Genome sequence of the chemolithoautotrophic nitrite-oxidizing bacterium Nitrobacter winogradskyi Nb-255

Influencing factors on the activity of an enriched Nitrospira culture with granular morphology

Nitrospira is a common genus of nitrite-oxidising bacteria (NOB) found in wastewater treatment plants (WWTPs). To identify the key factors influencing the composition of NOB communities, research was conducted using both sequencing batch reactor (SBR) and continuous flow reactor under different conditions. High-throughput 16S rRNA gene sequencing revealed that Nitrospira (18.79% in R1 and 25.77% in R3) was the dominant NOB under low dissolved oxygen (DO) and low nitrite (NO 2 - -N) concentrations, while Nitrobacter (21.26% in R2) was the dominant NOB under high DO and high NO 2 - -N concentrations. Flocculent and granule sludge were cultivated with Nitrospira as the dominant genus. Compared to Nitrospira flocculent sludge, Nitrospira granule sludge had higher inhibition threshold concentrations for free ammonia (FA) and free nitrous acid (FNA). It was more likely to resist adverse environmental disturbances. Furthermore, the effects of environmental factors such as temperature, pH, and DO on the activity of Nitrospira granular sludge were also studied. The results showed that the optimum temperature and pH for Nitrospira granular sludge were 36°C and 7.0, respectively. Additionally, Nitrospira granular sludge showed a higher dissolved oxygen half-saturation constant (Ko) of 3.67 ± 0.71 mg/L due to its morphological characteristics. However, the majority of WWTPs conditions do not meet the conditions for the Nitrospira granular sludge. Thus, it can be speculated that future development of aerobic partial nitrification granular sludge may automatically eliminate the influence of Nitrospira. This study provides a theoretical basis for a deeper understanding of Nitrospira and the development of future water treatment processes.

A positive contribution to nitrogen removal by a novel NOB in a full-scale duck wastewater treatment system

Nitrite-oxidizing bacteria (NOB) are undesirable in the anaerobic ammonium oxidation (anammox)-driven nitrogen removal technologies in the modern wastewater treatment plants (WWTPs). Diverse strategies have been developed to suppress NOB based on their physiological properties that we have understood. But our knowledge of the diversity and mechanisms employed by NOB for survival in the modern WWTPs remains limited. Here, Three NOB species (NOB01-03) were recovered from the metagenomic datasets of a full-scale WWTP treating duck breeding wastewater. Among them, NOB01 and NOB02 were classified as newly identified lineage VII, tentatively named Candidatus (Ca.) Nitrospira NOB01 and Ca. Nitrospira NOB02. Analyses of genomes and in situ transcriptomes revealed that these two novel NOB were active and showed a high metabolic versatility. The transcriptional activity of Ca. Nitrospira could be detected in all tanks with quite different dissolved oxygen (DO) (0.01-5.01 mg/L), illustrating Ca. Nitrospira can survive in fluctuating DO conditions. The much lower Ca. Nitrospira abundance on the anammox bacteria-enriched sponge carrier likely originated from the intensification substrate (NO2 -) competition from anammox and denitrifying bacteria. In particular, a highlight is that Ca. Nitrospira encoded and treanscribed cyanate hydratase (CynS), amine oxidase, urease (UreC), and copper-containing nitrite reductase (NirK) related to ammonium and NO production, driving NOB to interact with the co-existed AOB and anammox bacteria. Ca. Nitrospira strains NOB01 and NOB02 showed quite different niche preference in the same aerobic tank, which dominanted the NOB communities in activated sludge and biofilm, respectively. In addition to the common rTCA cycle for CO2 fixation, a reductive glycine pathway (RGP) was encoded and transcribed by NOB02 likely for CO2 fixation purpose. Additionally, a 3b group hydrogenase and respiratory nitrate reductase were uniquely encoded and transcribed by NOB02, which likely confer a survival advantage to this strain in the fluctuant activated sludge niche. The discovery of this new genus significantly broadens our understanding of the ecophysiology of NOB. Furthermore, the impressive metabolic versatility of the novel NOB revealed in this study advances our understanding of the survival strategy of NOB and provides valuable insight for suppressing NOB in the anammox-based WWTP.

Whole transcriptome analysis highlights nutrient limitation of nitrogen cycle bacteria in simulated microgravity

Regenerative life support systems (RLSS) will play a vital role in achieving self-sufficiency during long-distance space travel. Urine conversion into a liquid nitrate-based fertilizer is a key process in most RLSS. This study describes the effects of simulated microgravity (SMG) on Comamonas testosteroni, Nitrosomonas europaea, Nitrobacter winogradskyi and a tripartite culture of the three, in the context of nitrogen recovery for the Micro-Ecological Life Support System Alternative (MELiSSA). Rotary cell culture systems (RCCS) and random positioning machines (RPM) were used as SMG analogues. The transcriptional responses of the cultures were elucidated. For CO2-producing C. testosteroni and the tripartite culture, a PermaLifeTM PL-70 cell culture bag mounted on an in-house 3D-printed holder was applied to eliminate air bubble formation during SMG cultivation. Gene expression changes indicated that the fluid dynamics in SMG caused nutrient and O2 limitation. Genes involved in urea hydrolysis and nitrification were minimally affected, while denitrification-related gene expression was increased. The findings highlight potential challenges for nitrogen recovery in space.

Complex evolutionary history of photosynthesis in Bradyrhizobium

Bradyrhizobium comprises a diverse group of bacteria with various lifestyles. Although best known for their nodule-based nitrogen-fixation in symbiosis with legumes, a select group of bradyrhizobia are also capable of photosynthesis. This ability seems to be rare among rhizobia, and its origin and evolution in these bacteria remain a subject of substantial debate. Therefore, our aim here was to investigate the distribution and evolution of photosynthesis in Bradyrhizobium using comparative genomics and representative genomes from closely related taxa in the families Nitrobacteraceae, Methylobacteriaceae, Boseaceae and Paracoccaceae . We identified photosynthesis gene clusters (PGCs) in 25 genomes belonging to three different Bradyrhizobium lineages, notably the so-called Photosynthetic, B. japonicum and B. elkanii supergroups. Also, two different PGC architectures were observed. One of these, PGC1, was present in genomes from the Photosynthetic supergroup and in three genomes from a species in the B. japonicum supergroup. The second cluster, PGC2, was also present in some strains from the B. japonicum supergroup, as well as in those from the B. elkanii supergroup. PGC2 was largely syntenic to the cluster found in Rhodopseudomonas palustris and Tardiphaga . Bayesian ancestral state reconstruction unambiguously showed that the ancestor of Bradyrhizobium lacked a PGC and that it was acquired horizontally by various lineages. Maximum-likelihood phylogenetic analyses of individual photosynthesis genes also suggested multiple acquisitions through horizontal gene transfer, followed by vertical inheritance and gene losses within the different lineages. Overall, our findings add to the existing body of knowledge on Bradyrhizobium ’s evolution and provide a meaningful basis from which to explore how these PGCs and the photosynthesis itself impact the physiology and ecology of these bacteria.

High nitrite-nitrogen stress intensity drives nitrite anaerobic oxidation to nitrate and inhibits methanogenesis

Nitrite is an important intermediate in nitrogen metabolism. We explored the effect of nitrite-nitrogen stress intensity (NNSI) on nitrite metabolism and methanogenesis in anaerobic digestion. The results showed that the NNSI regulated microbial diversity, composition, and functions, and microbial community assembly was primarily shaped by stochastic processes. Moreover, the NNSI was negatively correlated with α-diversity and positively correlated with non-metric multi-dimensional scaling distance. Denitrification gradually increased with increasing NNSI; however, methanogenesis was gradually inhibited, which was primarily due to the inhibition of the aceticlastic methanogenesis pathway (i.e., Methanosaeta) and methylotrophic methanogenesis pathway (i.e., Candidatus_Methanofastidiosum). High NNSI (1882 ± 98.99 mg/L NO₂^--N) promoted nitrite anaerobic oxidation to nitrate and was favorable for dissimilatory nitrate reduction to ammonia (DNRA). We present evidence for the microbial transformation of nitrite under anaerobic conditions, with potential geochemical and evolutionary importance. As nitrogen oxides were already present on early Earth, our finding presents the possibility of a nitrogen cycle before the evolution of oxygenic photosynthesis.

