Temperature and salinity drive comammox community composition in mangrove ecosystems across southeastern China

Sediment depth-related variations of comammox Nitrospira: Evidence in the Three Gorges Reservoir, China

The recent discovery of comammox Nitrospira as complete ammonia-oxidizing microorganism has fundamentally revolutionized our understanding of nitrogen cycling in sediment environments. However, knowledge regarding their abundance, biodiversity, community structure, and interactions is predominantly limited to the upper layers (0-20 cm). To address this gap, we collected sediment samples along profiles ranging from 0 to 300 cm in depth at three locations within the middle segment of the Three Gorges Reservoir (TGR), China. Quantitative real-time PCR (qPCR) analyses suggested that comammox bacteria were not only ubiquitous in deep sediments but also more abundant than ammonia-oxidizing bacteria (AOB). Ammonia monooxygenases subunit A (amoA) gene amplicon sequencing illuminated that comammox bacteria were more sensitive to sedimental depth compared to AOB and ammonia-oxidizing archaea (AOA), as evidenced by a more significant decline in community diversity and similarity over distance along sediment vertical profiles. Notably, we discovered that the amoA gene abundance, alpha- and beta-diversity of comammox bacteria exerted an essential contribution to potential nitrification rates according to random forest model. Phylogenetic analysis indicted that most comammox bacteria within sediment samples belonged to clade A.2. Intriguingly, the average relative abundance of comammox clade A.2 displayed a noteworthy rise with sediment depth, whereas clade A.1 demonstrated a converse pattern, unveiling distinct ecological niche adaptations of these two clades along the sediment profile. Ecological network analysis further revealed closer interactions between comammox bacteria and canonical ammonia oxidizers in the superficial layer (0-40 cm), with the network structure gradually simplifying from superficial to deep sediment (200-300 cm). Overall, these findings broaden the current recognition of the geographic distribution and niche segregation of comammox bacteria at the fine scale of the sediments ecosystems and provide insights into sediment depth-related variations of their coexistence network patterns in large freshwater reservoirs.

Model of shrimp pond-mediated spatiotemporal dynamic distribution of antibiotic resistance genes in the mangrove habitat of a subtropical gulf

Aquacultures are the main reason for the environmental selection of antibiotic resistance genes (ARGs), resulting in the enrichment of ARGs. As a filter, a marine mangrove ecosystem can reduce antimicrobial resistance (AMR) or eliminate ARGs; however, its elimination mechanism remains unclear. This study investigated the spatiotemporal dynamic distribution of ARGs in two different types of mangrove habitats (shrimp ponds and virgin forests), within a subtropical gulf located in the Beibu Gulf, China, during dry and wet seasons by using metagenomics and real time quantitative polymerase chain reaction (RT-qPCR) analysis. As the key environmental factors, sulfide, salinity, and mobile genetic elements significantly were found to contribute to ARGs distribution, respectively. Wet and dry seasons influenced the dispersal of ARGs but did not affect the microbial community structure. Three potential biomarkers, TEM-116, smeD, and smeE, played key roles in seasonal differences. The key different genes in the biological relevance of absolute abundance were demonstrated by RT-qPCR. Co-occurrence network analysis indicated that high-abundance ARGs were distributed in a modular manner. For the first time, a risk index weighted by risk rank (RIR) was proposed and used to quantify the human risk of ARGs in the mangrove metagenome. The shrimp ponds during the wet season showed the highest RIR detected. In addition to offering a perspective on reducing AMR in mangrove wetlands, this study constructed the first spatiotemporal dynamic model of ARGs in the Beibu Gulf, China and contributed to revealing the global spread of ARGs. Meanwhile, this study proposes a new pipeline for assessing the risk of ARGs, while also exploring the concept of "One Health."

