Metabolite release by nitrifiers facilitates metabolic interactions in the ocean.

Microbial chemoautotroph-heterotroph interactions may play a pivotal role in the cycling of carbon in the deep ocean, reminiscent of phytoplankton-heterotroph associations in surface waters. Nitrifiers are the most abundant chemoautotrophs in the global ocean, yet very little is known about nitrifier metabolite production, release, and transfer to heterotrophic microbial communities. To elucidate which organic compounds are released by nitrifiers and potentially available to heterotrophs, we characterized the exo- and endometabolomes of the ammonia-oxidizing archaeon Nitrosopumilus adriaticus CCS1 and the nitrite-oxidizing bacterium Nitrospina gracilis Nb-211. Nitrifier endometabolome composition was not a good predictor of exometabolite availability, indicating that metabolites were predominately released by mechanisms other than cell death/lysis. Although both nitrifiers released labile organic compounds, N. adriaticus preferentially released amino acids, particularly glycine, suggesting that its cell membranes might be more permeable to small, hydrophobic amino acids. We further initiated co-culture systems between each nitrifier and a heterotrophic alphaproteobacterium, and compared exometabolite and transcript patterns of nitrifiers grown axenically to those in co-culture. In particular, B vitamins exhibited dynamic production and consumption patterns in nitrifier-heterotroph co-cultures. We observed an increased production of vitamin B2 and the vitamin B12 lower ligand dimethylbenzimidazole by N. adriaticus and N. gracilis, respectively. In contrast, the heterotroph likely produced vitamin B5 in co-culture with both nitrifiers and consumed the vitamin B7 precursor dethiobiotin when grown with N. gracilis. Our results indicate that B vitamins and their precursors could play a particularly important role in governing specific metabolic interactions between nitrifiers and heterotrophic microbes in the ocean.

Growth of complete ammonia oxidizers on guanidine.

Guanidine is a chemically stable nitrogen compound that is excreted in human urine and is widely used in manufacturing of plastics, as a flame retardant and as a component of propellants, and is well known as a protein denaturant in biochemistry1-3. Guanidine occurs widely in nature and is used by several microorganisms as a nitrogen source, but microorganisms growing on guanidine as the only substrate have not yet been identified. Here we show that the complete ammonia oxidizer (comammox) Nitrospira inopinata and probably most other comammox microorganisms can grow on guanidine as the sole source of energy, reductant and nitrogen. Proteomics, enzyme kinetics and the crystal structure of a N. inopinata guanidinase homologue demonstrated that it is a bona fide guanidinase. Incubation experiments with comammox-containing agricultural soil and wastewater treatment plant microbiomes suggested that guanidine serves as substrate for nitrification in the environment. The identification of guanidine as a growth substrate for comammox shows an unexpected niche of these globally important nitrifiers and offers opportunities for their isolation.

Co-occurring nitrifying symbiont lineages are vertically inherited and widespread in marine sponges.

Ammonia-oxidizing archaea and nitrite-oxidizing bacteria are common members of marine sponge microbiomes. They derive energy for carbon fixation and growth from nitrification-the aerobic oxidation of ammonia to nitrite and further to nitrate-and are proposed to play essential roles in the carbon and nitrogen cycling of sponge holobionts. In this study, we characterize two novel nitrifying symbiont lineages, Candidatus Nitrosokoinonia and Candidatus Nitrosymbion in the marine sponge Coscinoderma matthewsi using a combination of molecular tools, in situ visualization, and physiological rate measurements. Both represent a new genus in the ammonia-oxidizing archaeal class Nitrososphaeria and the nitrite-oxidizing bacterial order Nitrospirales, respectively. Furthermore, we show that larvae of this viviparous sponge are densely colonized by representatives of Ca. Nitrosokoinonia and Ca. Nitrosymbion indicating vertical transmission. In adults, the representatives of both symbiont genera are located extracellularly in the mesohyl. Comparative metagenome analyses and physiological data suggest that ammonia-oxidizing archaeal symbionts of the genus Ca. Nitrosokoinonia strongly rely on endogenously produced nitrogenous compounds (i.e. ammonium, urea, nitriles/cyanides, and creatinine) rather than on exogenous ammonium sources taken up by the sponge. Additionally, the nitrite-oxidizing bacterial symbionts of the genus Ca. Nitrosymbion may reciprocally support the ammonia-oxidizers with ammonia via the utilization of sponge-derived urea and cyanate. Comparative analyses of published environmental 16S rRNA gene amplicon data revealed that Ca. Nitrosokoinonia and Ca. Nitrosymbion are widely distributed and predominantly associated with marine sponges and corals, suggesting a broad relevance of our findings.