Rapid detection of nitrite based on nitrite-oxidizing bacteria biosensor and its application in surface water monitoring

Timely and sensitive detection of nitrite is of great significance for human health protection and water pollution treatment. However, many biosensors can only determine the comprehensive toxicity of the water, and there are few electroactive biofilm (EAB) sensors for the specific detection of pollutants. Biofilms formed by bacteria with specific functions can improve the specificity of nitrite identification by biosensors. This study developed a novel, rapidly responding, high sensitivity (958.6 μAμM^-1cm^-2), wide detection range and anti-interference electrochemical biosensor based on electroactive nitrite-oxidizing bacteria. The biosensor could accurately detect nitrite in the range of 0.3-100 mg/L within 3 min by the cyclic voltammetry (CV) method. The bioelectrode could perform stable detection of nitrite over 200 cycles. The specificity of the biosensor for detecting nitrite was demonstrated by the presence of nitrite oxidizing bacteria (NOB) and nitrite oxidase enzyme (NXR) on the electrode biofilm. The biosensor performed well in wetlands and rivers, with an RSD <14.8% in the detection of nitrite at low concentrations, and further revealed the nitrification occurrence. Our study provided a feasible way for the development of a highly sensitive, rapidly responding and stable electrochemical biosensor, which also exhibited potential applications for long-term detection of nitrite and assessment of ecological function in surface water (rivers, lakes, wetlands, marshes, etc.).

Differential Responses of the Catalytic Efficiency of Ammonia and Nitrite Oxidation to Changes in Temperature

Microbially mediated nitrification plays an important role in the nitrogen (N) cycle, and rates of activity have been shown to change significantly with temperature. Despite this, the substrate affinities of nitrifying bacteria and archaea have not been comprehensively measured and are often assumed to be static in mathematical models of environmental systems. In this study, we measured the oxidation kinetics of ammonia- (NH3) oxidizing archaea (AOA), NH3-oxidizing bacteria (AOB), and two distinct groups of nitrite (NO2 -)-oxidizing bacteria (NOB), of the genera Nitrobacter and Nitrospira, by measuring the maximum rates of apparent activity (V max(app)), the apparent half-saturation constant (K m(app)), and the overall catalytic efficiency (V max(app) /K m(app)) over a range of temperatures. Changes in V max(app) and K m(app) with temperature were different between groups, with V max(app) and catalytic efficiency increasing with temperature in AOA, while V max(app) , K m(app), and catalytic efficiency increased in AOB. In Nitrobacter NOB, V max(app) and K m(app) increased, but catalytic efficiency decreased significantly with temperature. Nitrospira NOB were variable, but V max(app) increased while catalytic efficiency and K m(app) remained relatively unchanged. Michaelis-Menten (MM) and Haldane (H) kinetic models of NH3 oxidation and NO2 - oxidation based on the collected data correctly predict nitrification potential in some soil incubation experiments, but not others. Despite previous observations of coupled nitrification in many natural systems, our results demonstrate significant differences in response to temperature strategies between the different groups of nitrifiers; and indicate the need to further investigate the response of nitrifiers to environmental changes.

Enriched microbial communities for ammonium and nitrite removal from recirculating aquaculture systems

The aim of this study was the enrichment of high-performance microbial communities in biofilters for removal of ammonium and nitrite from aquaculture water. Ammonium oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) were enriched from different environmental water samples. The microbial communities with higher ammonium and nitrite removal activity were selected and adapted to different temperatures [9 °C, 15 °C, room temperature (25 °C), and 30 °C]. The expression of genes involved in nitrification including ammonia monooxygenase (AMO) and nitrite oxidoreductase (NXR) were measured in temperature-adapted AOB and NOB microbiomes. The microbial species present in the selected microbiomes were identified via 16s rRNA sequencing. The microbial communities containing Nitrosomonas oligotropha and Nitrobacter winogradskyi showed the highest ammonium and nitrite removal activity at all temperatures used for adaptation. Furthermore, the microbial communities do not contain any pathogenic bacteria. They also exhibited the highest expression of AMO and NXR genes. Using the enriched microbial communities, we achieved a 288% and 181% improvement in ammonium and nitrite removal over the commonly used communities in biofilters at 9 °C, respectively. These results suggest that the selected microbiomes allowed for a significant improvement of water quality in a recirculating aquaculture system (RAS).

Genome Streamlining, Proteorhodopsin, and Organic Nitrogen Metabolism in Freshwater Nitrifiers

Microbial nitrification is a critical process governing nitrogen availability in aquatic systems. Freshwater nitrifiers have received little attention, leaving many unanswered questions about their taxonomic distribution, functional potential, and ecological interactions. Here, we reconstructed genomes to infer the metabolism and ecology of free-living picoplanktonic nitrifiers across the Laurentian Great Lakes, a connected series of five of Earth’s largest lakes. Surprisingly, ammonia-oxidizing bacteria (AOB) related to Nitrosospira dominated over ammonia-oxidizing archaea (AOA) at nearly all stations, with distinct ecotypes prevailing in the transparent, oligotrophic upper lakes compared to Lakes Erie and Ontario. Unexpectedly, one ecotype of Nitrosospira encodes proteorhodopsin, which could enhance survival under conditions where ammonia oxidation is inhibited or substrate limited. Nitrite-oxidizing bacteria (NOB) “Candidatus Nitrotoga” and Nitrospira fluctuated in dominance, with the latter prevailing in deeper, less-productive basins. Genome reconstructions reveal highly reduced genomes and features consistent with genome streamlining, along with diverse adaptations to sunlight and oxidative stress and widespread capacity for organic nitrogen use. Our findings expand the known functional diversity of nitrifiers and establish their ecological genomics in large lake ecosystems. By elucidating links between microbial biodiversity and biogeochemical cycling, our work also informs ecosystem models of the Laurentian Great Lakes, a critical freshwater resource experiencing rapid environmental change.

deepNEC: a novel alignment-free tool for the identification and classification of nitrogen biochemical network-related enzymes using deep learning

Nitrogen is essential for life and its transformations are an important part of the global biogeochemical cycle. Being an essential nutrient, nitrogen exists in a range of oxidation states from +5 (nitrate) to -3 (ammonium and amino-nitrogen), and its oxidation and reduction reactions catalyzed by microbial enzymes determine its environmental fate. The functional annotation of the genes encoding the core nitrogen network enzymes has a broad range of applications in metagenomics, agriculture, wastewater treatment and industrial biotechnology. This study developed an alignment-free computational approach to determine the predicted nitrogen biochemical network-related enzymes from the sequence itself. We propose deepNEC, a novel end-to-end feature selection and classification model training approach for nitrogen biochemical network-related enzyme prediction. The algorithm was developed using Deep Learning, a class of machine learning algorithms that uses multiple layers to extract higher-level features from the raw input data. The derived protein sequence is used as an input, extracting sequential and convolutional features from raw encoded protein sequences based on classification rather than traditional alignment-based methods for enzyme prediction. Two large datasets of protein sequences, enzymes and non-enzymes were used to train the models with protein sequence features like amino acid composition, dipeptide composition (DPC), conformation transition and distribution, normalized Moreau-Broto (NMBroto), conjoint and quasi order, etc. The k-fold cross-validation and independent testing were performed to validate our model training. deepNEC uses a four-tier approach for prediction; in the first phase, it will predict a query sequence as enzyme or non-enzyme; in the second phase, it will further predict and classify enzymes into nitrogen biochemical network-related enzymes or non-nitrogen metabolism enzymes; in the third phase, it classifies predicted enzymes into nine nitrogen metabolism classes; and in the fourth phase, it predicts the enzyme commission number out of 20 classes for nitrogen metabolism. Among all, the DPC + NMBroto hybrid feature gave the best prediction performance (accuracy of 96.15% in k-fold training and 93.43% in independent testing) with an Matthews correlation coefficient (0.92 training and 0.87 independent testing) in phase I; phase II (accuracy of 99.71% in k-fold training and 98.30% in independent testing); phase III (overall accuracy of 99.03% in k-fold training and 98.98% in independent testing); phase IV (overall accuracy of 99.05% in k-fold training and 98.18% in independent testing), the DPC feature gave the best prediction performance. We have also implemented a homology-based method to remove false negatives. All the models have been implemented on a web server (prediction tool), which is freely available at http://bioinfo.usu.edu/deepNEC/.