Abundance, diversity and physiological preferences of comammox Nitrospira in urban groundwater

Complete ammonia oxidizer (comammox Nitrospira), catalyze complete nitrification process in a single organism, are frequently detected in groundwater ecosystem. However, the ecological niches and environmental driving factors of comammox Nitrospira in urban groundwater are largely unknown. Here we investigated the communities of ammonia oxidizers in urban groundwater located in Shanghai city, China. Quantitative analysis demonstrated the dominance of comammox Nitrospira over classical ammonia oxidizers (ammonia-oxidizing archaea and bacteria, AOA and AOB). Phylogenetic analysis showed clades B and A2 comprise the majority of comammox Nitrospira groups. Temperature was one of the most vital factors affecting comammox Nitrospira community. Furthermore, clade A comammox Nitrospira can be enriched by urea substrate, which was in line with the ability of utilizing urea by the pure clade A comammox culture Nitrospira inopinata. In addition, we observed that relatively low temperature (<20 °C) and high copper levels (>0.04 mg L-1) can stimulate the growth of comammox Nitrospira. Overall, this study revealed the presence, diversity and physiological preferences of comammox Nitrospira in urban groundwater nitrification, shedding insights on the ecological roles of comammox Nitrospira in subsurface environment.

Community assembly of comammox Nitrospira in coastal wetlands across southeastern China

Complete ammonia oxidizers (comammox Nitrospira) are ubiquitous in coastal wetland sediments and play an important role in nitrification. Our study examined the impact of habitat modifications on comammox Nitrospira communities in coastal wetland sediments across tropical and subtropical regions of southeastern China. Samples were collected from 21 coastal wetlands in five provinces where native mudflats were invaded by Spartina alterniflora and subsequently converted to aquaculture ponds. The results showed that comammox Nitrospira abundances were mainly influenced by sediment grain size rather than by habitat modifications. Compared to S. alterniflora marshes and native mudflats, aquaculture pond sediments had lower comammox Nitrospira diversity, lower clade A.1 abundance, and higher clade A.2 abundance. Sulfate concentration was the most important factor controlling the diversity of comammox Nitrospira. The response of comammox Nitrospira community to habitat change varied significantly by location, and environmental variables accounted for only 11.2% of the variations in community structure across all sites. In all three habitat types, dispersal limitation largely controlled the comammox Nitrospira community assembly process, indicating the stochastic nature of these sediment communities in coastal wetlands. IMPORTANCE Comammox Nitrospira have recently gained attention for their potential role in nitrification and nitrous oxide (N2O) emissions in soil and sediment. However, their distribution and assembly in impacted coastal wetland are poorly understood, particularly on a large spatial scale. Our study provides novel evidence that the effects of habitat modification on comammox Nitrospira communities are dependent on the location of the wetland. We also found that the assembly of comammox Nitrospira communities in coastal wetlands was mainly governed by stochastic processes. Nevertheless, sediment grain size and sulfate concentration were identified as key variables affecting comammox Nitrospira abundance and diversity in coastal sediments. These findings are significant as they advance our understanding of the environmental adaptation of comammox Nitrospira and how future landscape modifications may impact their abundance and diversity in coastal wetlands.

Laboratory tidal microcosm deciphers responses of sediment archaeal and bacterial communities to microplastic exposure

Microplastics (MPs) are 1-5 mm plastic particles that are serious global contaminants distributed throughout marine ecosystems. However, their impact on intertidal sediment microbial communities is poorly understood. In this study, we conducted a 30-day laboratory tidal microcosm experiment to investigate the effects of MPs on microbial communities. Specifically, we used the biodegradable polymers polylactic acid (PLA) and polybutylene succinate (PBS), as well as the conventional polymers polyethylene terephthalate (PET), polycarbonate (PC), and polyethylene (PE). Treatments with different concentrations (1-5%, w/w) of PLA- and PE-MPs were also included. We analyzed taxonomic variations in archaeal and bacterial communities using 16S rRNA high-throughput sequencing. PLA-MPs at concentrations of 1% (w/w) rapidly altered microbiome composition. Total organic carbon and nitrite nitrogen were the key physicochemical factors and urease was the major enzyme shaping MP-exposed sediment microbial communities. Stochastic processes predominated in microbial assembly and the addition of biodegradable MPs enhanced the contribution of ecological selections. The major keystone taxa of archaea and bacteria were Nitrososphaeria and Alphaproteobacteria, respectively. MPs exposure had less effect on archaeal functions while nitrogen cycling decreased in PLA-MPs treatments. These findings expanded the current understanding of the mechanism and pattern that MPs affect sediment microbial communities.

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.

