Ammonia oxidizers in river sediments of the Qinghai-Tibet Plateau and their adaptations to high-elevation conditions

Long-neglected contribution of nitrification to N2O emissions in the Yellow River

Rivers play a significant role in the global nitrous oxide (N2O) budget. However, the microbial sources and sinks of N2O in river systems are not well understood or quantified, resulting in the prolonged neglect of nitrification. This study investigated the isotopic signatures of N2O, thereby quantifying the microbial source of N2O production and the degree of N2O reduction in the Yellow River. Although denitrification has long been considered to be the dominant pathway of N2O production in rivers, our findings indicated that denitrification only accounted for 18.3% (8.2%-43.0%) of the total contribution to N2O production in the Yellow River, with 50.2%-80.2% being concurrently reduced. The denitrification contribution to N2O production (R2 = 0.44, p < 0.01) and N2O reduction degree (R2 = 0.70, p < 0.01) were positively related to the dissolved organic carbon (DOC) content. Similar to urban rivers and eutrophic lakes, denitrification was the primary process responsible for N2O production (43.0%) in certain reaches with high organic content (DOC = 5.29 mg/L). Nevertheless, the denitrification activity was generally constrained by the availability of electron donors (average DOC = 2.51 mg/L) throughout the Yellow River basin. Consequently, nitrification emerged as the primary contributor in the well-oxygenated Yellow River. Additionally, our findings further distinguished the respective contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to N2O emissions. Although AOB dominated the N2O production in the Yellow River, the AOA specie abundance (AOA/(AOA + AOB)) contributed up to 32.6%, which resulted in 25.6% of the total nitrifier-produced N2O, suggesting a significant occurrence of AOA in the oligotrophic Yellow River. Overall, this study provided a non-invasive approach for quantifying the microbial sources and sinks to N2O emissions, and demonstrated the substantial role of nitrification in the large oligotrophic rivers.

Unveiling unique microbial nitrogen cycling and nitrification driver in coastal Antarctica

Largely removed from anthropogenic delivery of nitrogen (N), Antarctica has notably low levels of nitrogen. Though our understanding of biological sources of ammonia have been elucidated, the microbial drivers of nitrate (NO3-) cycling in coastal Antarctica remains poorly understood. Here, we explore microbial N cycling in coastal Antarctica, unraveling the biological origin of NO3- via oxygen isotopes in soil and lake sediment, and through the reconstruction of 1968 metagenome-assembled genomes from 29 microbial phyla. Our analysis reveals the metabolic potential for microbial N2 fixation, nitrification, and denitrification, but not for anaerobic ammonium oxidation, signifying a unique microbial N-cycling dynamic. We identify the predominance of complete ammonia oxidizing (comammox) Nitrospira, capable of performing the entire nitrification process. Their adaptive strategies to the Antarctic environment likely include synthesis of trehalose for cold stress, high substrate affinity for resource utilization, and alternate metabolic pathways for nutrient-scarce conditions. We confirm the significant role of comammox Nitrospira in the autotrophic, nitrification process via 13C-DNA-based stable isotope probing. This research highlights the crucial contribution of nitrification to the N budget in coastal Antarctica, identifying comammox Nitrospira clade B as a nitrification driver.

Climate and local environment co-mediate the taxonomic and functional diversity of bacteria and archaea in the Qinghai-Tibet Plateau rivers

Understanding the environmental response patterns of riverine microbiota is essential for predicting the potential impact of future environmental change on river ecosystems. Vulnerable plateau ecosystems are particularly sensitive to climate and local environmental changes, however, the environmental response patterns of the taxonomic and functional diversity of riverine microbiota remain unclear. Here, we conducted a systematic investigation of the taxonomic and functional diversity of bacteria and archaea from riparian soils, sediments, and water across the elevation of 1800- 4800 m in the Qinghai-Tibet Plateau rivers. We found that within the elevation range of 1800 to 3800 m, riparian soils and sediments exhibited similarities and stabilities in microbial taxonomic and functional diversity, and water microbiomes were more sensitive with great fluctuations in microbial diversity. Beyond the elevation of 3800 m, microbial diversity declined across all riverine matrixes. Local environmental conditions can influence the sensitivity of microbiomes to climate change. The combination of critical climate and local environmental factors, including total nitrogen, total organic carbon, as well as climate variables associated with temperature and precipitation, provided better explanations for microbial diversity than single-factor analyses. Under the extremely adverse scenario of high greenhouse gas emission concentrations (SSP585), we anticipate that by the end of this century, the bacterial, archaeal, and microbial functional diversity across the river network of the Yangtze and Yellow source basin would potentially change by -16.9- 5.2 %, -16.1- 5.7 %, and -9.3- 6.4 %, respectively. Overall, climate and local environments jointly shaped the microbial diversity in plateau river ecosystems, and water microbiomes would provide early signs of environmental changes. Our study provides effective theoretical foundations for the conservation of river biodiversity and functional stability under environmental changes.