Determination of 15N/14N of Ammonium, Nitrite, Nitrate, Hydroxylamine, and Hydrazine Using Colorimetric Reagents and Matrix-Assisted Laser Desorption Ionization-Time-of-Flight Mass Spectrometry (MALDI-TOF MS)

In the nitrogen (N) cycle, nitrogenous compounds are chemically and biologically converted to various aqueous and gaseous N species. The ¹⁵N-labeling approach is a powerful culture-dependent technique to obtain insights into the complex nitrogen transformation reactions that occur in cultures. In the ¹⁵N-labeling approach, the fates of supplemented ¹⁵N- and/or unlabeled gaseous and aqueous compounds are tracked by mass spectrometry (MS) analysis, whereas MS analysis of aqueous N species requires laborious sample preparation steps and is performed using isotope-ratio mass spectrometry, which requires an expensive mass spectrometer. We developed a simple and high-throughput MS method for determining the ¹⁵N atoms percent of NH₄⁺, NO₂^-, NO₃^-, NH₂OH, and N₂H₄, where liquid samples (<0.5 mL) were mixed with colorimetric reagents (naphthylethylenediamine for NO₂^-, indophenol for NH₄⁺, and p-aminobenzaldehyde for N₂H₄), and the mass spectra of the formed N complex dyes were obtained by matrix-assisted laser desorption ionization-time-of-flight (MALDI-TOF) MS. NH₂OH and NO₃^- were chemically converted to NO₂^- by iodine oxidation and copper/hydrazine reduction reaction, respectively, prior to the above colorimetric reaction. The intensity of the isotope peak (M + 1 or M + 2) increased when the N complex dye was formed by coupling with a ¹⁵N-labeled compound, and a linear relationship was found between the determined ¹⁵N/¹⁴N peak ratio and ¹⁵N atom% for the tested N species. The developed method was applied to bacterial cultures to examine their N-transformation reactions, enabling us to observe the occurrence of NO₂^- oxidation and NO₃^- reduction in a hypoxic Nitrobacter winogradskyi culture. IMPORTANCE ¹⁵N/¹⁴N analysis for aqueous N species is a powerful tool for obtaining insights into the global N cycle, but the procedure is cumbersome and laborious. The combined use of colorimetric reagents and MALDI-TOF MS, designated color MALDI-TOF MS, enabled us to determine the ¹⁵N atom% of common aqueous N species without laborious sample preparation and chromatographic separation steps; for instance, the ¹⁵N atom% of NO₂^- can be determined from >1,000 liquid samples daily at <$1 (U.S.) per 384 samples for routine analysis. This convenient MS method is a powerful tool that will advance our ability to explore the N-transformation reactions that occur in various environments and biological samples.

Metaproteomics, Heterotrophic Growth, and Distribution of Nitrosomonas europaea and Nitrobacter winogradskyi after Long-Term Operation of an Autotrophic Nitrifying Biofilm Reactor

Bioregenerative life support systems (BLSS) are currently in development to tackle low recovery efficiencies, high energy demands, as well as food, water, and oxygen production challenges through the regeneration of nutrients from waste streams. The MELiSSA pilot plant has been developed as a testbed for regenerative life support system bioreactor operation and characterization. As nitrogen is a vital resource in such systems, we studied the functional composition of a new packed-bed nitrifying bioreactor inoculated with a co-culture of Nitrosomonas europaea (ATCC 25978) and Nitrobacter winogradskyi (ATCC 25391). After 840 days of autotrophic continuous cultivation, the packed-bed was sampled at five vertical positions, each with three horizontal positions, and the biomass at each position was characterized via qPCR, 16S amplicon sequencing, and liquid chromatography tandem mass spectrometry. The total number of cells within the different sections fluctuated around 8.95 ± 5.10 × 107 cells/mL of beads. Based on 16S amplicons and protein content, N. europaea and N. winogradskyi constituted overall 44.07 ± 11.75% and 57.53 ± 12.04% of the nitrifying bioreactor, respectively, indicating the presence of a heterotrophic population that, even after such a long operation time, did not affect the nitrification function of the bioreactor. In addition, DNA-based abundance estimates showed that N. europaea was slightly more abundant than N. winogradskyi, whereas protein-based abundance estimates indicated a much higher abundance of N. europaea. This highlights that single-method approaches need to be carefully interpreted in terms of overall cell abundance and metabolic activity.

Treatment of municipal wastewater by employing membrane bioreactors combined with efficient nitration microbial communities isolated by Isolation Chip with Plate Streaking technology

In this research, we developed a method so-called Isolation Chip with Plate Streaking (ICPS) to selectively enrich nitrifying microbial consortium for treating municipal wastewater. In batch experiment, these bacterial communities were able to remove NH₃ -N in 72 h with an efficiency of 96%. Firmicutes, Bacteroidetes, and Proteobacteria species are dominant bacteria in these communities. When the bacterial communities were used in the membrane bioreactor under typical condition, the removal efficiency was 81.0%. In contrast, under the actual wastewater condition, the efficiency could reach 91.2%. All above results showed clearly that the consortium selected by our ICPS method could achieve high-efficient NH₃ -N removal, thus offering a reliable technique for screening functional microorganisms in the field of water treatment. PRACTITIONER POINTS: ICPS technology was designed and used for screening specialized NH₃ -N-removing isolates. The screening process benefited the growth of the dominant nitrifying bacteria Firmicutes and Bacteroidetes. When the functional bacteria applied into the MBR, the NH₃ -N removal efficiency was 91.2% under actual wastewater conditions.

Development of Nitrogen Recycling Strategies for Bioregenerative Life Support Systems in Space

To enable long-distance space travel, the development of a highly efficient and robust system to recover nutrients from waste streams is imperative. The inability of the current physicochemical-based environmental control and life support system (ECLSS) on the ISS to produce food in situ and to recover water and oxygen at high enough efficiencies results in the need for frequent resupply missions from Earth. Therefore, alternative strategies like biologically-based technologies called bioregenerative life support systems (BLSSs) are in development. These systems aim to combine biological and physicochemical processes, which enable in situ water, oxygen, and food production (through the highly efficient recovery of minerals from waste streams). Hence, minimalizing the need for external consumables. One of the BLSS initiatives is the European Space Agency's (ESA) Micro-Ecological Life Support System Alternative (MELiSSA). It has been designed as a five-compartment bioengineered system able to produce fresh food and oxygen and to recycle water. As such, it could sustain the needs of a human crew for long-term space exploration missions. A prerequisite for the self-sufficient nature of MELiSSA is the highly efficient recovery of valuable minerals from waste streams. The produced nutrients can be used as a fertilizer for food production. In this review, we discuss the need to shift from the ECLSS to a BLSS, provide a summary of past and current BLSS programs and their unique approaches to nitrogen recovery and processing of urine waste streams. In addition, compartment III of the MELiSSA loop, which is responsible for nitrogen recovery, is reviewed in-depth. Finally, past, current, and future related ground and space demonstration and the space-related challenges for this technology are considered.

Selective enrichment of comammox from activated sludge using antibiotics

While the ubiquitous presence of comammox in engineered systems provides the foundation of developing a novel biological nitrogen removal process, factors contributing to the comammox dynamics in engineered systems have not been well resolved. Here, we investigate the long-term effects of ten different antibiotics on microbial community dynamics in activated sludge and the results show that both types and concentrations of antibiotics affect the taxonomic composition of nitrifiers, including comammox, ammonia-oxidizing bacteria, and canonical nitrite-oxidizing bacteria. Specifically, phylogenetically different comammox Nitrospira were selectively enriched by four types of antibiotics (i.e., ampicillin, kanamycin, lincomycin, and trimethoprim). Comparative genomic analysis of the four newly identified comammox clade A Nitrospira revealed that the comammox enriched by antibiotics shared the conserved key metabolic potentials, such as carbon fixation, complete ammonia oxidation, and utilization of hydrogen as alternative electron donors, among the known comammox organisms. Comammox strains enriched in this study also encoded genes involved in formate and cyanate metabolism that were recently reported in comammox clade A organisms from wastewater treatment systems. Our findings highlight that the comammox in activated sludge ecosystems possess high metabolic versatility than previously recognized and could be selectively enriched by some antibiotics.

Biological nitrification inhibition in the rhizosphere: determining interactions and impact on microbially mediated processes and potential applications

Nitrification is the microbial conversion of reduced forms of nitrogen (N) to nitrate (NO3-), and in fertilized soils it can lead to substantial N losses via NO3- leaching or nitrous oxide (N2O) production. To limit such problems, synthetic nitrification inhibitors have been applied but their performance differs between soils. In recent years, there has been an increasing interest in the occurrence of biological nitrification inhibition (BNI), a natural phenomenon according to which certain plants can inhibit nitrification through the release of active compounds in root exudates. Here, we synthesize the current state of research but also unravel knowledge gaps in the field. The nitrification process is discussed considering recent discoveries in genomics, biochemistry and ecology of nitrifiers. Secondly, we focus on the 'where' and 'how' of BNI. The N transformations and their interconnections as they occur in, and are affected by, the rhizosphere, are also discussed. The NH4+ and NO3- retention pathways alternative to BNI are reviewed as well. We also provide hypotheses on how plant compounds with putative BNI ability can reach their targets inside the cell and inhibit ammonia oxidation. Finally, we discuss a set of techniques that can be successfully applied to solve unresearched questions in BNI studies.