Effects of dam building on the occurrence and activity of comammox bacteria in river sediments and their contribution to nitrification

The recent discovery of complete ammonia oxidizers (comammox) has fundamentally changed our understanding of nitrification. However, studies on the occurrence and activity of comammox bacteria and their contribution to nitrification remain unclear. Here, we investigated the abundance, activity, and diversity of comammox bacteria and their contribution to nitrification in sediments from dammed rivers in winter and summer. Our results indicated that comammox clade A was ubiquitous in all sediment samples and the community structure in comammox varied between the upper and lower reaches, but not on the time scale (winter and summer). Comammox activity in the dammed river sediments in summer was prominently higher than in winter (summer: 1.08 ± 0.52; winter: 0.197 ± 0.148 mg N kg-1 day-1). Furthermore, the activity of comammox bacteria in summer appeared higher in the vicinity of the dammed river and in the Sanjiang estuary, which is located downstream of the dammed river. The activity of ammonia-oxidizing bacteria (AOB) (0.77 ± 0.478 mg N kg-1 day-1) was higher compared to comammox (0.639 ± 0.588 mg N kg-1 day-1) and ammonia-oxidizing archaea (AOA) (0.026 ± 0.022 mg N kg-1 day-1) in both winter and summer. In terms of contribution to the nitrification process, AOB (winter: 67.13 ± 12.21 %; summer: 50.57 ± 16.14 %) outperformed comammox (winter: 28.59 ± 12.51 %; summer: 48.38 ± 16.62 %) and AOA (winter: <7.39 %; summer: <2.09 %). These findings indicated that the nitrification process in dammed river sediments was mainly dominated by AOB. Additionally, comammox activity was significantly affected by temperature and NH4+, suggesting that these variables were key determinants of the niche partitioning of comammox. Collectively, our findings provide novel perspectives into the widespread distribution and contribution of comammox to nitrification in dammed river ecosystems, thus broadening our understanding of the nitrification processes.

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.

Differential responses of canonical nitrifiers and comammox Nitrospira to long-term fertilization in an Alfisol of Northeast China

The newly identified complete ammonia oxidizer (comammox) that converts ammonia directly into nitrate has redefined the long-held paradigm of two-step nitrification mediated by two distinct groups of nitrifiers. However, exploration of the niche differentiation of canonical nitrifiers and comammox Nitrospira and their ecological importance in agroecosystems is still limited. Here, we adopted quantitative PCR (qPCR) and Illumina MiSeq sequencing to investigate the effects of five long-term fertilization regimes in the variations of ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), nitrite-oxidizing bacteria (NOB), and comammox Nitrospira abundances and comammox community composition in two soil layers (0-20 cm, topsoil; 20-40 cm, subsoil) in an Alfisol in Northeast China. The fertilization treatments included no fertilizer (CK); chemical nitrogen (N) fertilizer; chemical N; phosphorus (P) and potassium (K) fertilizers (NPK); recycled organic manure (M) and chemical N, P, K plus recycled manure (MNPK). Compared with CK, manure and/or chemical fertilizer significantly increased the AOB amoA gene abundance. Long-term recycled manure increased soil organic matter (SOM) contents and maintained the soil pH, but decreased the NH₄ ⁺-N concentrations, which markedly promoted the nxrA and nxrB gene abundances of NOB and the amoA gene abundances of comammox Nitrospira clade A and AOA. Although the comammox Nitrospira clade B abundance tended to decrease after fertilization, the structural equation modeling analysis showed that comammox clade B had direct positive impacts on soil potential ammonia oxidation (PAO; λ = 0.59, p < 0.001). The long-term fertilization regime altered the community composition of comammox Nitrospira. Additionally, comammox Nitrospira clades A and B had individual response patterns to the soil layer. The relative abundance of clade A was predominant in the topsoil in the N (86.5%) and MNPK (76.4%) treatments, while clade B appeared to be dominant in the subsoil (from 78.7 to 88.1%) with lower ammonium contents, implying niche separation between these clades. Soil pH, NH₄ ⁺-N and SOM content were crucial factors shaping the soil nitrifying microbial abundances and the comammox Nitrospira community. Together, these findings expand the current understanding of the niche specialization and the important role of comammox Nitrospira in terrestrial ecosystems.