The abundance of comammox bacteria was higher than that of ammonia-oxidizing archaea and bacteria in rhizosphere of emergent macrophytes in a typical shallow lake riparian

Complete ammonia oxidation (comammox) bacteria can complete the whole nitrification process independently, which not only challenges the classical two-step nitrification theory but also updates long-held perspective of microbial ecological relationship in nitrification process. Although comammox bacteria have been found in many ecosystems in recent years, there is still a lack of research on the comammox process in rhizosphere of emergent macrophytes in lakeshore zone. Sediment samples were collected in this study from rhizosphere, far-rhizosphere, and non-rhizosphere of emergent macrophytes along the shore of Lake Liangzi, a shallow lake. The diversity of comammox bacteria and amoA gene abundance of comammox bacteria, ammonia-oxidizing archaea (AOA), and ammonia-oxidizing bacteria (AOB) in these samples were measured. The results showed that comammox bacteria widely existed in the rhizosphere of emergent macrophytes and fell into clade A.1, clade A.2, and clade B, and clade A was the predominant community in all sampling sites. The abundance of comammox amoA gene (6.52 × 106-2.45 × 108 copies g-1 dry sediment) was higher than that of AOB amoA gene (6.58 × 104-3.58 × 106 copies g-1 dry sediment), and four orders of magnitude higher than that of AOA amoA gene (7.24 × 102-6.89 × 103 copies g-1 dry sediment), suggesting that the rhizosphere of emergent macrophytes is more favorable for the growth of comammox bacteria than that of AOB and AOA. Our study indicated that the comammox bacteria may play important roles in ammonia-oxidizing processes in all different rhizosphere regions.

Nitrogen fertilization rate affects communities of ammonia-oxidizing archaea and bacteria in paddy soils across different climatic zones of China

Nitrogen fertilization has important effects on nitrification. However, how the rate of nitrogen fertilization affects nitrification potential, as well as the communities of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), remains unclear. We performed a large-scale investigation of nitrification potential and ammonia-oxidizer communities in Chinese paddy fields at different nitrogen fertilization rates across different climatic zones. It was found that the nitrification potential at the high nitrogen fertilization rate (≥150 kg-1 N ha-1) was 23.35 % higher than that at the intermediate rate (100-150 kg-1 N ha-1) and 20.77 % higher than that at the low rate (< 100 kg-1 N ha-1). The nitrification potential showed no significant variation among different nitrogen fertilization rates across climatic zones. Furthermore, the AOA and AOB amoA gene abundance at the high nitrogen fertilization rate was 481.67 % and 292.74 % higher (p < 0.05) than that at the intermediate rate, respectively. Correlation analysis demonstrated a significant positive correlation between AOB abundance and nitrification potential. AOA and AOB community composition differed significantly among nitrogen fertilization rates. Moreover, soil NH4+ content, pH, water content, bulk density, and annual average temperature were regarded as key environmental factors influencing the community structure of ammonia-oxidizers. Taken together, the nitrogen fertilization rate had a significant impact on the communities of AOA and AOB but did not significantly alter the nitrification potential. Our findings provide new insights into the impact of nitrogen fertilization management on nitrification in rice paddy fields.

Abundance, community and driving factor of nitrifiers in western China plateau

Complete ammonia oxidation (comammox) is one of the most important biogeochemical processes, with recent studies showing that comammox process dominates nitrification in many ecosystems. However, the abundance, community and driving factor of comammox bacteria and other nitrifying microorganisms in plateau wetland is still unclear. Here, the abundances and community features of comammox bacteria, ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in the wetland sediments of western China plateaus were examined using qPCR and high-throughput sequencing. The results indicate that comammox bacteria were more abundant than AOA and AOB, and dominated the nitrification process. Compared with low-elevation samples (below 3000 m: samples 6-10, 12, 13, 15, 16), the abundance of comammox bacteria was much higher at high-elevation samples (above 3000 m: samples 1-5, 11, 14, 17, 18). The key species of AOA, AOB, and comammox bacteria were Nitrososphaera viennensis, Nitrosomonas europaea, and Nitrospira nitrificans, respectively. The key factor affecting comammox bacteria community was elevation. Elevation could increase the interaction links of key species Nitrospira nitrificans, resulting in high comammox bacterial abundance. The results of this study advance our knowledge of comammox bacteria in natural ecosystems.

Temperature drives elevational diversity patterns of different types of organisms in Qinghai-Tibetan Plateau wetlands

The spatial pattern and driving mechanism of biodiversity along elevational gradients are key topics in ecology. However, it is still unclear whether the multidimensional diversity of different types of organisms shows a similar response to elevation changes. Here, we measured the species and phylogenetic diversity of plants, bacteria, fungi, and microbial functional groups (nitrifiers, denitrifiers, methanogens, and methanotrophs) in 36 wetland sites on the Qinghai-Tibetan Plateau. The results showed that both species and phylogenetic diversity of plants, bacteria, and fungi exhibited a significant elevational gradient, in direct contrast to no significant diversity changes observed for denitrifiers, methanogens, and methanotrophs along the same altitude gradient. Our findings suggest that elevation and temperature were more likely to associate with the diversity of plants, bacteria, and fungi than the diversity of microbial functional groups, with important implications for assessing the effect of ongoing climate warming on biodiversity in Qinghai-Tibetan alpine wetlands.