Light as a Novel Inhibitor of Nitrite-Oxidizing Bacteria (NOB) for the Mainstream Partial Nitrification of Wastewater Treatment

Conventional biological nutrient removal processes in municipal wastewater treatment plants are energy-consuming, with oxygen supply accounting for 45–75% of the energy expenditure. Many recent studies examined the implications of the anammox process in sidestream wastewater treatment to reduce energy consumption, however, the process did not successfully remove nitrogen in mainstream wastewater treatment with relatively low ammonia concentrations. In this study, blue light was applied as an inhibitor of nitrite-oxidizing bacteria (NOB) in a photo sequencing batch reactor (PSBR) containing raw wastewater. This simulated a biological nitrogen removal system for the investigation of its application potential in nitrite accumulation and nitrogen removal. It was found that blue light illumination effectively inhibited NOB rather than ammonia-oxidizing bacteria due to their different sensitivity to light, resulting in partial nitrification. It was also observed that the NOB inhibition rates were affected by other operational parameters like mixed liquor suspended solids (MLSS) concentration and sludge retention time (SRT). According to the obtained results, it was concluded that the process efficiency of partial nitrification and anammox (PN/A) could be significantly enhanced by blue light illumination with appropriate MLSS concentration and SRT conditions.

Function of quorum sensing and cell signaling in wastewater treatment systems

Quorum sensing (QS) is a communication mode between microorganisms to regulate bacteria ecological relations and physiological behaviors, thus achieve the physiological function that single bacteria cannot complete. This phenomenon plays important roles in the formation of biofilm and granular sludge, and may be related to enhancement of some functional bacteria activity in wastewater treatment systems. There is a need to better understand bacterial QS in engineered reactors, and to assess how designs and operations might improve the removal efficiency. This article reviewed the recent advances of QS in several environmental systems and mainly analyzed the regulation mechanism of QS-based strategies for biofilm, granular sludge, functional bacteria, and biofouling control. The co-existences of multiple signal molecules in wastewater treatment (WWT) processes were also summarized, which provide basis for the future research on the QS mechanism of multiple signal molecules' interaction in WWT. This review would present some prospects and suggestions which are of practical significance for further application.

Effects of Spartina alterniflora Invasion on Soil Microbial Community Structure and Ecological Functions

It has been reported that the invasion of Spartina alterniflora changed the soil microbial community in the mangrove ecosystem in China, especially the bacterial community, although the response of soil fungal communities and soil microbial ecological functions to the invasion of Spartina alterniflora remains unclear. In this study, we selected three different communities (i.e., Spartina alterniflora community (SC), Spartina alterniflora-mangrove mixed community (TC), and mangrove community (MC)) in the Zhangjiangkou Mangrove Nature Reserve in China. High-throughput sequencing technology was used to analyze the impact of Spartina alterniflora invasion on mangrove soil microbial communities. Our results indicate that the invasion of Spartina alterniflora does not cause significant changes in microbial diversity, but it can alter the community structure of soil bacteria. The results of the LEfSe (LDA Effect Size) analysis show that the relative abundance of some bacterial taxa is not significantly different between the MC and SC communities, but different changes have occurred during the invasion process (i.e., TC community). Different from the results of the bacterial community, the invasion of Spartina alterniflora only cause a significant increase in few fungal taxa during the invasion process, and these taxa are at some lower levels (such as family, genus, and species) and classified into the phylum Ascomycota. Although the invasion of Spartina alterniflora changes the taxa with certain ecological functions, it may not change the potential ecological functions of soil microorganisms (i.e., the potential metabolic pathways of bacteria, nutritional patterns, and fungal associations). In general, the invasion of Spartina alterniflora changes the community structure of soil microorganisms, but it may not affect the potential ecological functions of soil microorganisms.

Physiological and genomic characterization of a new 'Candidatus Nitrotoga' isolate

Oxidation of nitrite to nitrate is an important process in the global nitrogen cycle. Recent molecular biology-based studies have revealed that the widespread nitrite-oxidizing bacteria (NOB) belonging to the genus 'Candidatus Nitrotoga' may be highly important for the environment. However, the insufficient availability of pure Nitrotoga cultures has limited our understanding of their physiological and genomic characteristics. Here, we isolated the 'Ca. Nitrotoga' sp. strain AM1P, from a previously enriched Nitrotoga culture, using an improved isolation strategy. Although 'Ca. Nitrotoga' have been recognized as cold-adapted NOB, the strain AM1P had a slightly higher optimum growth temperature at 23°C. Strain AM1P showed a pH optimum of 8.3 and was not inhibited even at high nitrite concentrations (20 mM). We obtained the complete genome of the strain and compared the genome profile to five previously sequenced 'Ca. Nitrotoga' strains. Comparative genomics suggested that lactate dehydrogenase may be only encoded in the strain AM1P and closely related genomes. While the growth yield of AM1P did not change, we observed faster growth in the presence of lactate in comparison to purely chemolithoautotrophic growth. The characterization of the new strain AM1P sheds light on the physiological adaptation of this environmentally important, but understudied genus 'Ca. Nitrotoga'.

Change of abundance and correlation of Nitrospira inopinata-like comammox and populations in nitrogen cycle during different seasons

Complete-nitrifying bacteria (comammox) play important roles in nitrogen-overloading aquatic systems. However, the understanding of the environmental relevance is still limited. Here, we studied the responses of comammox bacteria (Nitrospira inopinata) in a tributary of the Yellow River, with the water and sediment, microbial, seasonal, and chemical variations considered. Illumina sequencing indicated that the predominant phyla in the river sediment were Proterobacteria, Bacteroidetes, Actinobacteria, and Chloroflex. Quantitative PCR revealed that N. inopinata-like comammox were approximately twice as abundant in the water during the wet season and in the sediment during the dry season than that of other conditions. Significant correlations were found between the abundance of N. inopinata-like comammox and pH (r = 0.58), temperature (r = 0.63), and dissolved oxygen (r = - 0.77). The abundance of N. inopinata-like comammox was higher than that of ammonia oxidizing archaea (AOA), and lower than that of ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB). Furthermore, a significant correlation was discovered between N. inopinata-like comammox and NOB (r = 0.60), and so was anammox bacteria (r = 0.358). Interestingly, N. inopinata-like comammox also showed positive relationships with denitrifying microbes (r = 0.559).

Dissolved oxygen/free ammonia (DO/FA) ratio manipulation to gain distinct proportions of nitrogen species in effluent of entrapped-cell-based reactors

Oxygen-limiting and/or free ammonia (FA)-accumulating conditions are two common operating strategies for partial nitrification in wastewater. Controlling either bulk dissolved oxygen (DO) or free ammonia (FA) concentration to maintain partial nitrification can be challenging due to the strong interdependency between these two parameters as substrates for ammonia oxidation. In this study, DO/FA ratio is proposed as a controlling parameter for partial nitrification by entrapped-cell-based reactors. At DO/FA >1.5, both ammonia and nitrite oxidation proceeded without inhibition leading to complete oxidation of ammonia to nitrate. An effluent containing nitrate as the main nitrogen species can be produced at these ratios. At a DO/FA ratio range of 0.2-1.5, ammonia oxidation proceeded without efficiency deterioration, while nitrite oxidation decreased with decreasing DO/FA ratio. At the ratios of 0.2-0.6, an effluent containing mainly nitrite can be generated. At DO/FA <0.2, both ammonia oxidation and nitrite oxidation were inhibited and the effluent with nearly equal molar of ammonia and nitrite was obtained. By controlling DO/FA ratio, effluents with different proportions of nitrogen species can be produced allowing the entrapped-cell-based system to be adaptable as an initial reactor for various nitrogen removal approaches.

Achieving high nitrogen removal efficiency by optimizing nitrite-dependent anaerobic methane oxidation process with growth factors

Nitrite-dependent anaerobic methane oxidation (N-DAMO) is a newly discovered bioprocess which uses methane as electron donor to reduce nitrite into dinitrogen. It is a promising clean bioprocess for denitrification in wastewater treatment. However, the low reaction rate and slow growth rate of N-DAMO bacteria within NC10 phylum limit the application of the process. In this study, we chose vitamin, heme, nucleobase and betaine to investigate their short- and long-term effects on N-DAMO bacteria. The concentrations of the growth factors of medium were improved according to the short-term experiments. The results were subsequently verified via long-term inoculations and were applied in a magnetically stirred gas lift reactor (MSGLR). The results indicated that nucleobase and betaine (5.0 and 200 μg L^-1, respectively) significantly stimulated the N-DAMO activity, whereas vitamin and heme had no significant effects in the tested concentration ranges. During the long-term incubation, N-DAMO bacteria continuously increased and finally achieved a relative abundance of 14.4% on day 300. Notably, larger aggregates of N-DAMO bacteria were observed at the end of the long-term incubation. And the nitrogen removal rate of the MSGLR increased to 70 mg N L^-1 day^-1, with the total nitrogen removal efficiency over 99.0%. However, the addition of betaine introduced methyl into the reactors and this made methylotrophs account a considerable part of the bacterial community, which limited the enrichment degree of N-DAMO bacteria. This work will contribute to the engineering application and enrichment of N-DAMO bacteria.