Plant Species-Driven Distribution of Individual Clades of Comammox Nitrospira in a Subtropical Estuarine Wetland

Plant species play a crucial role in mediating the activity and community structure of soil microbiomes through differential inputs of litter and rhizosphere exudates, but we have a poor understanding of how plant species influence comammox Nitrospira, a newly discovered ammonia oxidizer with pivotal functionality. Here, we investigate the abundance, diversity, and community structure of comammox Nitrospira underneath five plant species and a bare tidal flat at three soil depths in a subtropical estuarine wetland. Plant species played a critical role in driving the distribution of individual clades of comammox Nitrospira, explaining 59.3% of the variation of community structure. Clade A.1 was widely detected in all samples, while clades A.2.1, A.2.2, A.3 and B showed plant species-dependent distribution patterns. Compared with the native species Cyperus malaccensis, the invasion of Spartina alterniflora increased the network complexity and changed the community structure of comammox Nitrospira, while the invasive effects from Kandelia obovata and Phragmites australis were relatively weak. Soil depths significantly influenced the community structure of comammox Nitrospira, but the effect was much weaker than that from plant species. Altogether, our results highlight the previously unrecognized critical role of plant species in driving the distribution of comammox Nitrospira in a subtropical estuarine wetland.

Nitrogen addition and warming rapidly alter microbial community compositions in the mangrove sediment

The mangrove ecosystem is an important CO₂ sink with an extraordinarily high primary productivity. However, it is vulnerable to the impact of climate warming and eutrophication. While there has been extensive research on plant growth and greenhouse gas emission in mangrove ecosystems, microbial communities, the primary biogeochemical cycling drivers, are much less understood. Here, we examined whether short-term experimental treatments: (1) eutrophication with a supplement of 185 g N m^-2·year^-1 (N), (2) 3°C warming (W), and (3) the dual treatment of N and W (NW) were sufficient to alter microbial communities in the sediment. After 4 months of experiments, most environmental factors remained unchanged. However, N had significant, strong effects on bacterial, fungal, and functional community compositions, while the effects of W on microbial communities were weaker. N increased bacterial richness, phylogenetic diversity, and evenness, owing to stronger stochastic processes induced by eutrophication. There were no interactive effects of N and W on bacterial, fungal, and functional community compositions, suggesting that joint effects of N and W were additive. Concomitant with higher N₂O efflux induced by N, the relative abundances of most bacterial nitrogen cycling genes were increased or remained changed by N. In contrast, N decreased or did not change those of most bacterial carbon degradation genes, while W increased or did not change the relative abundances of most of bacterial and fungal carbon degradation genes, implying higher carbon degradation potentials. As the most abundant inorganic nitrogenous species in mangrove sediment, ammonium was a key factor in shaping microbial functional communities. Collectively, our findings showed that microbial community compositions in the mangrove sediment were highly sensitive to short-term N and W treatments, giving rise to ecological consequences such as higher N₂O efflux.

Genomic profiling of Nitrospira species reveals ecological success of comammox Nitrospira

BACKGROUND

The discovery of microorganisms capable of complete ammonia oxidation to nitrate (comammox) has prompted a paradigm shift in our understanding of nitrification, an essential process in N cycling, hitherto considered to require both ammonia oxidizing and nitrite oxidizing microorganisms. This intriguing metabolism is unique to the genus Nitrospira, a diverse taxon previously known to only contain canonical nitrite oxidizers. Comammox Nitrospira have been detected in diverse environments; however, a global view of the distribution, abundance, and diversity of Nitrospira species is still incomplete.

RESULTS

In this study, we retrieved 55 metagenome-assembled Nitrospira genomes (MAGs) from newly obtained and publicly available metagenomes. Combined with publicly available MAGs, this constitutes the largest Nitrospira genome database to date with 205 MAGs, representing 132 putative species, most without cultivated representatives. Mapping of metagenomic sequencing reads from various environments against this database enabled an analysis of the distribution and habitat preferences of Nitrospira species. Comammox Nitrospira's ecological success is evident as they outnumber and present higher species-level richness than canonical Nitrospira in all environments examined, except for marine and wastewaters samples. The type of environment governs Nitrospira species distribution, without large-scale biogeographical signal. We found that closely related Nitrospira species tend to occupy the same habitats, and that this phylogenetic signal in habitat preference is stronger for canonical Nitrospira species. Comammox Nitrospira eco-evolutionary history is more complex, with subclades achieving rapid niche divergence via horizontal transfer of genes, including the gene encoding hydroxylamine oxidoreductase, a key enzyme in nitrification.