Anthropogenic land-use activities within watersheds reduce comammox activity and diversity in rivers

Nitrogen cycling plays a key role in maintaining river ecological functions which are threatened by anthropogenic activities. The newly discovered complete ammonia oxidation, comammox, provides novel insights into the ecological effects of nitrogen on that it oxidizes ammonia directly to nitrate without releasing nitrite as canonical ammonia oxidization conducted by AOA or AOB which is believed to play an important role in greenhouse gas generation. Theoretically, contribution of commamox, AOA and AOB to ammonia oxidization in rivers might be impacted by anthropogenic land-use activities through alterations in flow regime and nutrient input. While how land use pattern affects comammox and other canonical ammonia oxidizers remains elusive. In this study, we examined the ecological effects of land use practices on the activity and contribution of three distinctive groups of ammonia oxidizers (AOA, AOB, comammox) as well as the composition of comammox bacterial communities from 15 subbasins covering an area of 6166 km² in North China. The results showed that comammox dominated nitrification (55.71%-81.21%) in less disturbed basins characterized by extensive forests and grassland, while AOB became the major player (53.83%-76.43%) in highly developed basins with drastic urban and agricultural development. In addition, increasing anthropogenic land use activities within the watershed lowered the alpha diversity of comammox communities and simplified the comammox network. Additionally, the alterations of NH₄⁺-N, pH and C/N induced by land use change were found to be crucial drivers in determining the distribution and activity of AOB and comammox. Together, our findings cast a new light on aquatic-terrestrial linkages from the view of microorganism-mediated nitrogen cycling and can further be applied to target watershed land use management.

Community composition and abundance of complete ammonia oxidation (comammox) bacteria in the Lancang River cascade reservoir

The construction of the reservoir has changed the nitrogen migration and transformation processes in the river, and a large amount of sediment deposition in the reservoir may also lead to the spatial differentiation of complete ammonia oxidation (comammox) bacteria. The study investigated the abundance and diversity of comammox bacteria in the sediments of three cascade reservoirs, namely, Xiaowan, Manwan, and Nuozhadu on the Lancang River in China. In these reservoirs, the average amoA gene abundance of clade A and clade B of comammox bacteria, ammonia-oxidizing archaea (AOA), and ammonia-oxidizing bacteria (AOB) was 4.16 ± 0.85 × 10⁵, 1.15 ± 0.33 × 10⁵, 7.39 ± 2.31 × 10⁴, and 3.28 ± 0.99 × 10⁵ copies g^-1, respectively. The abundance of clade A was higher than that of other ammonia oxidizing microorganisms. The spatial variation of comammox bacteria abundance differed among different reservoirs, but the spatial variation trends of the two clades of comammox bacteria in the same reservoir were similar. At each sampling point, clade A1, clade A2, and clade B coexisted, and clade A2 was usually the dominant species. The connection between comammox bacteria in the pre-dam sediments was looser than that in non-pre-dam sediments, and comammox bacteria in pre-dam sediments exhibited a simpler network structure. The main factor affecting comammox bacteria abundance was NH₄⁺-N, while altitude, temperature, and conductivity of overlying water were the main factors affecting comammox bacteria diversity. Environmental changes caused by differences in the spatial distribution of these cascade reservoirs may be the main driver of the changes of community composition and abundance of comammox bacteria. This study confirms that the construction of cascade reservoirs results in niche spatial differentiation of comammox bacteria.

Soil moisture is a primary driver of comammox Nitrospira abundance in New Zealand soils

The objectives of this study were to investigate the abundance and community composition of comammox Nitrospira under: (i) pasture-based dairy farms from different regions, and (ii) different land uses from the same region and soil type. The results clearly showed that comammox Nitrospira were most abundant (3.0 × 10⁶ copies) under the west coast dairy farm conditions, where they were also significantly more abundant than canonical ammonia oxidisers. This was also true in the Canterbury dairy farm. The six land uses investigated were pine monoculture, a long term no input ecological trial, sheep + beef and Dairy, both irrigated and non-irrigated. It was concluded that comammox Nitrospira was most abundant under the irrigated dairy farm (2.7 × 10⁶ copies). Contrary to the current industry opinion, the relatively high abundance of comammox Nitrospira under fertile irrigated dairy land suggests that comammox Nitrospira found in terrestrial ecosystems may be copiotrophic. it was also determined that comammox Nitrospira was more abundant under irrigated land use than their non-irrigated counterparts, suggesting that soil moisture is a key environmental parameter influencing comammox abundance. Comammox abundance was also positively correlated with annual rainfall, further supporting this theory. Phylogenetic analysis of the comammox Nitrospira detected determined that 17 % of the comammox community belonged to a newly distinguished subclade, clade B.2. The remaining 83 % belonged to clade B.1. No sequences from clade A were found.

Nitrifiers Cooperate to Produce Nitrous Oxide in Plateau Wetland Sediments

The thawing of dormant plateau permafrost emits nitrous oxide (N₂O) through wetlands; however, the N₂O production mechanism in plateau wetlands is still unclear. Here, we used the ¹⁵N-¹⁸O double tracer technique and metagenomic sequencing to analyze the N₂O production mechanism in the Yunnan-Kweichow and Qinghai-Tibet plateau wetlands during the summer of 2020. N₂O production activity was detected in all 16 sediment samples (elevation 1020-4601 m: 2.55 ± 0.42-26.38 ± 3.25 ng N g^-1 d^-1) and was promoted by nitrifier denitrification (ND). The key functional genes of ND (amoA, hao, and nirK) belonged to complete ammonia oxidizing (comammox) bacteria, and the key ND species was the comammox bacterium Nitrospira nitrificans. We found that the comammox bacterial species N. nitrificans and the ammonia oxidizing bacterial (AOB) species Nitrosomonas europaea cooperate to produce N₂O in the plateau wetland sediments. Furthermore, we inferred that environmental factors (elevation and total organic matter (TOM)) influence the cooperation pattern via N. nitrificans, thus affecting the N₂O production activity in the plateau wetland sediments. Our findings advance the mechanistic understanding of nitrifiers in biogeochemical cycles and global climate change.