Cultivation and Transcriptional Analysis of a Canonical Nitrospira Under Stable Growth Conditions

Nitrite-oxidizing bacteria (NOB) are vital players in the global nitrogen cycle that convert nitrite to nitrate during the second step of nitrification. Within this functional guild, members of the genus Nitrospira are most widespread, phylogenetically diverse, and physiologically versatile, and they drive nitrite oxidation in many natural and engineered ecosystems. Despite their ecological and biotechnological importance, our understanding of their energy metabolism is still limited. A major bottleneck for a detailed biochemical characterization of Nitrospira is biomass production, since they are slow-growing and fastidious microorganisms. In this study, we cultivated Nitrospira moscoviensis under nitrite-oxidizing conditions in a continuous stirred tank reactor (CSTR) system. This cultivation setup enabled accurate control of physicochemical parameters and avoided fluctuating levels of their energy substrate nitrite, thus ensuring constant growth conditions and furthermore allowing continuous biomass harvesting. Transcriptomic analyses under these conditions supported the predicted core metabolism of N. moscoviensis, including expression of all proteins required for carbon fixation via the reductive tricarboxylic acid cycle, assimilatory nitrite reduction, and the complete respiratory chain. Here, simultaneous expression of multiple copies of respiratory complexes I and III suggested functional differentiation. The transcriptome also indicated that the previously assumed membrane-bound nitrite oxidoreductase (NXR), the enzyme catalyzing nitrite oxidation, is formed by three soluble subunits. Overall, the transcriptomic data greatly refined our understanding of the metabolism of Nitrospira. Moreover, the application of a CSTR to cultivate Nitrospira is an important foundation for future proteomic and biochemical characterizations, which are crucial for a better understanding of these fascinating microorganisms.

Indications for enzymatic denitrification to N2O at low pH in an ammonia-oxidizing archaeon

Nitrous oxide (N₂O) is a key climate change gas and nitrifying microbes living in terrestrial ecosystems contribute significantly to its formation. Many soils are acidic and global change will cause acidification of aquatic and terrestrial ecosystems, but the effect of decreasing pH on N₂O formation by nitrifiers is poorly understood. Here, we used isotope-ratio mass spectrometry to investigate the effect of acidification on production of N₂O by pure cultures of two ammonia-oxidizing archaea (AOA; Nitrosocosmicus oleophilus and Nitrosotenuis chungbukensis) and an ammonia-oxidizing bacterium (AOB; Nitrosomonas europaea). For all three strains acidification led to increased emission of N₂O. However, changes of ¹⁵N site preference (SP) values within the N₂O molecule (as indicators of pathways for N₂O formation), caused by decreasing pH, were highly different between the tested AOA and AOB. While acidification decreased the SP value in the AOB strain, SP values increased to a maximum value of 29‰ in N. oleophilus. In addition, ¹⁵N-nitrite tracer experiments showed that acidification boosted nitrite transformation into N₂O in all strains, but the incorporation rate was different for each ammonia oxidizer. Unexpectedly, for N. oleophilus more than 50% of the N₂O produced at pH 5.5 had both nitrogen atoms from nitrite and we demonstrated that under these conditions expression of a putative cytochrome P450 NO reductase is strongly upregulated. Collectively, our results indicate that N. oleophilus might be able to enzymatically denitrify nitrite to N₂O at low pH.

Cobalt-dependent inhibition of nitrite oxidation in Nitrobacter winogradskyi

Nitrobacter winogradskyi is an abundant, intensively studied autotrophic nitrite-oxidizing bacterium, which is frequently used as a model strain in the two-step nitrification of ammonia (NH₃) to nitrate (NO₃^-) via nitrite (NO₂^-), either in activated sludge, agricultural field studies or more recently in artificial microbial consortia for organic hydroponics. We observed a hitherto unknown cobalt ion-dependent inhibition of cell growth and NO₂^- oxidation activity of N. winogradskyi in a mineral medium, which strongly depended on accompanying Ca²⁺ and Mg²⁺ concentrations. This inhibition was bacteriostatic, but susceptible to natural chelators. l-Histidine effectively restored cell growth and NO₂^- oxidation activity of N. winogradskyi in mineral media containing Co²⁺ with >90% recovery. Our results suggest that Co²⁺ competed with alkaline earth metals during uptake and that its toxicity was significantly reduced by complexation.

Nitrogen cycling during wastewater treatment

Many wastewater treatment plants in the world do not remove reactive nitrogen from wastewater prior to release into the environment. Excess reactive nitrogen not only has a negative impact on human health, it also contributes to air and water pollution, and can cause complex ecosystems to collapse. In order to avoid the deleterious effects of excess reactive nitrogen in the environment, tertiary wastewater treatment practices that ensure the removal of reactive nitrogen species need to be implemented. Many wastewater treatment facilities rely on chemicals for tertiary treatment, however, biological nitrogen removal practices are much more environmentally friendly and cost effective. Therefore, interest in biological treatment is increasing. Biological approaches take advantage of specific groups of microorganisms involved in nitrogen cycling to remove reactive nitrogen from reactor systems by converting ammonia to nitrogen gas. Organisms known to be involved in this process include autotrophic ammonia-oxidizing bacteria, heterotrophic ammonia-oxidizing bacteria, ammonia-oxidizing archaea, anaerobic ammonia oxidizing bacteria (anammox), nitrite-oxidizing bacteria, complete ammonia oxidizers, and dissimilatory nitrate reducing microorganisms. For example, in nitrifying-denitrifying reactors, ammonia- and nitrite-oxidizing bacteria convert ammonia to nitrate and then denitrifying microorganisms reduce nitrate to nonreactive dinitrogen gas. Other nitrogen removal systems (anammox reactors) take advantage of anammox bacteria to convert ammonia to nitrogen gas using NO as an oxidant. A number of promising new biological treatment technologies are emerging and it is hoped that as the cost of these practices goes down more wastewater treatment plants will start to include a tertiary treatment step.

Metatranscriptomic Investigation of Adaptation in NO and N2O Production From a Lab-Scale Nitrification Process Upon Repeated Exposure to Anoxic-Aerobic Cycling

The molecular mechanisms of microbial adaptation to repeated anoxic-aerobic cycling were investigated by integrating whole community gene expression (metatranscriptomics) and physiological responses, including the production of nitric (NO) and nitrous (N2O) oxides. Anoxic-aerobic cycling was imposed for 17 days in a lab-scale full-nitrification mixed culture system. Prior to cycling, NO and N2O levels were sustained at 0.097 ± 0.006 and 0.054 ± 0.019 ppmv, respectively. Once the anoxic-aerobic cycling was initiated, peak emissions were highest on the first day (9.8 and 1.3 ppmv, respectively). By the end of day 17, NO production returned to pre-cycling levels (a peak of 0.12 ± 0.007 ppmv), while N2O production reached a new baseline (a peak of 0.32 ± 0.05 ppmv), one order of magnitude higher than steady-state conditions. Concurrently, post-cycling transcription of norBQ and nosZ returned to pre-cycling levels after an initial 5.7- and 9.5-fold increase, while nirK remained significantly expressed (1.6-fold) for the duration of and after cycling conditions. The imbalance in nirK and nosZ mRNA abundance coupled with continuous conversion of NO to N2O might explain the elevated post-cycling baseline for N2O. Metatranscriptomic investigation notably indicated possible NO production by NOB under anoxic-aerobic cycling through a significant increase in nirK expression. Opposing effects on AOB (down-regulation) and NOB (up-regulation) CO2 fixation were observed, suggesting that nitrifying bacteria are differently impacted by anoxic-aerobic cycling. Genes encoding the terminal oxidase of the electron transport chain (ccoNP, coxBC) were the most significantly transcribed, highlighting a hitherto unexplored pathway to manage high electron fluxes resulting from increased ammonia oxidation rates, and leading to overall, increased NO and N2O production. In sum, this study identified underlying metabolic processes and mechanisms contributing to NO and N2O production through a systems-level interrogation, which revealed the differential ability of specific microbial groups to adapt to sustained operational conditions in engineered biological nitrogen removal processes.

