea to guanidine to carboxyguanidine to allophanate to ammonia and carbon dioxide. The first enzyme, guanylurea hydrolase, is a member of the isochorismatase-like hydrolase protein family that includes biuret hydrolase and triuret hydrolase. Although homologs, the three enzymes show distinct substrate specificities. Pairwise sequence comparisons and the use of sequence similarity networks allowed fine structure discrimination between the three homologous enzymes and provided insights into the evolutionary origins of guanylurea hydrolase. IMPORTANCE Metformin is a pharmaceutical most prescribed for type 2 diabetes and is now being examined for potential benefits to COVID-19 patients. People taking the drug pass it largely unchanged and it subsequently enters wastewater treatment plants. Metformin has been known to be metabolized to guanylurea. The levels of guanylurea often exceed that of metformin, leading to the former being considered a “dead end” metabolite. Metformin and guanylurea are water pollutants of emerging concern as they persist to reach non-target aquatic life and humans, the latter if it remains in treated water. The present study has identified a Pseudomonas mendocina strain that completely degrades guanylurea. The genome was sequenced and the genes involved in guanylurea metabolism were identified in three widely separated genomic regions. This knowledge advances the idea that guanylurea is not a dead end product and will allow for bioinformatic identification of the relevant genes in wastewater treatment plant microbiomes and other environments subjected to metagenomic sequencing.

scholarly journals Cultivation and genomic analysis ofCandidatusNitrosocaldus islandicus, a novel obligately thermophilic ammonia-oxidizingThaumarchaeon

2017 ◽  
Author(s):  
Anne Daebeler ◽  
Craig Herbold ◽  
Julia Vierheilig ◽  
Christopher J. Sedlacek ◽  
Petra Pjevac ◽  
...  
Keyword(s):  
Ammonia Oxidation ◽  
Hot Spring ◽  
Enrichment Culture ◽  
Genomic Analysis ◽  
Molecular Data ◽  
Ammonia Oxidizer ◽  
Genomic Feature ◽  
Acid Fermentation ◽  
Ammonia Oxidizing Archaea ◽  
Metabolic Versatility

AbstractAmmonia-oxidizing archaea (AOA) within the phylumThaumarchaeaare the only known aerobic ammonia oxidizers in geothermal environments. Although molecular data indicate the presence of phylogenetically diverse AOA from theNitrosocaldusclade, group 1.1b and group 1.1aThaumarchaeain terrestrial high-temperature habitats, only one enrichment culture of an AOA thriving above 50 °C has been reported and functionally analyzed. In this study, we physiologically and genomically characterized a novelThaumarchaeonfrom the deep-branchingNitrosocaldaceaefamily of which we have obtained a high (∼85 %) enrichment from biofilm of an Icelandic hot spring (73 °C). This AOA, which we provisionally refer to as “CandidatusNitrosocaldus islandicus”, is an obligately thermophilic, aerobic chemolithoautotrophic ammonia oxidizer, which stoichiometrically converts ammonia to nitrite at temperatures between 50 °C and 70 °C.Ca.N. islandicus encodes the expected repertoire of enzymes proposed to be required for archaeal ammonia oxidation, but unexpectedly lacks anirKgene and also possesses no identifiable other enzyme for nitric oxide (NO) generation. Nevertheless, ammonia oxidation by this AOA appears to be NO-dependent asCa.N. islandicus is, like all other tested AOA, inhibited by the addition of an NO scavenger. Furthermore, comparative genomics revealed thatCa.N. islandicus has the potential for aromatic amino acid fermentation as its genome encodes an indolepyruvate oxidoreductase(iorAB)as well as a type 3b hydrogenase, which are not present in any other sequenced AOA. A further surprising genomic feature of this thermophilic ammonia oxidizer is the absence of DNA polymerase D genes - one of the predominant replicative DNA polymerases in all other ammonia-oxidizingThaumarchaea.Collectively, our findings suggest that metabolic versatility and DNA replication might differ substantially between obligately thermophilic and other AOA.

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