Comammox biogeography subject to anthropogenic interferences along a high-altitude river

The recent discovery of comammox Nitrospira performing complete ammonia oxidation to nitrate has overturned the long-held dogma of two-step nitrification on Earth, yet little is known about the effect of urbanization interference on their distribution. Using gene-centric metagenomics, we provided the first blueprints about comammox community, biogeography, and environmental drivers along a high-elevation (> 2000 m) river flowing through the largest city on the vulnerable Qinghai-Tibetan Plateau. Our study confirmed a wide presence and diversity of yet-uncultured comammox clade B across wet and dry seasons, with average 3.0 and 2.0 times as abundant as clade-A amoA genes in water and sediments, respectively. Species identified from freshwater and drinking water treatment plants dominated the comammox guilds (58∼100%), suggesting this plateau river shared a similar comammox assemblage with the above habitat types. Compared with the urban area harboring more abundant canonical Nitrospira identified in wastewater (average 24%), the upstream suburban reach had a smaller human population but larger proportions of comammox in ammonia-oxidizing prokaryotes (24∼72% of abundances) and Nitrospira sublineages I/II. Higher contents of nitrate and nitrite in water, and antibiotics in water and sediments, may restrain comammox niches in nitrifiers over the urban area. Further random forest analysis revealed that lincosamides and quinolones were the most important antibiotic predictors for the niche differentiations between comammox and canonical nitrifiers in water, while macrolides for those in sediments. Finally, by incubation experiments, we demonstrated higher activity contributions of benthic comammox in the suburban area (36.2∼92.8% of potential ammonia-oxidation rates) than in the urban reach, and that the contribution variation had significant negative relations with macrolides and their major components. Overall, this study highlighted that anthropogenic activities hampered the advantage of riverine complete nitrifiers over the canonical two-step ones.

Diversity distribution and characteristics of comammox in different ecosystems

The discovery of complete ammonia oxidizers (comammox), which can oxidize ammonia into nitrate, has recently changed the concept of traditional nitrification. However, comparative studies on the analysis of comammox microbial community in different ecosystems are still scarce. In this study, the distribution and diversity of the comammox microbial community in farmlands, riparian zones, and river sediments in summer and winter were investigated by high-throughput sequencing. And the relative abundance of ammonia-oxidizing microorganisms was measured via their amoA genes of real-time quantitative polymerase chain reaction (qPCR). The relationships between ammonia oxidation microorganisms and the environmental factors were further analyzed. The abundance of comammox clade A was one order of magnitude lower than that of ammonia-oxidizing archaea (AOA) but higher than that of ammonia-oxidizing bacteria (AOB). The abundance of comammox was higher in summer than in winter and higher in farmland soils (1.81 ± 0.95 × 10⁷ copies g^-1) than in riparian zones and river sediments. Meanwhile, Candidatus Nitrospira nitrosa were the most widespread comammox in most samples (up to 86.31%), followed by Candidatus Nitrospira nitrificans, with a low abundance of Candidatus Nitrospira inopinata (lower than 0.61%). Furthermore, the abundance of comammox clade A had a significantly negative correlation with pH and NH₄⁺ concentration (P < 0.05). The study revealed the potential advantages of comammox in farmlands and may be conducive to further research on comammox in microbial nitrogen cycling.

Time-series monitoring of river hydrochemistry and multiple isotope signals in the Yarlung Tsangpo River reveals a hydrological domination of fluvial nitrate fluxes in the Tibetan Plateau

Nutrient element cycling in the Tibetan Plateau, the highest and largest plateau in the world, is sensitive to anthropogenic disturbances and climate change. Studying the spatiotemporal dynamics of reactive nitrogen (N) - predominantly in the form of nitrate (NO₃^-) - in the plateau is crucial to understand the regional and global N cycles and their feedbacks with climate change. We conducted the first weekly frequency hydro-geochemical monitoring (i.e., discharge, water chemistry, and multiple isotopes) from the upper to the lower reaches of the Yarlung Tsangpo River, the largest river in the plateau, in pronounced wet/dry cycles to reveal the biogeochemical transformations and fluvial fluxes of NO₃^- response to hydrologic condition. Relative stable NO₃^- concentration and significant linear correlations between the fluvial NO₃^- fluxes and the discharge were observed, suggesting that a significant potential NO₃^- source counterbalanced the diluting effects during the rainy season. The negative correlations between δ¹⁵N-NO₃^- and discharge/NO₃^- fluxes suggested that the increasing NO₃^- flux respond to the increasing discharge was mainly from water leaching of ¹⁵N-depleted soil sources, rather than ¹⁵N-enriched sewage. The isotopic mixing model calculation showed that NO₃^- fluxes were largely generated in the relatively densely populated middle reaches (56%), of which 74% were from soil sources. The fluxes of the soil sources showed large seasonal variation and peaked in August, with hydrological condition as the primary driver. Based on the critical findings, we put forward a NO₃^- export conceptual model that integrated anthropogenic and climatic forcings and classified NO₃^- export mechanisms in river basins into transport-limited and generation-limited regimes. In a transport-limited regime that characterized most river basins in the Tibetan Plateau, fluvial NO₃^- flux presented a linearly relationship in response to runoff variation. In contrast, in a generation-limited regime, the flux would be largely dependent on the thermodynamic of nitrification.