Metabolic and phylogenetic diversity in the phylum Nitrospinota revealed by comparative genome analyses.

The most abundant known nitrite-oxidizing bacteria in the marine water column belong to the phylum Nitrospinota. Despite their importance in marine nitrogen cycling and primary production, there are only few cultured representatives that all belong to the class Nitrospinia. Moreover, although Nitrospinota were traditionally thought to be restricted to marine environments, metagenome-assembled genomes have also been recovered from groundwater. Over the recent years, metagenomic sequencing has led to the discovery of several novel classes of Nitrospinota (UBA9942, UBA7883, 2-12-FULL-45-22, JACRGO01, JADGAW01), which remain uncultivated and have not been analyzed in detail. Here, we analyzed a nonredundant set of 98 Nitrospinota genomes with focus on these understudied Nitrospinota classes and compared their metabolic profiles to get insights into their potential role in biogeochemical element cycling. Based on phylogenomic analysis and average amino acid identities, the highly diverse phylum Nitrospinota could be divided into at least 33 different genera, partly with quite distinct metabolic capacities. Our analysis shows that not all Nitrospinota are nitrite oxidizers and that members of this phylum have the genomic potential to use sulfide and hydrogen for energy conservation. This study expands our knowledge of the phylogeny and potential ecophysiology of the phylum Nitrospinota and offers new avenues for the isolation and cultivation of these elusive bacteria.

Cultivation and genomic characterization of novel and ubiquitous marine nitrite-oxidizing bacteria from the Nitrospirales.

Nitrospirales, including the genus Nitrospira, are environmentally widespread chemolithoautotrophic nitrite-oxidizing bacteria. These mostly uncultured microorganisms gain energy through nitrite oxidation, fix CO2, and thus play vital roles in nitrogen and carbon cycling. Over the last decade, our understanding of their physiology has advanced through several new discoveries, such as alternative energy metabolisms and complete ammonia oxidizers (comammox Nitrospira). These findings mainly resulted from studies of terrestrial species, whereas less attention has been given to marine Nitrospirales. In this study, we cultured three new marine Nitrospirales enrichments and one isolate. Three of these four NOB represent new Nitrospira species while the fourth represents a novel genus. This fourth organism, tentatively named "Ca. Nitronereus thalassa", represents the first cultured member of a Nitrospirales lineage that encompasses both free-living and sponge-associated nitrite oxidizers, is highly abundant in the environment, and shows distinct habitat distribution patterns compared to the marine Nitrospira species. Partially explaining this, "Ca. Nitronereus thalassa" harbors a unique combination of genes involved in carbon fixation and respiration, suggesting differential adaptations to fluctuating oxygen concentrations. Furthermore, "Ca. Nitronereus thalassa" appears to have a more narrow substrate range compared to many other marine nitrite oxidizers, as it lacks the genomic potential to utilize formate, cyanate, and urea. Lastly, we show that the presumed marine Nitrospirales lineages are not restricted to oceanic and saline environments, as previously assumed.

Nitrification in acidic and alkaline environments.

Aerobic nitrification is a key process in the global nitrogen cycle mediated by microorganisms. While nitrification has primarily been studied in near-neutral environments, this process occurs at a wide range of pH values, spanning ecosystems from acidic soils to soda lakes. Aerobic nitrification primarily occurs through the activities of ammonia-oxidising bacteria and archaea, nitrite-oxidising bacteria, and complete ammonia-oxidising (comammox) bacteria adapted to these environments. Here, we review the literature and identify knowledge gaps on the metabolic diversity, ecological distribution, and physiological adaptations of nitrifying microorganisms in acidic and alkaline environments. We emphasise that nitrifying microorganisms depend on a suite of physiological adaptations to maintain pH homeostasis, acquire energy and carbon sources, detoxify reactive nitrogen species, and generate a membrane potential at pH extremes. We also recognize the broader implications of their activities primarily in acidic environments, with a focus on agricultural productivity and nitrous oxide emissions, as well as promising applications in treating municipal wastewater.