CONCLUSIONS

Our study expands the genomic inventory of the Nitrospira genus, exposes the ecological success of complete ammonia oxidizers within a wide range of habitats, identifies the habitat preferences of (sub)lineages of canonical and comammox Nitrospira species, and proposes that horizontal transfer of genes involved in nitrification is linked to niche separation within a sublineage of comammox Nitrospira. Video Abstract.

Temperature drives the assembly of Bacillus community in mangrove ecosystem

Mangroves are located at the interface of terrestrial and marine environments, and experience fluctuating conditions, creating a need to better explore the relative role of the bacterial community. Bacillus has been reported to be the dominant group in the mangrove ecosystem and plays a key role in maintaining the biodiversity and function of the mangrove ecosystem. However, studies on bacterial and Bacillus community across four seasons in the mangrove ecosystem are scarce. Here, we employed seasonal large-scale sediment samples collected from the mangrove ecosystem in southeastern China and utilized 16S rRNA gene amplicon sequencing to reveal bacterial and Bacillus community structure changes across seasons. Compared with the whole bacterial community, we found that Bacillus community was greatly affected by season (temperature) rather than site. The key factors, NO₃-N and NH₄-N showed opposite interaction with superabundant taxa Bacillus taxa (SAT) and three rare Bacillus taxa including high rare taxa (HRT), moderate rare taxa (MRT) and low rare taxa (LRT). Network analysis suggested the co-occurrence of Bacillus community and Bacillus-bacteria, and revealed SAT had closer relationship compared with rare Bacillus taxa. HRT might act crucial response during the temperature decreasing process across seasons. This study fills a gap in addressing the assembly of Bacillus community and their role in maintaining microbial diversity and function in mangrove ecosystem.

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.

Existence and distribution of novel phylotypes of Nitrospira in water columnsof the South China Sea

In the biological nitrogen cycle, nitrite oxidation is performed by nitrite oxidation bacteria, of which Nitrospira is widespread and diverse. Communities of Nitrospira were collected at 25-1500 m depths in the South China Sea. Phylogenetic diversity, community composition, and environmental factors were investigated using high-throughput sequencing targeting the nxrB gene and statistical analyses. The community composition of Nitrospira varied spatially and by depth. Among the 24 OTUs with relatively high abundance, 70% were unclassified and not affiliated with the known Nitrospira genus, suggesting a previously unrecognized high diversity of marine Nitrospira. Five known Nitrospira genera were detected, of which the common marine Nitrospira marina was not the dominant species, whereas Candidatus Nitrospira lenta and Candidatus Nitrospira defluvii dominated in shallow habitats. Comammox Candidatus Nitrospira nitrosa was discovered in the marine ecosystem. The niche differentiation of versatile Nitrospira species was mainly shaped by nitrate, temperature, and DO.

Deterministic Factors Determine the Comammox Community Composition in the Pearl River Estuary Ecosystem

Complete ammonia oxidizers (comammox) have been widely detected in riverine and estuarine ecosystems. However, knowledge about the process of comammox community assembly from freshwater to marine environments is still limited. Here, based on deep sequencing, we investigated the community composition of comammox along a salinity gradient in the Pearl River Estuary (PRE), South China. Our results showed that comammox microorganisms in the PRE sediments were extremely diverse and displayed distinct distributional patterns between upstream and downstream habitats. Quantitative PCR demonstrated that comammox was the dominant ammonia-oxidizing microorganism (AOM) in the PRE upstream sediments, and ammonia-oxidizing archaea (AOA) dominated the PRE downstream sediments, while ammonia-oxidizing bacteria (AOB) were not dominant in any section of the PRE. Neutral modeling revealed that stochastic processes explained a limited part of the variation in the comammox community. The majority of beta nearest-taxon index values were higher than 2, indicating that comammox community assembly in the PRE sediments was better explained through a deterministic process than through a stochastic process. Salinity and total nitrogen were the most important contributing factors that shaped the comammox community. This study expanded the current knowledge of the diversity and niche preference of comammox in the estuarine ecosystem, and further enhances our understanding of the assembly of comammox community from freshwater to marine environments.