Spatial turnover of core and occasional bacterial taxa in the plastisphere from a plateau river, China

Plastic surfaces in the environment are a comparatively new niche for microbial colonization, also known as the "plastisphere". However, our understanding of the core and occasional bacterial taxa in the plastisphere is limited. Here, environmental plastic, water, and sediment samples were collected from 10 sites in a plateau river (Lhasa River, China) in September of 2019. The composition and spatial turnover of core and occasional bacterial taxa in the plastisphere were revealed via 16S rRNA gene sequencing and compared with water and sediment. The results indicated that deterministic processes dominated the habitat specialization that shaped the formation of core and occasional taxa in the plastisphere, water, and sediment of the Lhasa River because the decline in zeta diversity in the plastisphere, water, and sediment was more fitted to a power-law form rather than an exponential form. Proteobacteria (65.9%), Bacteroidetes (16.0%), and Cyanobacteria (11.7%) dominated the plastic core taxa. Core taxa rather than occasional taxa in the plastisphere had a lower (21.7%) proportion of OTUs and a higher (81.7%) proportion of average relative abundance than water and sediment, which were dominant in plastic bacterial communities. The spatial turnover of core and occasional bacterial taxa in the plastisphere was governed by abiotic as well as biotic factors. Specifically, the spatial turnover of core taxa in the plastisphere with high connectivity but low functional redundancy was easily affected by geographical distance, altitude, and heavy metals. Furthermore, strong drug resistance was found in the spatially persistent core taxa in the plastisphere. This study provides empirical support for the spatial turnover (species variation) and potential ecological mechanisms of bacterial communities in the plastisphere from river ecosystems.

Comammox Nitrospira Bacteria Are Dominant Ammonia Oxidizers in Mainstream Nitrification Bioreactors Emended with Sponge Carriers

Complete ammonia oxidation (i.e., comammox) is a newly discovered microbial process performed by a subset of the Nitrospira genus, and this unique microbial process has been ubiquitously detected in various wastewater treatment units. However, the operational conditions favoring comammox prevalence remain unclear. In this study, the dominance of comammox Nitrospira in four sponge biofilm reactors fed with low-strength ammonium (NH₄⁺ = 23 ± 3 mg N/L) wastewater was proved by coupling 16S rRNA gene amplicon sequencing, quantitative polymerase chain reaction (qPCR), and metagenomic sequencing. The results showed that comammox Nitrospira dominated in the nitrifying guild over canonical ammonia-oxidizing bacteria (AOB) constantly, despite the significant variation in the residual ammonium concentration (0.01-15 mg N/L) under different sets of operating conditions. This result indicates that sponge biofilms greatly favor retaining comammox Nitrospira in wastewater treatment and highlights an essential role of biomass retention in the comammox prevalence. Moreover, analyses of the assembled metagenomic sequences revealed that the retrieved amoA gene sequences affiliated with comammox Nitrospira (53.9-66.0% read counts of total amoA gene reads) were always higher than those (28.4-43.4%) related to β-proteobacterial AOB taxa. The comammox Nitrospira bacteria detected in the present biofilm systems were close to clade A Candidatus Nitrospira nitrosa.

Distribution of ammonia oxidizers and their role in N2 O emissions in the reservoir riparian zone

As a transitional boundary between terrestrial and aquatic ecosystems, the riparian zone is considered a hotspot for N₂ O production because of the active nitrogen processes. Ammoxidation is an important microbial pathway for N₂ O production, but the distribution of ammonia oxidizers under different land-use types in the reservoir riparian zone and what role they played in N₂ O emissions are still not clear. We investigated spatiotemporal distributions of ammonia-oxidizing archaea (AOA) and bacteria (AOB) and their role in N₂ O emissions in different land-use types along the riparian zone of Miyun Reservoir: grassland, sparse woods, and woodland. We found significant differences in both AOA abundance and AOB diversity indices among land-use types. AOA and AOB communities were significantly separated by different land-use types. The main drivers to determine the distribution of ammonia-oxidizing microbial community were soil water content, NH₄ ⁺ , NO₃ ^- , and total organic carbon (TOC). In situ N₂ O flux was highest in woodland with a mean value of 12.28 μg/m² ·h, and it was substantially decreased by 121% and 123% in sparse woods and grassland. TOC content was decreased by 20% and 40% in sparse woods and grassland compared with woodland, and it was significantly positively correlated with in situ N₂ O flux. Meanwhile, AOB diversity indices were significantly correlated with in situ N₂ O flux. These results showed that the heterogeneity of physicochemical properties among different land-use types affected the community of AOA and AOB in riparian zones. AOB not AOA, and community diversity rather than abundance, played a role in N₂ O emissions.