Hot spring distribution and survival mechanisms of thermophilic comammox Nitrospira.

The recent discovery of Nitrospira species capable of complete ammonia oxidation (comammox) in non-marine natural and engineered ecosystems under mesothermal conditions has changed our understanding of microbial nitrification. However, little is known about the occurrence of comammox bacteria or their ability to survive in moderately thermal and/or hyperthermal habitats. Here, we report the wide distribution of comammox Nitrospira in five terrestrial hot springs at temperatures ranging from 36 to 80°C and provide metagenome-assembled genomes of 11 new comammox strains. Interestingly, the identification of dissimilatory nitrate reduction to ammonium (DNRA) in thermophilic comammox Nitrospira lineages suggests that they have versatile ecological functions as both sinks and sources of ammonia, in contrast to the described mesophilic comammox lineages, which lack the DNRA pathway. Furthermore, the in situ expression of key genes associated with nitrogen metabolism, thermal adaptation, and oxidative stress confirmed their ability to survive in the studied hot springs and their contribution to nitrification in these environments. Additionally, the smaller genome size and higher GC content, less polar and more charged amino acids in usage profiles, and the expression of a large number of heat shock proteins compared to mesophilic comammox strains presumably confer tolerance to thermal stress. These novel insights into the occurrence, metabolic activity, and adaptation of comammox Nitrospira in thermal habitats further expand our understanding of the global distribution of comammox Nitrospira and have significant implications for how these unique microorganisms have evolved thermal tolerance strategies.

Complete Genome Sequence of Nitrospina watsonii 347, Isolated from the Black Sea.

Here, we present the complete genome sequence of Nitrospina watsonii 347, a nitrite-oxidizing bacterium isolated from the Black Sea at a depth of 100 m. The genome has a length of 3,011,914 bp with 2,895 predicted coding sequences. Its predicted metabolism is similar to that of Nitrospina gracilis with differences in defense against reactive oxygen species.

Rapid nitrification involving comammox and canonical Nitrospira at extreme pH in saline-alkaline lakes.

Nitrite-oxidizing bacteria (NOB) catalyse the second nitrification step and are the main biological source of nitrate. The most diverse and widespread NOB genus is Nitrospira, which also contains complete ammonia oxidizers (comammox) that oxidize ammonia to nitrate. To date, little is known about the occurrence and biology of comammox and canonical nitrite oxidizing Nitrospira in extremely alkaline environments. Here, we studied the seasonal distribution and diversity, and the effect of short-term pH changes on comammox and canonical Nitrospira in sediments of two saline, highly alkaline lakes. We identified diverse canonical and comammox Nitrospira clade A-like phylotypes as the only detectable NOB during more than a year, suggesting their major importance for nitrification in these habitats. Gross nitrification rates measured in microcosm incubations were highest at pH 10 and considerably faster than reported for other natural, aquatic environments. Nitrification could be attributed to canonical and comammox Nitrospira and to Nitrososphaerales ammonia-oxidizing archaea. Furthermore, our data suggested that comammox Nitrospira contributed to ammonia oxidation at an extremely alkaline pH of 11. These results identify saline, highly alkaline lake sediments as environments of uniquely strong nitrification with novel comammox Nitrospira as key microbial players.

Enhanced nitrogen and carbon removal in natural seawater by electrochemical enrichment in a bioelectrochemical reactor.