IMPORTANCE Microbial communities are shaped by stochastic (emigration, immigration, birth, death, and genetic drift of species) and deterministic (e.g., environmental factors) processes. However, it remains unknown as to which type of process is more important in influencing the comammox community assembly from freshwater to marine environments. In this study, we compared the relative importance of stochastic and deterministic processes in shaping the assembly of the comammox community, which demonstrated that the deterministic process was more important in determining the community assembly patterns in the PRE ecosystem.

Effects of Microplastics on Microbial Community in Zhanjiang Mangrove Sediments

Microplastics are easily consumed by marine animals, thereby entering the food chain and endangering animal health. However, there are few studies focusing on the effects of microplastics in mangrove sediments on microbial communities. In order to study the influence of microplastics on microorganisms, microplastics and microorganisms were extracted from Zhanjiang (Guangdong Province, China) mangrove sediments and analyzed. The results showed that there were differences in Shannon and Simpson indices of the microbial community in microplastics (p < 0.05), and there were also differences between JG30_KF_CM45 and Natranaerovirga at the genus level, indicating that microplastics may affect the diversity and composition of microorganisms in sediments. In addition, FAPROTAX function prediction analysis showed that microplastics may affect the nitrification of microbial communities. The results from this study indicate that microplastics affected the diversity and richness of microorganisms in mangrove sediments, which provides an experimental basis for the relationship between microplastics and microorganisms.

Microbial diversity and ecological interactions of microorganisms in the mangrove ecosystem: Threats, vulnerability, and adaptations

Mangroves are among the world's most productive ecosystems and a part of the "blue carbon" sink. They act as a connection between the terrestrial and marine ecosystems, providing habitat to countless organisms. Among these, microorganisms (e.g., bacteria, archaea, fungi, phytoplankton, and protozoa) play a crucial role in this ecosystem. Microbial cycling of major nutrients (carbon, nitrogen, phosphorus, and sulfur) helps maintain the high productivity of this ecosystem. However, mangrove ecosystems are being disturbed by the increasing concentration of greenhouse gases within the atmosphere. Both the anthropogenic and natural factors contribute to the upsurge of greenhouse gas concentration, resulting in global warming. Changing climate due to global warming and the increasing rate of human interferences such as pollution and deforestation are significant concerns for the mangrove ecosystem. Mangroves are susceptible to such environmental perturbations. Global warming, human interventions, and its consequences are destroying the ecosystem, and the dreadful impacts are experienced worldwide. Therefore, the conservation of mangrove ecosystems is necessary for protecting them from the changing environment-a step toward preserving the globe for better living. This review highlights the importance of mangroves and their microbial components on a global scale and the degree of vulnerability of the ecosystems toward anthropic and climate change factors. The future scenario of the mangrove ecosystem and the resilience of plants and microbes have also been discussed.

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.

Omics-based ecosurveillance uncovers the influence of estuarine macrophytes on sediment microbial function and metabolic redundancy in a tropical ecosystem

Vertical zonation within estuarine ecosystems can strongly influence microbial diversity and function by regulating competition, predation, and environmental stability. The degree to which microbial communities exhibit horizontal patterns through an estuary has received comparatively less attention. Here, we take a multi-omics ecosurveillance approach to study environmental gradients created by the transition between dominant vegetation types along a near pristine tropical river system (Wenlock River, Far North Queensland, Australia). The study sites included intertidal mudflats fringed by saltmarsh, mangrove or mixed soft substrata habitats. Collected sediments were analyzed for eukaryotes and prokaryotes using small sub-unit (SSU) rRNA gene amplicons to profile the relative taxonomic composition. Central carbon metabolism metabolites and other associated organic polar metabolites were analyzed using established metabolomics-based approaches, coupled with total heavy metals analysis. Eukaryotic taxonomic information was found to be more informative of habitat type. Bacterial taxonomy and community composition also showed habitat-specificity, with phyla Proteobacteria and Cyanobacteria strongly linked to mangroves and saltmarshes, respectively. In contrast, metabolite profiling was critical for understanding the biochemical pathways and expressed functional outputs in these systems that were tied to predicted microbial gene function (16S rRNA). A high degree of metabolic redundancy was observed in the bacterial communities, with the metabolomics data suggesting varying degrees of metabolic criticality based on habitat type. The predicted functions of the bacterial taxa combined with annotated metabolites accounted for the conservative perspective of microbial community redundancy against the putative metabolic pathway impacts in the metabolomics data. Coupling these data demonstrates that habitat-mediated estuarine gradients drive patterns of community diversity and metabolic function and highlights the real redundancy potential of habitat microbiomes. This information is useful as a point of comparison for these sensitive ecosystems and provides a framework for identifying potentially vulnerable or at-risk systems before they are significantly degraded.