Niche differentiation of comammox Nitrospira in sediments of the Three Gorges Reservoir typical tributaries, China

Complete ammonia oxidizer (Comammox) can complete the whole nitrification process independently, whose niche differentiation is important guarantee for its survival and ecological function. This study investigated the niche differentiation of comammox Nitrospira in the sediments of three typical tributaries of the Three Gorges Reservoir (TGR). Clade A and clade B of comammox Nitrospira coexisted in all sampling sites simultaneously. The amoA gene abundance of clade A and B was gradually increased or decreased along the flow path of the three tributaries with obvious spatial differentiation. The amoA gene abundance of comammox Nitrospira clade A (6.36 × 10³ - 5.06 × 10⁴ copies g^-1 dry sediment) was higher than that of clade B (6.26 × 10² - 6.27 × 10³ copies g^-1 dry sediment), and the clade A amoA gene abundance was one order of magnitude higher than that of AOA (7.24 × 10² - 6.89 × 10³ copies g^-1 dry sediment) and AOB (1.44 × 10² - 1.46 × 10³ copies g^-1 dry sediment). A significant positive correlation was observed between comammox Nitrospira clade A amoA gene abundance and flow distance (P < 0.05). The number of operational taxonomic units (OTUs) in two sub-clades of clade A accounted for the majority in different tributaries, indicating that clade A also had population differentiation among different tributaries. This study revealed that comammox Nitrospira in the sediments of TGR tributaries have niche differentiation and clade A.2 played a more crucial role in comammox Nitrospira community.

Saline lakes on the Qinghai-Tibet Plateau harbor unique viral assemblages mediating microbial environmental adaption

The highest plateau on Earth, Qinghai-Tibet Plateau, contains thousands of lakes with broad salinity and diverse and unique microbial communities. However, little is known about their co-occurring viruses. Herein, we identify 4,560 viral Operational Taxonomic Units (vOTUs) from six viromes of three saline lakes on Qinghai-Tibet Plateau, with less than 1% that could be classified. Most of the predicted vOTUs were associated with the dominant bacterial and archaeal phyla. Virus-encoded auxiliary metabolic genes suggest that viruses influence microbial metabolisms of carbon, nitrogen, sulfur, and lipid; the antibiotic resistance mediation; and their salinity adaption. The six viromes clustered together with the ice core viromes and bathypelagic ocean viromes and might represent a new viral habitat. This study has revealed the unique characteristics and potential ecological roles of DNA viromes in the lakes of the highest plateau and established a foundation for the recognition of the viral roles in plateau lake ecosystems.

Comammox activity dominates nitrification process in the sediments of plateau wetland

The recent discovery of complete ammonia oxidation (comammox) has increased our understanding of nitrification. Although comammox has been shown to play an important role in plain wetland ecosystems, studies of comammox contribution are still limited in plateau wetland ecosystems. Here, we analyzed the abundance, activity, community and biogeochemical mechanisms of the comammox bacteria in Yunnan-kweichow and Qinghai-Tibet plateau wetlands from elevations of 1000-5000 m. Comammox bacteria were widely distributed in all 16 sediment samples with abundances higher than 0.96 ± 0.26 × 10⁷ copies g^-1 (n = 16). Comammox showed high activity (1.18 ± 0.17 to 1.98 ± 0.08 mg N kg^-1 d^-1) at high-elevation (3000-5000 m) and dominated the nitrification process (activity contribution: 37.20 - 60.62%). The activity contribution of ammonia-oxidizing bacteria (1.07 ± 0.08 to 2.79 ± 0.35 mg N kg^-1 d^-1) dominated the nitrification process (44.55 - 64.15%) in low-elevation (1000-3000 m) samples. All detected comammox Nitrospira belonged to clade A, while clade B was not detected. Elevation always had a strongest effect on key comammox species. Thus, we infer that elevation may drive the high relative abundance of the species Candidatus Nitrospira nitrificans (avg. 12.40%) and the low relative abundance of the species Nitrospira sp. SG-bin2 (avg. 4.75%) in high-elevation samples that showed a high comammox activity (avg. 1.62 mg N kg^-1 d^-1) and high contribution (avg. 46.08%) to the nitrification process. These results indicate that comammox may be an important and currently underestimated microbial nitrification process in plateau wetland ecosystems.

Distinct strategies of abundant and rare bacterioplankton in river-reservoir system: Evidence from a 2800 km plateau river

Riverine bacterioplankton are highly responsive to river alterations and their abundant and rare sub-communities may have different roles in biogeochemical cycling. However, with the rapid development of dam constructions, our knowledge on adaptation mechanism of these sub-communities in regulated river ecosystem was still limited, especially with regard to their functional traits. Here, our study was conducted in the 2800 km Yarlung Tsangpo River on the Tibetan Plateau to address the question of how abundant and rare bacterioplankton would respond taxonomically and functionally to river damming using 16S rRNA gene sequencing combined with Geochip microarray technique. Our results showed that abundant sub-community dominated taxonomic composition while rare sub-community largely determined functional composition. It is also observed that taxonomic diversity of abundant sub-community was significantly stimulated in the reservoir while that of rare sub-community was markedly inhibited. Moreover, abundant sub-community exhibited functional redundancy under damming disturbances since altered taxonomic composition and unaltered functional composition co-occurred simultaneously. Meanwhile, due to portfolio effect, rare sub-community maintained a greater stability under damming disturbances with little variation in taxonomic and functional compositions. In addition, the Stegen null model analysis revealed that stochastic process governed community assembly in both abundant and rare sub-communities. However, according to source tracking analysis, the taxonomic dispersion of abundant sub-community was less significantly impeded by the dam while the functional dispersion of rare sub-community was less strongly interrupted, indicating that the dispersal process in the dominated sub-community was less susceptible to damming. Therefore, by considering bacterioplankton functional traits, our study provided comprehensive evidences for the distinct strategies of abundant and rare sub-communities in response to damming.