Municipal and industrial wastewater discharges in coastal and marine environments are of major concern due to their high carbon and nitrogen loads and the resulted phenomenon of eutrophication. Bioelectrochemical reactors (BERs) for simultaneous nitrogen and carbon removal have gained attention owing to their cost efficiency and versatility, as well as the possibility of electrochemical enrich specific groups. This study presented a scalable two-chamber BERs using graphite granules as electrode material. BERs were inoculated and operated for 37 days using natural seawater with high concentrations of ammonium and acetate. The BERs demonstrated a maximum current density of 0.9 A m-3 and removal rates of 7.5 mg NH4+-N L-1 d-1 and 99.5 mg L-1 d-1 for total organic carbon (TOC). Removals observed for NH4+-N and TOC were 96.2% and 68.7%, respectively. The results of nutrient removal (i.e., ammonium, nitrate, nitrite and TOC) and microbial characterization (i.e., next-generation sequencing of the 16S rRNA gene and fluorescence in situ hybridization) showed that BERs operated with a poised cathode at -260 mV (vs. Ag/AgCl) significantly enriched nitrifying microorganisms in the anode and denitrifying microorganisms and planctomycetes in the cathode. Interestingly, the electrochemical enrichment did not increase the total number of microorganisms in the formed biofilms but controlled their composition. Thus, this work shows the first successful attempt to electrochemically enrich marine nitrifying and denitrifying microorganisms and presents a technique to accelerate the start-up process of BERs to remove dissolved inorganic nitrogen and total organic carbon from seawater.

Specific Localization and Quantification of the Oligo-Mouse-Microbiota (OMM12 ) by Fluorescence In Situ Hybridization (FISH).

The oligo-mouse-microbiota (OMM12 ) is a widely used syncom that colonizes gnotobiotic mice in a stable manner. It provides several fundamental functions to its murine host, including colonization resistance against enteric pathogens. Here, we designed and validated specific fluorescence in situ hybridization (FISH) probes to detect and quantify OMM12 strains on intestinal tissue cross sections. 16S rRNA-specific probes were designed, and specificity was validated on fixed pure cultures. A hybridization protocol was optimized for sensitive detection of the individual bacterial cells in cryosections. Using this method, we showed that the intestinal mucosal niche of Akkermansia muciniphila can be influenced by global gut microbial community context. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol: Localization and quantification of OMM12 single strains in mouse cecum cross section Support Protocol: Establishment of specific FISH probe set for OMM12 syncom.

A nitrite-oxidising bacterium constitutively consumes atmospheric hydrogen.

Chemolithoautotrophic nitrite-oxidising bacteria (NOB) of the genus Nitrospira contribute to nitrification in diverse natural environments and engineered systems. Nitrospira are thought to be well-adapted to substrate limitation owing to their high affinity for nitrite and capacity to use alternative energy sources. Here, we demonstrate that the canonical nitrite oxidiser Nitrospira moscoviensis oxidises hydrogen (H2) below atmospheric levels using a high-affinity group 2a nickel-iron hydrogenase [Km(app) = 32 nM]. Atmospheric H2 oxidation occurred under both nitrite-replete and nitrite-deplete conditions, suggesting low-potential electrons derived from H2 oxidation promote nitrite-dependent growth and enable survival during nitrite limitation. Proteomic analyses confirmed the hydrogenase was abundant under both conditions and indicated extensive metabolic changes occur to reduce energy expenditure and growth under nitrite-deplete conditions. Thermodynamic modelling revealed that H2 oxidation theoretically generates higher power yield than nitrite oxidation at low substrate concentrations and significantly contributes to growth at elevated nitrite concentrations. Collectively, this study suggests atmospheric H2 oxidation enhances the growth and survival of NOB amid variability of nitrite supply, extends the phenomenon of atmospheric H2 oxidation to an eighth phylum (Nitrospirota), and reveals unexpected new links between the global hydrogen and nitrogen cycles. Long classified as obligate nitrite oxidisers, our findings suggest H2 may primarily support growth and survival of certain NOB in natural environments.

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

Nitrification, 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 cultivated phylogenetic lineages were determined using microrespirometry. Members of the genus Nitrosocosmicus have the lowest affinity for both ammonia and total ammonium of any characterized AOA, and these values are similar to previously determined ammonia and total ammonium 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 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.