Ecological Role of Bacteria Involved in the Biogeochemical Cycles of Mangroves Based on Functional Genes Detected through GeoChip 5.0

Mangroves provide a variety of ecosystem services and contribute greatly to the global biogeochemical cycle. Microorganisms play important roles in biogeochemical cycles and maintain the dynamic balance of mangroves. However, the roles of bacteria in the biogeochemical cycles of mangroves and their ecological distribution and functions remain largely uncharacterized. This study thus sought to analyze and compare the ecological distributions and potential roles of bacteria in typical mangroves using 16S rRNA gene amplicon sequencing and GeoChip. Interestingly, the bacterial community compositions were largely similar in the studied mangroves, including Shenzhen, Yunxiao, Zhanjiang, Hainan, Hongkong, Fangchenggang, and Beihai mangroves. Moreover, gamma-proteobacterium_uncultured and Woeseia were the most abundant microorganisms in the mangroves. Furthermore, most of the bacterial communities were significantly correlated with phosphorus levels (P < 0.05; −0.93 < R < 0.93), suggesting that this nutrient is a vital driver of bacterial community composition. Additionally, GeoChip analysis indicated that the functional genes amyA, narG, dsrA, and ppx were highly abundant in the studied mangroves, suggesting that carbon degradation, denitrification, sulfite reduction, and polyphosphate degradation are crucial processes in typical mangroves. Moreover, several genera were found to synergistically participate in biogeochemical cycles in mangroves. For instance, Neisseria, Ruegeria, Rhodococcus, Desulfotomaculum, and Gordonia were synergistically involved in the carbon, nitrogen, and sulfur cycles, whereas Neisseria and Treponema were synergistically involved in the nitrogen cycle and the sulfur cycle. Taken together, our findings provide novel insights into the ecological roles of bacteria in the biogeochemical cycles of mangroves.

IMPORTANCE Bacteria have important functions in biogeochemical cycles, but studies on their function in an important ecosystem, mangroves, are still limited. Here, we investigated the ecological role of bacteria involved in biogeochemical cycles in seven representative mangroves of southern China. Furthermore, various functional genes from bacteria involved in biogeochemical cycles were identified by GeoChip 5.0. The functional genes associated with the carbon cycle (particularly carbon degradation) were the most abundant, suggesting that carbon degradation is the most active process in mangroves. Additionally, some high-abundance bacterial populations were found to synergistically mediate key biogeochemical cycles in the mangroves, including Neisseria, Pseudomonas, Treponema, Desulfotomaculum, and Nitrosospira. In a word, our study gives novel insights into the function of bacteria in biogeochemical cycles in mangroves.

Salinity gradients shape the nitrifier community composition in Nanliu River Estuary sediments and the ecophysiology of comammox Nitrospira inopinata

The recent discovery of complete ammonia oxidizers (comammox), which convert ammonia to nitrate in a single organism, revolutionized the conventional understanding that two types of nitrifying microorganisms have to be involved in the nitrification process for more than 100 years. However, how different types of nitrifiers in response to salinity change remains largely unclear. This study not only investigated nitrifier community (including ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), comammox and nitrite-oxidizing Nitrospira) in the Nanliu estuary to find the ecological relationship between salinity and functional communities and also studied the physiology of a typical comammox Nitrospira inopinata in response to a salinity gradient. Based on sequences retrieved with four sets of functional gene primes, comammox Nitrospira was in general, mainly composed of clade A, with a clear separation of clade A1 subgroup in all samples and clade A2 subgroup in low salinity ones. As expected, group I.1b and group I.1a AOA dominated the AOA community in low- and high-salinity samples, respectively. Nitrosomonas-AOB were detected in all samples while Nitrosospira-AOB were mainly found in relatively high-salinity samples. Regarding general Nitrospira, lineages II and IV were the major groups in most of the samples, while lineage I Nitrospira was only detected in low-salinity samples. Furthermore, the comammox pure culture of N. inopinata showed an optimal salinity at 0.5‰ and ceased to grow at 12.8‰ for ammonia oxidation, but remained active for nitrite oxidation. These results show new evidence regarding niche specificity of different nitrifying microorganisms modulated mainly by salinity, and also a clear response by comammox N. inopinata to a wide range of simulated salinity levels.