Genomic profiling of four cultivated Candidatus Nitrotoga spp. predicts broad metabolic potential and environmental distribution

Nitrite-oxidizing bacteria (NOB) play a critical role in the mitigation of nitrogen pollution by metabolizing nitrite to nitrate, which is removed via assimilation, denitrification, or anammox. Recent studies showed that NOB are phylogenetically and metabolically diverse, yet most of our knowledge of NOB comes from only a few cultured representatives. Using cultivation and genomic sequencing, we identified four putative Candidatus Nitrotoga NOB species from freshwater sediments and water column samples in Colorado, USA. Genome analyses indicated highly conserved 16S rRNA gene sequences, but broad metabolic potential including genes for nitrogen, sulfur, hydrogen, and organic carbon metabolism. Genomic predictions suggested that Ca. Nitrotoga can metabolize in low oxygen or anoxic conditions, which may support an expanded environmental niche for Ca. Nitrotoga similar to other NOB. An array of antibiotic and metal resistance genes likely allows Ca. Nitrotoga to withstand environmental pressures in impacted systems. Phylogenetic analyses highlighted a deeply divergent nitrite oxidoreductase alpha subunit (NxrA), suggesting a novel evolutionary trajectory for Ca. Nitrotoga separate from any other NOB and further revealing the complex evolutionary history of nitrite oxidation in the bacterial domain. Ca. Nitrotoga-like 16S rRNA gene sequences were prevalent in globally distributed environments over a range of reported temperatures. This work considerably expands our knowledge of the Ca. Nitrotoga genus and suggests that their contribution to nitrogen cycling should be considered alongside other NOB in wide variety of habitats.

Metabolic and Proteomic Responses to Salinity in Synthetic Nitrifying Communities of Nitrosomonas spp. and Nitrobacter spp

Typically, nitrification is a two-stage microbial process and is key in wastewater treatment and nutrient recovery from waste streams. Changes in salinity represent a major stress factor that can trigger response mechanisms, impacting the activity and the physiology of bacteria. Despite its pivotal biotechnological role, little information is available on the specific response of nitrifying bacteria to varying levels of salinity. In this study, synthetic communities of ammonia-oxidizing bacteria (AOB Nitrosomonas europaea and/or Nitrosomonas ureae) and nitrite-oxidizing bacteria (NOB Nitrobacter winogradskyi and/or Nitrobacter vulgaris) were tested at 5, 10, and 30 mS cm^-1 by adding sodium chloride to the mineral medium (0, 40, and 200 mM NaCl, respectively). Ammonia oxidation activity was less affected by salinity than nitrite oxidation. AOB, on their own or in combination with NOB, showed no significant difference in the ammonia oxidation rate among the three conditions. However, N. winogradskyi improved the absolute ammonia oxidation rate of both N. europaea and N. ureae. N. winogradskyi’s nitrite oxidation rate decreased to 42% residual activity upon exposure to 30 mS cm^-1, also showing a similar behavior when tested with Nitrosomonas spp. The nitrite oxidation rate of N. vulgaris, as a single species, was not affected when adding sodium chloride up to 30 mS cm^-1, however, its activity was completely inhibited when combined with Nitrosomonas spp. in the presence of ammonium/ammonia. The proteomic analysis of a co-culture of N. europaea and N. winogradskyi revealed the production of osmolytes, regulation of cell permeability and an oxidative stress response in N. europaea and an oxidative stress response in N. winogradskyi, as a result of increasing the salt concentration from 5 to 30 mS cm^-1. A specific metabolic response observed in N. europaea suggests the role of carbon metabolism in the production of reducing power, possibly to meet the energy demands of the stress response mechanisms, induced by high salinity. For the first time, metabolic modifications and response mechanisms caused by the exposure to salinity were described, serving as a tool toward controllability and predictability of nitrifying systems exposed to salt fluctuations.

Characterization of the First "Candidatus Nitrotoga" Isolate Reveals Metabolic Versatility and Separate Evolution of Widespread Nitrite-Oxidizing Bacteria

Nitrification is a key process of the biogeochemical nitrogen cycle and of biological wastewater treatment. The second step, nitrite oxidation to nitrate, is catalyzed by phylogenetically diverse, chemolithoautotrophic nitrite-oxidizing bacteria (NOB). Uncultured NOB from the genus "Candidatus Nitrotoga" are widespread in natural and engineered ecosystems. Knowledge about their biology is sparse, because no genomic information and no pure "Ca Nitrotoga" culture was available. Here we obtained the first "Ca Nitrotoga" isolate from activated sludge. This organism, "Candidatus Nitrotoga fabula," prefers higher temperatures (>20°C; optimum, 24 to 28°C) than previous "Ca Nitrotoga" enrichments, which were described as cold-adapted NOB. "Ca Nitrotoga fabula" also showed an unusually high tolerance to nitrite (activity at 30 mM NO2-) and nitrate (up to 25 mM NO3-). Nitrite oxidation followed Michaelis-Menten kinetics, with an apparent Km (Km(app)) of ~89 µM nitrite and a Vmax of ~28 µmol of nitrite per mg of protein per h. Key metabolic pathways of "Ca Nitrotoga fabula" were reconstructed from the closed genome. "Ca Nitrotoga fabula" possesses a new type of periplasmic nitrite oxidoreductase belonging to a lineage of mostly uncharacterized proteins. This novel enzyme indicates (i) separate evolution of nitrite oxidation in "Ca Nitrotoga" and other NOB, (ii) the possible existence of phylogenetically diverse, unrecognized NOB, and (iii) together with new metagenomic data, the potential existence of nitrite-oxidizing archaea. For carbon fixation, "Ca Nitrotoga fabula" uses the Calvin-Benson-Bassham cycle. It also carries genes encoding complete pathways for hydrogen and sulfite oxidation, suggesting that alternative energy metabolisms enable "Ca Nitrotoga fabula" to survive nitrite depletion and colonize new niches.IMPORTANCE Nitrite-oxidizing bacteria (NOB) are major players in the biogeochemical nitrogen cycle and critical for wastewater treatment. However, most NOB remain uncultured, and their biology is poorly understood. Here, we obtained the first isolate from the environmentally widespread NOB genus "Candidatus Nitrotoga" and performed a detailed physiological and genomic characterization of this organism ("Candidatus Nitrotoga fabula"). Differences between key phenotypic properties of "Ca Nitrotoga fabula" and those of previously enriched "Ca Nitrotoga" members reveal an unexpectedly broad range of physiological adaptations in this genus. Moreover, genes encoding components of energy metabolisms outside nitrification suggest that "Ca Nitrotoga" are ecologically more flexible than previously anticipated. The identification of a novel nitrite-oxidizing enzyme in "Ca Nitrotoga fabula" expands our picture of the evolutionary history of nitrification and might lead to discoveries of novel nitrite oxidizers. Altogether, this study provides urgently needed insights into the biology of understudied but environmentally and biotechnologically important microorganisms.

Advances in algal-prokaryotic wastewater treatment: A review of nitrogen transformations, reactor configurations and molecular tools

The synergistic activity of algae and prokaryotic microorganisms can be used to improve the efficiency of biological wastewater treatment, particularly with regards to nitrogen removal. For example, algae can provide oxygen through photosynthesis needed for aerobic degradation of organic carbon and nitrification and harvested algal-prokaryotic biomass can be used to produce high value chemicals or biogas. Algal-prokaryotic consortia have been used to treat wastewater in different types of reactors, including waste stabilization ponds, high rate algal ponds and closed photobioreactors. This review addresses the current literature and identifies research gaps related to the following topics: 1) the complex interactions between algae and prokaryotes in wastewater treatment; 2) advances in bioreactor technologies that can achieve high nitrogen removal efficiencies in small reactor volumes, such as algal-prokaryotic biofilm reactors and enhanced algal-prokaryotic treatment systems (EAPS); 3) molecular tools that have expanded our understanding of the activities of algal and prokaryotic communities in wastewater treatment processes.

Genome-Scale, Constraint-Based Modeling of Nitrogen Oxide Fluxes during Coculture of Nitrosomonas europaea and Nitrobacter winogradskyi

Modern agriculture is sustained by application of inorganic nitrogen (N) fertilizer in the form of ammonium (NH₄⁺). Up to 60% of NH₄⁺-based fertilizer can be lost through leaching of nitrifier-derived nitrate (NO₃⁻), and through the emission of N oxide gases (i.e., nitric oxide [NO], N dioxide [NO₂], and nitrous oxide [N₂O] gases), the latter being a potent greenhouse gas. Our approach to modeling of nitrification suggests that both biotic and abiotic mechanisms function as important sources and sinks of N oxides during microaerobic conditions and that previous models might have underestimated gross NO production during nitrification.