In-situ expressions of comammox Nitrospira along the Yangtze River

The recent discovery of comammox Nitrospira as complete nitrifiers has significantly enriched our understanding on the nitrogen cycle, yet little is known about their metabolic transcripts in natural aquatic ecosystems. Using the genome-centric metatranscriptomics, we provided the first in-situ expression patterns of comammox Nitrospira along the Yangtze River. Our study confirmed widespread expressions of comammox Nitrospira, with the highest transcription accounting for 33.3% and 63.8% of amoA and nxrAB genes expressed in ammonia-oxidizing prokaryotes (AOPs) and Nitrospira sublineages I/II, respectively. Moreover, comammox two clades differed in nitrification, with clade A acting as the dominator to ammonia oxidation in comammox, and clade B contributing more transcripts to nitrite oxidation than to ammonia oxidation. Compared to canonical Nitrospira, comammox community had lower expressions of ammonia/nitrite transporters and nitrogen assimilatory genes, but far higher expressions in urea transport and hydrolysis, facilitating to derivation of ammonia and energy mainly through intracellular ureolytic metabolism. This suggests no need for "reciprocal-feeding" between canonical Nitrospira and AOPs in a natural river. Aerobic mixotrophy of comammox bacteria was suggested by expressions of genes coding for respiratory complexes I-V, oxidative/reductive TCA cycle, oxygen stress defenses, and transport/catabolism of simple carbohydrates and low-biosynthetic-cost amino acids. Intriguingly, significant positive correlations among expressions of ammonia monooxygenases, hydroxylamine dehydrogenase and copper-dependent nitrite reductase indicated that comammox Nitrospira had the potential of converting nitrite to nitric oxide accompanied by ammonia oxidation under low-C/N and aerobic conditions, while gene expressions in this pathway were significantly and positively associated with pH. Overall, this study illustrated novel transcriptional characteristics of comammox Nitrospira, and highlighted the necessity of reassessing their contributions to biogeochemical carbon and nitrogen cycling with perspective of in-situ meta-omics as well as culture experiments.

Distinctive Patterns and Controls of Nitrous Oxide Concentrations and Fluxes from Urban Inland Waters

Inland waters are significant sources of nitrous oxide (N₂O), a powerful greenhouse gas. However, considerable uncertainty exists in the estimates of N₂O efflux from global inland waters due to a lack of direct measurements in urban inland waters, which are generally characterized by high carbon and nitrogen concentrations and low carbon-to-nitrogen ratios. Herein, we present direct measurements of N₂O concentrations and fluxes in lakes and rivers of Beijing, China, during 2018-2020. N₂O concentrations and fluxes in the waters of Beijing exceeded previous estimates of global rivers due to the high carbon and nutrient concentrations and high aquatic productivity. In contrast, the N₂O emission factor (N₂O-N/DIN, median 0.0005) was lower than global medians and the N₂O yield (ΔN₂O/(ΔN₂O + ΔN₂), average 1.6%) was higher than those typically observed in rivers and streams. The positive relationship between N₂O emissions and denitrifying bacteria as well as the Michaelis-Menten relationship between N₂O emissions and NO₃^--N concentrations suggested that bacteria control the net production of N₂O in waters of Beijing with N saturation, leading to a low N₂O emission factor. However, low carbon-to-nitrogen ratios are beneficial for N₂O accumulation during denitrification, resulting in high N₂O yields. This study demonstrates the significant N₂O emissions and their distinctive patterns and controls in urban inland waters and suggests that N₂O emission estimates based on nitrogen loads and simple emission factor values are not appropriate for urban inland water systems.

Biotransformation of lincomycin and fluoroquinolone antibiotics by the ammonia oxidizers AOA, AOB and comammox: A comparison of removal, pathways, and mechanisms

In this study, we evaluated the biotransformation mechanisms of lincomycin (LIN) and three fluoroquinolone antibiotics (FQs), ciprofloxacin (CFX), norfloxacin (NFX), and ofloxacin (OFX), which regularly enter aquatic environments through human activities, by different ammonia-oxidizing microorganisms (AOM). The organisms included a pure culture of the complete ammonia oxidizer (comammox) Nitrospira inopinata, an ammonia oxidizing archaeon (AOA) Nitrososphaera gargensis, and an ammonia-oxidizing bacterium (AOB) Nitrosomonas nitrosa Nm90. The removal of these antibiotics by the pure microbial cultures and the protein-normalized biotransformation rate constants indicated that LIN was significantly co-metabolically biotransformed by AOA and comammox, but not by AOB. CFX and NFX were significantly co-metabolized by AOA and AOB, but not by comammox. None of the tested cultures transformed OFX effectively. Generally, AOA showed the best biotransformation capability for LIN and FQs, followed by comammox and AOB. The transformation products and their related biotransformation mechanisms were also elucidated. i) The AOA performed hydroxylation, S-oxidation, and demethylation of LIN, as well as nitrosation and cleavage of the piperazine moiety of CFX and NFX; ii) the AOB utilized nitrosation to biotransform CFX and NFX; and iii) the comammox carried out hydroxylation, demethylation, and demethylthioation of LIN. Hydroxylamine, an intermediate of ammonia oxidation, chemically reacted with LIN and the selected FQs, with removals exceeding 90%. Collectively, these findings provide important fundamental insights into the roles of different ammonia oxidizers and their intermediates on LIN and FQ biotransformation in nitrifying environments including wastewater treatment systems.