Nitrogen Kinetic Isotope Effects of Nitrification by the Complete Ammonia Oxidizer Nitrospira inopinata.

Analysis of nitrogen isotope fractionation effects is useful for tracing biogeochemical nitrogen cycle processes. Nitrification can cause large nitrogen isotope effects through the enzymatic oxidation of ammonia (NH3) via nitrite (NO2-) to nitrate (NO3-) (15εNH4+→NO2- and 15εNO2-→NO3-). The isotope effects of ammonia-oxidizing bacteria (AOB) and archaea (AOA) and of nitrite-oxidizing bacteria (NOB) have been analyzed previously. Here, we studied the nitrogen isotope effects of the complete ammonia oxidizer (comammox) Nitrospira inopinata that oxidizes NH3 to NO3-. At high ammonium (NH4+) availability (1 mM) and pH between 6.5 and 8.5, its 15εNH4+→NO2- ranged from -33.1 to -27.1‰ based on substrate consumption (residual substrate isotopic composition) and -35.5 to -31.2‰ based on product formation (cumulative product isotopic composition), while the 15εNO2-→NO3- ranged from 6.5 to 11.1‰ based on substrate consumption. These values resemble isotope effects of AOB and AOA and of NOB in the genus Nitrospira, suggesting the absence of fundamental mechanistic differences between key enzymes for ammonia and nitrite oxidation in comammox and canonical nitrifiers. However, ambient pH and initial NH4+ concentrations influenced the isotope effects in N. inopinata. The 15εNH4+→NO2- based on product formation was smaller at pH 6.5 (-31.2‰) compared to pH 7.5 (-35.5‰) and pH 8.5 (-34.9‰), while 15εNO2-→NO3- was smaller at pH 8.5 (6.5‰) compared to pH 7.5 (8.8‰) and pH 6.5 (11.1‰). Isotopic fractionation via 15εNH4+→NO2- and 15εNO2-→NO3- was smaller at 0.1 mM NH4+ compared to 0.5 to 1.0 mM NH4+. Environmental factors, such as pH and NH4+ availability, therefore need to be considered when using isotope effects in 15N isotope fractionation models of nitrification. IMPORTANCE Nitrification is an important nitrogen cycle process in terrestrial and aquatic environments. The discovery of comammox has changed the view that canonical AOA, AOB, and NOB are the only chemolithoautotrophic organisms catalyzing nitrification. However, the contribution of comammox to nitrification in environmental and technical systems is far from being completely understood. This study revealed that, despite a phylogenetically distinct enzymatic repertoire for ammonia oxidation, nitrogen isotope effects of 15εNH4+→NO2- and 15εNO2-→NO3- in comammox do not differ significantly from those of canonical nitrifiers. Thus, nitrogen isotope effects are not suitable indicators to decipher the contribution of comammox to nitrification in environmental samples. Moreover, this is the first systematic study showing that the ambient pH and NH4+ concentration influence the isotope effects of nitrifiers. Hence, these key parameters should be considered in comparative analyses of isotope effects of nitrifiers across different growth conditions and environmental samples.

Sustained nitrogen loss in a symbiotic association of Comammox Nitrospira and Anammox bacteria.

The discovery of anaerobic ammonia-oxidizing bacteria (Anammox) and, more recently, aerobic bacteria common in many natural and engineered systems that oxidize ammonia completely to nitrate (Comammox) have significantly altered our understanding of the global nitrogen cycle. A high affinity for ammonia (Km(app),NH3 ≈ 63nM) and oxygen place Comammox Nitrospira inopinata, the first described isolate, in the same trophic category as organisms such as some ammonia-oxidizing archaea. However, N. inopinata has a relatively low affinity for nitrite (Km,NO2 ≈ 449.2µM) suggesting it would be less competitive for nitrite than other nitrite-consuming aerobes and anaerobes. We examined the ecological relevance of the disparate substrate affinities by coupling it with the Anammox bacterium Candidatus Brocadia anammoxidans. Synthetic communities of the two were established in hydrogel granules in which Comammox grew in the aerobic outer layer to provide Anammox with nitrite in the inner anoxic core to form dinitrogen gas. This spatial organization was confirmed with FISH imaging, supporting a mutualistic or commensal relationship. The functional significance of interspecies spatial organization was informed by the hydrogel encapsulation format, broadening our limited understanding of the interplay between these two species. The resulting low nitrate formation and the competitiveness of Comammox over other aerobic ammonia- and nitrite-oxidizers sets this ecological cooperation apart and points to potential biotechnological applications. Since nitrate is an undesirable product of wastewater treatment effluents, the Comammox-Anammox symbiosis may be of economic and ecological importance to reduce nitrogen contamination of receiving waters.