Nitrification, the aerobic oxidation of ammonia to nitrate via nitrite, emits nitrogen (N) oxide gases (NO, NO₂, and N₂O), which are potentially hazardous compounds that contribute to global warming. To better understand the dynamics of nitrification-derived N oxide production, we conducted culturing experiments and used an integrative genome-scale, constraint-based approach to model N oxide gas sources and sinks during complete nitrification in an aerobic coculture of two model nitrifying bacteria, the ammonia-oxidizing bacterium Nitrosomonas europaea and the nitrite-oxidizing bacterium Nitrobacter winogradskyi. The model includes biotic genome-scale metabolic models (iFC578 and iFC579) for each nitrifier and abiotic N oxide reactions. Modeling suggested both biotic and abiotic reactions are important sources and sinks of N oxides, particularly under microaerobic conditions predicted to occur in coculture. In particular, integrative modeling suggested that previous models might have underestimated gross NO production during nitrification due to not taking into account its rapid oxidation in both aqueous and gas phases. The integrative model may be found at https://github.com/chaplenf/microBiome-v2.1.

IMPORTANCE Modern agriculture is sustained by application of inorganic nitrogen (N) fertilizer in the form of ammonium (NH₄⁺). Up to 60% of NH₄⁺-based fertilizer can be lost through leaching of nitrifier-derived nitrate (NO₃⁻), and through the emission of N oxide gases (i.e., nitric oxide [NO], N dioxide [NO₂], and nitrous oxide [N₂O] gases), the latter being a potent greenhouse gas. Our approach to modeling of nitrification suggests that both biotic and abiotic mechanisms function as important sources and sinks of N oxides during microaerobic conditions and that previous models might have underestimated gross NO production during nitrification.

Physiological and Metagenomic Characterizations of the Synergistic Relationships between Ammonia- and Nitrite-Oxidizing Bacteria in Freshwater Nitrification

Nitrification plays a crucial role in global nitrogen cycling and treatment processes. However, the relationships between the nitrifier guilds of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) are still poorly understood, especially in freshwater habitats. This study examined the physiological interactions between the AOB and NOB present in a freshwater aquarium biofilter by culturing them, either together or separately, in a synthetic medium. Metagenomic and 16S rRNA gene sequencing revealed the presence and the draft genomes of Nitrosomonas-like AOB as well as Nitrobacter-like NOB in the cultures, including the first draft genome of Nitrobacter vulgaris. The nitrifiers exhibited different growth rates with different ammonium (NH₄⁺) or nitrite concentrations (50-1,500 μM) and the growth rates were elevated under a high bicarbonate (HCO₃^-) concentration. The half-saturation constant (K_s for NH₄⁺), the maximum growth rate (μ_max), and the lag duration indicated a strong dependence on the synergistic relationships between the two guilds. Overall, the ecophysiological and metagenomic results in this study provided insights into the phylogeny of the key nitrifying players in a freshwater biofilter and showed that interactions between the two nitrifying guilds in a microbial community enhanced nitrification.

Genomic Analysis of Two Phylogenetically Distinct Nitrospira Species Reveals Their Genomic Plasticity and Functional Diversity

The genus Nitrospira represents a dominant group of nitrite-oxidizing bacteria in natural and engineered ecosystems. This genus is phylogenetically divided into six lineages, for which vast phylogenetic and functional diversity has been revealed by recent molecular ecophysiological analyses. However, the genetic basis underlying these phenotypic differences remains largely unknown because of the lack of genome sequences representing their diversity. To gain a more comprehensive understanding of Nitrospira, we performed genomic comparisons between two Nitrospira strains (ND1 and NJ1 belonging to lineages I and II, respectively) previously isolated from activated sludge. In addition, the genomes of these strains were systematically compared with previously reported six Nitrospira genomes to reveal their similarity and presence/absence of several functional genes/operons. Comparisons of Nitrospira genomes indicated that their genomic diversity reflects phenotypic differences and versatile nitrogen metabolisms. Although most genes involved in key metabolic pathways were conserved between strains ND1 and NJ1, assimilatory nitrite reduction pathways of the two Nitrospira strains were different. In addition, the genomes of both strains contain a phylogenetically different urease locus and we confirmed their ureolytic activity. During gene annotation of strain NJ1, we found a gene cluster encoding a quorum-sensing system. From the enriched supernatant of strain NJ1, we successfully identified seven types of acyl-homoserine lactones with a range of C10–C14. In addition, the genome of strain NJ1 lacks genes relevant to flagella and the clustered regularly interspaced short palindromic repeat (CRISPR)-Cas (CRISPR-associated genes) systems, whereas most nitrifying bacteria including strain ND1 possess these genomic elements. These findings enhance our understanding of genomic plasticity and functional diversity among members of the genus Nitrospira.

Acyl-Homoserine Lactone Production in Nitrifying Bacteria of the Genera Nitrosospira, Nitrobacter, and Nitrospira Identified via a Survey of Putative Quorum-Sensing Genes

The genomes of many bacteria that participate in nitrogen cycling through the process of nitrification contain putative genes associated with acyl-homoserine lactone (AHL) quorum sensing (QS). AHL QS or bacterial cell-cell signaling is a method of bacterial communication and gene regulation and may be involved in nitrogen oxide fluxes or other important phenotypes in nitrifying bacteria. Here, we carried out a broad survey of AHL production in nitrifying bacteria in three steps. First, we analyzed the evolutionary history of AHL synthase and AHL receptor homologs in sequenced genomes and metagenomes of nitrifying bacteria to identify AHL synthase homologs in ammonia-oxidizing bacteria (AOB) of the genus Nitrosospira and nitrite-oxidizing bacteria (NOB) of the genera Nitrococcus, Nitrobacter, and Nitrospira Next, we screened cultures of both AOB and NOB with uncharacterized AHL synthase genes and AHL synthase-negative nitrifiers by a bioassay. Our results suggest that an AHL synthase gene is required for, but does not guarantee, cell density-dependent AHL production under the conditions tested. Finally, we utilized mass spectrometry to identify the AHLs produced by the AOB Nitrosospira multiformis and Nitrosospira briensis and the NOB Nitrobacter vulgaris and Nitrospira moscoviensis as N-decanoyl-l-homoserine lactone (C₁₀-HSL), N-3-hydroxy-tetradecanoyl-l-homoserine lactone (3-OH-C₁₄-HSL), a monounsaturated AHL (C_10:1-HSL), and N-octanoyl-l-homoserine lactone (C₈-HSL), respectively. Our survey expands the list of AHL-producing nitrifiers to include a representative of Nitrospira lineage II and suggests that AHL production is widespread in nitrifying bacteria.IMPORTANCE Nitrification, the aerobic oxidation of ammonia to nitrate via nitrite by nitrifying microorganisms, plays an important role in environmental nitrogen cycling from agricultural fertilization to wastewater treatment. The genomes of many nitrifying bacteria contain genes associated with bacterial cell-cell signaling or quorum sensing (QS). QS is a method of bacterial communication and gene regulation that is well studied in bacterial pathogens, but less is known about QS in environmental systems. Our previous work suggested that QS might be involved in the regulation of nitrogen oxide gas production during nitrite metabolism. This study characterized putative QS signals produced by different genera and species of nitrifiers. Our work lays the foundation for future experiments investigating communication between nitrifying bacteria, the purpose of QS in these microorganisms, and the manipulation of QS during nitrification.

Ecological Energetic Perspectives on Responses of Nitrogen-Transforming Chemolithoautotrophic Microbiota to Changes in the Marine Environment

Transformation and mobilization of bioessential elements in the biosphere, lithosphere, atmosphere, and hydrosphere constitute the Earth’s biogeochemical cycles, which are driven mainly by microorganisms through their energy and material metabolic processes. Without microbial energy harvesting from sources of light and inorganic chemical bonds for autotrophic fixation of inorganic carbon, there would not be sustainable ecosystems in the vast ocean. Although ecological energetics (eco-energetics) has been emphasized as a core aspect of ecosystem analyses and microorganisms largely control the flow of matter and energy in marine ecosystems, marine microbial communities are rarely studied from the eco-energetic perspective. The diverse bioenergetic pathways and eco-energetic strategies of the microorganisms are essentially the outcome of biosphere-geosphere interactions over evolutionary times. The biogeochemical cycles are intimately interconnected with energy fluxes across the biosphere and the capacity of the ocean to fix inorganic carbon is generally constrained by the availability of nutrients and energy. The understanding of how microbial eco-energetic processes influence the structure and function of marine ecosystems and how they interact with the changing environment is thus fundamental to a mechanistic and predictive understanding of the marine carbon and nitrogen cycles and the trends in global change. By using major groups of chemolithoautotrophic microorganisms that participate in the marine nitrogen cycle as examples, this article examines their eco-energetic strategies, contributions to carbon cycling, and putative responses to and impacts on the various global change processes associated with global warming, ocean acidification, eutrophication, deoxygenation, and pollution. We conclude that knowledge gaps remain despite decades of tremendous research efforts. The advent of new techniques may bring the dawn to scientific breakthroughs that necessitate the multidisciplinary combination of eco-energetic, biogeochemical and “omics” studies in this field.