Abundance and Functional Importance of Complete Ammonia Oxidizers and Other Nitrifiers in a Riparian Ecosystem

The discovery of complete ammonia oxidation (comammox) has altered our understanding of nitrification, which is the rate-limiting process in the global nitrogen cycle. However, understanding the ecological role of comammox or its contribution to nitrification in both natural and artificial ecosystems is still in its infancy. Here, we investigated the community distribution and function of comammox bacteria in riparian ecosystems and analyzed interactions between comammox and other nitrogen cycling microorganisms. The comammox bacterial abundance and rate were higher in summer than in winter and higher in nonrhizosphere soils than in the rhizosphere. Fringe soils in the riparian zone comprise a comammox hotspot, where the abundance (2.58 × 10⁸ copies g^-1) and rate (0.86 mg N kg^-1 d^-1) of comammox were not only higher than at other sampling sites but also higher than those of other ammonia oxidation processes. The comammox rate correlated significantly positively with relative abundance of the comammox species Candidatus Nitrospira nitrificans but not with that of the species Candidatus Nitrospira nitrosa. Analysis of comammox interaction with other ammonia-oxidizing processes revealed ammonia-oxidizing archaea to dominate interface soils, comammox to dominate in fringe soils, and anaerobic ammonium oxidation (anammox) to dominate in interface sediments of the riparian zone. These results indicate that comammox may constitute an important and currently underestimated process of microbial nitrification in riparian zone ecosystems.

Both microbial abundance and community composition mattered for N2 production rates of the overlying water in one high-elevation river

Overlying water is another potential hotspot of nitrogen removal through anammox and denitrification reactions in river systems. However, N₂ production and the controlling factors have rarely been investigated in the overlying water of high-elevation rivers. This study analyzed the abundance and community of denitrifying and anammox bacteria as well as their effects on N₂ production rates in the overlying water of the Yellow River source region (elevation range: 2687-4223 m). Higher suspended particle concentrations remarkably promoted functional gene abundances of both denitrifying and anammox bacteria (r > 0.9, p < 0.01). N₂ production rates in overlying water samples ranged from 0.25 to 4.22 μmol N₂ L^-1 d^-1. The overlying water was estimated to contribute to 36.8% (on average) of riverine N₂ emission flux. Higher temperatures markedly accelerated N₂ production rates (p = 0.051). Moreover, N₂ production rates were positively related to both anammox and denitrifying bacterial abundances (p < 0.05), and such relationships were markedly affected by corresponding community compositions. The explanatory power of denitrifier abundance (R² = 0.56) for N₂ production rate variations was greatly elevated when it was integrated with community composition (R² = 0.92). This study highlights the significance of overlying water nitrogen removal in the Yellow River source region; moreover, the effects of both microbial abundance and community composition on riverine N₂ production rates should be considered in future research.

Alternative strategies of nutrient acquisition and energy conservation map to the biogeography of marine ammonia-oxidizing archaea

Ammonia-oxidizing archaea (AOA) are among the most abundant and ubiquitous microorganisms in the ocean, exerting primary control on nitrification and nitrogen oxides emission. Although united by a common physiology of chemoautotrophic growth on ammonia, a corresponding high genomic and habitat variability suggests tremendous adaptive capacity. Here, we compared 44 diverse AOA genomes, 37 from species cultivated from samples collected across diverse geographic locations and seven assembled from metagenomic sequences from the mesopelagic to hadopelagic zones of the deep ocean. Comparative analysis identified seven major marine AOA genotypic groups having gene content correlated with their distinctive biogeographies. Phosphorus and ammonia availabilities as well as hydrostatic pressure were identified as selective forces driving marine AOA genotypic and gene content variability in different oceanic regions. Notably, AOA methylphosphonate biosynthetic genes span diverse oceanic provinces, reinforcing their importance for methane production in the ocean. Together, our combined comparative physiological, genomic, and metagenomic analyses provide a comprehensive view of the biogeography of globally abundant AOA and their adaptive radiation into a vast range of marine and terrestrial habitats.

Environmental Factors, More Than Spatial Distance, Explain Community Structure of Soil Ammonia-Oxidizers in Wetlands on the Qinghai-Tibetan Plateau

In wetland ecosystems, ammonia oxidation highly depends on the activity of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), which are, therefore, important for studying nitrogen cycling. However, the ammonia-oxidizer communities in the typical high-elevation wetlands are poorly understood. Here, we examined ammonia-oxidizer communities in soils from three wetland types and 31 wetland sites across the Qinghai-Tibetan Plateau. The amoA gene of AOA and AOB was widespread across all wetland types. Nitrososphaera clade (Group I.1b) overwhelmingly dominated in AOA community (90.36%), while Nitrosospira was the principal AOB type (64.96%). The average abundances of AOA and AOB were 2.63 × 104 copies g-1 and 9.73 × 103 copies g-1. The abundance of AOA amoA gene was higher in riverine and lacustrine wetlands, while AOB amoA gene dominated in palustrine wetlands. The environmental conditions, but not spatial distance, have a dominant role in shaping the pattern of ammonia-oxidizer communities. The AOA community composition was influenced by mean annual temperature (MAT) and mean annual precipitation (MAP), while MAT, conductivity and plant richness, pH, and TN influenced the AOB community composition. The net nitrification rate had a significant correlation to AOB, but not AOA abundance. Our results suggest a dominant role for climate factors (MAT and MAP) in shaping community composition across a wide variety of wetland sites and conditions.