Genomic and kinetic analysis of novel Nitrospinae enriched by cell sorting.

Chemolithoautotrophic nitrite-oxidizing bacteria (NOB) are key players in global nitrogen and carbon cycling. Members of the phylum Nitrospinae are the most abundant, known NOB in the oceans. To date, only two closely affiliated Nitrospinae species have been isolated, which are only distantly related to the environmentally abundant uncultured Nitrospinae clades. Here, we applied live cell sorting, activity screening, and subcultivation on marine nitrite-oxidizing enrichments to obtain novel marine Nitrospinae. Two binary cultures were obtained, each containing one Nitrospinae strain and one alphaproteobacterial heterotroph. The Nitrospinae strains represent two new genera, and one strain is more closely related to environmentally abundant Nitrospinae than previously cultured NOB. With an apparent half-saturation constant of 8.7 ± 2.5 µM, this strain has the highest affinity for nitrite among characterized marine NOB, while the other strain (16.2 ± 1.6 µM) and Nitrospina gracilis (20.1 ± 2.1 µM) displayed slightly lower nitrite affinities. The new strains and N. gracilis share core metabolic pathways for nitrite oxidation and CO2 fixation but differ remarkably in their genomic repertoires of terminal oxidases, use of organic N sources, alternative energy metabolisms, osmotic stress and phage defense. The new strains, tentatively named "Candidatus Nitrohelix vancouverensis" and "Candidatus Nitronauta litoralis", shed light on the niche differentiation and potential ecological roles of Nitrospinae.

A refined set of rRNA-targeted oligonucleotide probes for in situ detection and quantification of ammonia-oxidizing bacteria.

Ammonia-oxidizing bacteria (AOB) of the betaproteobacterial genera Nitrosomonas and Nitrosospira are key nitrifying microorganisms in many natural and engineered ecosystems. Since many AOB remain uncultured, fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes has been one of the most widely used approaches to study the community composition, abundance, and other features of AOB directly in environmental samples. However, the established and widely used AOB-specific 16S rRNA-targeted FISH probes were designed up to two decades ago, based on much smaller rRNA gene sequence datasets than available today. Several of these probes cover their target AOB lineages incompletely and suffer from a weak target specificity, which causes cross-hybridization of probes that should detect different AOB lineages. Here, a set of new highly specific 16S rRNA-targeted oligonucleotide probes was developed and experimentally evaluated that complements the existing probes and enables the specific detection and differentiation of the known, major phylogenetic clusters of betaproteobacterial AOB. The new probes were successfully applied to visualize and quantify AOB in activated sludge and biofilm samples from seven pilot- and full-scale wastewater treatment systems. Based on its improved target group coverage and specificity, the refined probe set will facilitate future in situ analyses of AOB.

Exploring the upper pH limits of nitrite oxidation: diversity, ecophysiology, and adaptive traits of haloalkalitolerant Nitrospira.