Integrated metabolism in sponge-microbe symbiosis revealed by genome-centered metatranscriptomics

Despite an increased understanding of functions in sponge microbiomes, the interactions among the symbionts and between symbionts and host are not well characterized. Here we reconstructed the metabolic interactions within the sponge Cymbastela concentrica microbiome in the context of functional features of symbiotic diatoms and the host. Three genome bins (CcPhy, CcNi and CcThau) were recovered from metagenomic data of C. concentrica, belonging to the proteobacterial family Phyllobacteriaceae, the Nitrospira genus and the thaumarchaeal order Nitrosopumilales. Gene expression was estimated by mapping C. concentrica metatranscriptomic reads. Our analyses indicated that CcPhy is heterotrophic, while CcNi and CcThau are chemolithoautotrophs. CcPhy expressed many transporters for the acquisition of dissolved organic compounds, likely available through the sponge's filtration activity and symbiotic carbon fixation. Coupled nitrification by CcThau and CcNi was reconstructed, supported by the observed close proximity of the cells in fluorescence in situ hybridization. CcPhy facultative anaerobic respiration and assimilation by diatoms may consume the resulting nitrate. Transcriptional analysis of diatom and sponge functions indicated that these organisms are likely sources of organic compounds, for example, creatine/creatinine and dissolved organic carbon, for other members of the symbiosis. Our results suggest that organic nitrogen compounds, for example, creatine, creatinine, urea and cyanate, fuel the nitrogen cycle within the sponge. This study provides an unprecedented view of the metabolic interactions within sponge-microbe symbiosis, bridging the gap between cell- and community-level knowledge.

Draft Genome Sequence of Nitrobacter vulgaris Strain Ab1, a Nitrite-Oxidizing Bacterium

Here, we present the 3.9-Mb draft genome sequence of Nitrobacter vulgaris strain Ab₁, which was isolated from a sewage system in Hamburg, Germany. The analysis of its genome sequence will contribute to our knowledge of nitrite-oxidizing bacteria and acyl-homoserine lactone quorum sensing in nitrifying bacteria.

Molecular and Kinetic Characterization of Planktonic Nitrospira spp. Selectively Enriched from Activated Sludge

Nitrospira spp. are chemolithoautotrophic nitrite-oxidizing bacteria (NOB), which are ubiquitous in natural and engineered environments. However, there exist few independent biokinetic studies on Nitrospira spp., likely because their isolation and selective enrichment from environmental consortia such as activated sludge can be challenging. Herein, planktonic Nitrospira spp. cultures closely related to Candidatus Nitrospira defluvii (Nitrospira lineage I) were successfully enriched from activated sludge in a sequencing batch reactor by maintaining sustained limiting extant nitrite and dissolved oxygen concentrations. Morphologically, the enrichment consisted largely of planktonic cells with an average characteristic diameter of 1.3 ± 0.6 μm. On the basis of respirometric assays, estimated maximum specific growth rate (μ_max), nitrite half saturation coefficient (K_S), oxygen half saturation coefficient (K_O), and biomass yield coefficient (Y) of the enriched cultures were 0.69 ± 0.10 d^-1, 0.52 ± 0.14 mg-N/L, 0.33 ± 0.14 mg-O₂/L, and 0.14 ± 0.02 mg-COD/mg-N, respectively. These parameters collectively reflect not just higher affinities of this enrichment for nitrite and oxygen, respectively, but also a higher biomass yield and energy transfer efficiency relative to Nitrobacter spp. Used in combination, these kinetic and thermodynamic parameters can help toward the development and application of energy-efficient biological nutrient removal processes through effective Nitrospira out-selection.

Detection and Diversity of the Nitrite Oxidoreductase Alpha Subunit (nxrA) Gene of Nitrospina in Marine Sediments

Nitrite-oxidizing bacteria (NOB) are chemolithoautotrophs that catalyze the oxidation of nitrite to nitrate, which is the second step of aerobic nitrification. In marine ecosystems, Nitrospina is assumed to be a major contributor to nitrification. To date, two strains of Nitrospina have been isolated from marine environments. Despite their ecological relevance, their ecophysiology and environmental distribution are understudied owing to fastidious cultivation techniques and the lack of a sufficient functional gene marker. To estimate the abundance, diversity, and distribution of Nitrospina in various marine sediments, we used nxrA, which encodes the alpha subunit of nitrite oxidoreductase, as a functional and phylogenetic marker. We observed that Nitrospina diversity in polar sediments was significantly lower than that of non-polar samples. Moreover, nxrA-like sequences revealed an unexpected diversity of Nitrospina, with approximately 41,000 different sequences based on a 95% similarity cutoff from six marine sediments. We detected nxrA gene copy numbers of up to 3.57 × 10⁴ per gram of marine sediment sample. The results of this study provide insight into the distribution and diversity of Nitrospina, which is fundamentally important for understanding their contribution to the nitrogen cycle in marine sediments.

Quorum Quenching of Nitrobacter winogradskyi Suggests that Quorum Sensing Regulates Fluxes of Nitrogen Oxide(s) during Nitrification

Quorum sensing (QS) is a widespread process in bacteria used to coordinate gene expression with cell density, diffusion dynamics, and spatial distribution through the production of diffusible chemical signals. To date, most studies on QS have focused on model bacteria that are amenable to genetic manipulation and capable of high growth rates, but many environmentally important bacteria have been overlooked. For example, representatives of proteobacteria that participate in nitrification, the aerobic oxidation of ammonia to nitrate via nitrite, produce QS signals called acyl-homoserine lactones (AHLs). Nitrification emits nitrogen oxide gases (NO, NO₂, and N₂O), which are potentially hazardous compounds that contribute to global warming. Despite considerable interest in nitrification, the purpose of QS in the physiology/ecology of nitrifying bacteria is poorly understood. Through a quorum quenching approach, we investigated the role of QS in a well-studied AHL-producing nitrite oxidizer, Nitrobacter winogradskyi We added a recombinant AiiA lactonase to N. winogradskyi cultures to degrade AHLs to prevent their accumulation and to induce a QS-negative phenotype and then used mRNA sequencing (mRNA-Seq) to identify putative QS-controlled genes. Our transcriptome analysis showed that expression of nirK and nirK cluster genes (ncgABC) increased up to 19.9-fold under QS-proficient conditions (minus active lactonase). These data led to us to query if QS influenced nitrogen oxide gas fluxes in N. winogradskyi Production and consumption of NO_x increased and production of N₂O decreased under QS-proficient conditions. Quorum quenching transcriptome approaches have broad potential to identify QS-controlled genes and phenotypes in organisms that are not genetically tractable.

IMPORTANCE

Bacterial cell-cell signaling, or quorum sensing (QS), is a method of bacterial communication and gene regulation that is well studied in bacteria. However, little is known about the purpose of QS in many environmentally important bacteria. Here, we demonstrate quorum quenching coupled with mRNA-Seq to identify QS-controlled genes and phenotypes in Nitrobacter winogradskyi, a nitrite-oxidizing bacterium. Nitrite oxidizers play an important role in the nitrogen cycle though their participation in nitrification, the aerobic oxidation of ammonia to nitrate via nitrite. Our quorum quenching approach revealed that QS influences production and consumption of environmentally important nitrogen oxide gases (NO, NO₂, and N₂O) in N. winogradskyi This study demonstrated a novel technique for studying QS in difficult-to-work-with microorganisms and showed that nitrite oxidizers might also contribute to nitrification-dependent production of nitrogen oxide gases that contribute to global warming.

SAR11 bacteria linked to ocean anoxia and nitrogen loss

Bacteria of the SAR11 clade constitute up to one half of all microbial cells in the oxygen-rich surface ocean. SAR11 bacteria are also abundant in oxygen minimum zones (OMZs), where oxygen falls below detection and anaerobic microbes have vital roles in converting bioavailable nitrogen to N2 gas. Anaerobic metabolism has not yet been observed in SAR11, and it remains unknown how these bacteria contribute to OMZ biogeochemical cycling. Here, genomic analysis of single cells from the world's largest OMZ revealed previously uncharacterized SAR11 lineages with adaptations for life without oxygen, including genes for respiratory nitrate reductases (Nar). SAR11 nar genes were experimentally verified to encode proteins catalysing the nitrite-producing first step of denitrification and constituted ~40% of OMZ nar transcripts, with transcription peaking in the anoxic zone of maximum nitrate reduction activity. These results link SAR11 to pathways of ocean nitrogen loss, redefining the ecological niche of Earth's most abundant organismal group.