Nitrite-oxidizing bacteria of the genus Nitrospira are key players of the biogeochemical nitrogen cycle. However, little is known about their occurrence and survival strategies in extreme pH environments. Here, we report on the discovery of physiologically versatile, haloalkalitolerant Nitrospira that drive nitrite oxidation at exceptionally high pH. Nitrospira distribution, diversity, and ecophysiology were studied in hypo- and subsaline (1.3-12.8 g salt/l), highly alkaline (pH 8.9-10.3) lakes by amplicon sequencing, metagenomics, and cultivation-based approaches. Surprisingly, not only were Nitrospira populations detected, but they were also considerably diverse with presence of members from  Nitrospira lineages I, II and IV. Furthermore, the ability of Nitrospira enrichment cultures to oxidize nitrite at neutral to highly alkaline pH of 10.5 was demonstrated. Metagenomic analysis of a newly enriched Nitrospira lineage IV species, "Candidatus Nitrospira alkalitolerans", revealed numerous adaptive features of this organism to its extreme environment. Among them were a sodium-dependent N-type ATPase and NADH:quinone oxidoreductase next to the proton-driven forms usually found in Nitrospira. Other functions aid in pH and cation homeostasis and osmotic stress defense. "Ca. Nitrospira alkalitolerans" also possesses group 2a and 3b [NiFe] hydrogenases, suggesting it can use hydrogen as alternative energy source. These results reveal how Nitrospira cope with strongly fluctuating pH and salinity conditions and expand our knowledge of nitrogen cycling in extreme habitats.

Activity and Metabolic Versatility of Complete Ammonia Oxidizers in Full-Scale Wastewater Treatment Systems.

The recent discovery of complete ammonia oxidizers (comammox) contradicts the paradigm that chemolithoautotrophic nitrification is always catalyzed by two different microorganisms. However, our knowledge of the survival strategies of comammox in complex ecosystems, such as full-scale wastewater treatment plants (WWTPs), remains limited. Analyses of genomes and in situ transcriptomes of four comammox organisms from two full-scale WWTPs revealed that comammox were active and showed a surprisingly high metabolic versatility. A gene cluster for the utilization of urea and a gene encoding cyanase suggest that comammox may use diverse organic nitrogen compounds in addition to free ammonia as the substrates. The comammox organisms also encoded the genomic potential for multiple alternative energy metabolisms, including respiration with hydrogen, formate, and sulfite as electron donors. Pathways for the biosynthesis and degradation of polyphosphate, glycogen, and polyhydroxyalkanoates as intracellular storage compounds likely help comammox survive unfavorable conditions and facilitate switches between lifestyles in fluctuating environments. One of the comammox strains acquired from the anaerobic tank encoded and transcribed genes involved in homoacetate fermentation or in the utilization of exogenous acetate, both pathways being unexpected in a nitrifying bacterium. Surprisingly, this strain also encoded a respiratory nitrate reductase which has not yet been found in any other Nitrospira genome and might confer a selective advantage to this strain over other Nitrospira strains in anoxic conditions.IMPORTANCE The discovery of comammox in the genus Nitrospira changes our perception of nitrification. However, genomes of comammox organisms have not been acquired from full-scale WWTPs, and very little is known about their survival strategies and potential metabolisms in complex wastewater treatment systems. Here, four comammox metagenome-assembled genomes and metatranscriptomic data sets were retrieved from two full-scale WWTPs. Their impressive and-among nitrifiers-unsurpassed ecophysiological versatility could make comammox Nitrospira an interesting target for optimizing nitrification in current and future bioreactor configurations.

Single cell analyses reveal contrasting life strategies of the two main nitrifiers in the ocean.

Nitrification, the oxidation of ammonia via nitrite to nitrate, is a key process in marine nitrogen (N) cycling. Although oceanic ammonia and nitrite oxidation are balanced, ammonia-oxidizing archaea (AOA) vastly outnumber the main nitrite oxidizers, the bacterial Nitrospinae. The ecophysiological reasons for this discrepancy in abundance are unclear. Here, we compare substrate utilization and growth of Nitrospinae to AOA in the Gulf of Mexico. Based on our results, more than half of the Nitrospinae cellular N-demand is met by the organic-N compounds urea and cyanate, while AOA mainly assimilate ammonium. Nitrospinae have, under in situ conditions, around four-times higher biomass yield and five-times higher growth rates than AOA, despite their ten-fold lower abundance. Our combined results indicate that differences in mortality between Nitrospinae and AOA, rather than thermodynamics, biomass yield and cell size, determine the abundances of these main marine nitrifiers. Furthermore, there is no need to invoke yet undiscovered, abundant nitrite oxidizers to explain nitrification rates in the ocean.