L-Cysteine Synthase Enhanced Sulfide Biotransformation in Subtropical Marine Mangrove Sediments as Revealed by Metagenomics Analysis

High sulfides concentrations can be poisonous to environment because of anthropogenic waste production or natural occurrences. How to elucidate the biological transformation mechanisms of sulfide pollutants in the subtropical marine mangrove ecosystem has gained increased interest. Thus, in the present study, the sulfide biotransformation in subtropical mangroves ecosystem was accurately evaluated using metagenomic sequencing and quantitative polymerase chain reaction analysis. Most abundant genes were related to the organic sulfur transformation. Furthermore, an ecological model of sulfide conversion was constructed. Total phosphorus was the dominant environmental factor that drove the sulfur cycle and microbial communities. We compared mangrove and non-mangrove soils and found that the former enhanced metabolism that was related to sulfate reduction when compared to the latter. Total organic carbon, total organic nitrogen, iron, and available sulfur were the key environmental factors that effectively influenced the dissimilatory sulfate reduction. The taxonomic assignment of dissimilatory sulfate-reducing genes revealed that Desulfobacterales and Chromatiales were mainly responsible for sulfate reduction. Chromatiales were most sensitive to environmental factors. The high abundance of cysE and cysK could contribute to the coping of the microbial community with the toxic sulfide produced by Desulfobacterales. Collectively, these findings provided a theoretical basis for the mechanism of the sulfur cycle in subtropical mangrove ecosystems.

L-Cysteine Synthase Enhanced Sulfide Biotransformation in Subtropical Marine Mangrove Sediments as Revealed by Metagenomics Analysis

High sulfides concentrations can be poisonous to environment because of anthropogenic waste production or natural occurrences. How to elucidate the biological transformation mechanisms of sulfide pollutants in the subtropical marine mangrove ecosystem has gained increased interest. Thus, in the present study, the sulfide biotransformation in subtropical mangroves ecosystem was accurately evaluated using metagenomic sequencing and quantitative polymerase chain reaction analysis. Most abundant genes were related to the organic sulfur transformation. Furthermore, an ecological model of sulfide conversion was constructed. Total phosphorus was the dominant environmental factor that drove the sulfur cycle and microbial communities. We compared mangrove and non-mangrove soils and found that the former enhanced metabolism that was related to sulfate reduction when compared to the latter. Total organic carbon, total organic nitrogen, iron, and available sulfur were the key environmental factors that effectively influenced the dissimilatory sulfate reduction. The taxonomic assignment of dissimilatory sulfate-reducing genes revealed that Desulfobacterales and Chromatiales were mainly responsible for sulfate reduction. Chromatiales were most sensitive to environmental factors. The high abundance of cysE and cysK could contribute to the coping of the microbial community with the toxic sulfide produced by Desulfobacterales. Collectively, these findings provided a theoretical basis for the mechanism of the sulfur cycle in subtropical mangrove ecosystems.

Temperature Influenced the Comammox Community Composition in Drinking Water and Wastewater Treatment Plants

Nitrification is a pivotal step applied in water engineered systems for nitrogen removal. Temperature variation due to seasonal changes is a great challenge for maintaining nitrogen removal efficiency in water engineered ecosystems by affecting nitrifier activities. Research on the abundance, activity, and metabolic characteristics of nitrifiers can provide information for selecting suitable design parameters to ensure efficient nitrogen removal in different seasons. To date, the temperature-related niche separation of comammox, a newly discovered nitrifier with potential high-growth yield, has been rarely investigated. This study addressed the distribution of comammox and canonical nitrifying guilds in drinking water treatment plants (DWTPs) and wastewater treatment plants (WWTPs) in different seasons. qPCR-based surveys showed that comammox ubiquitously distributed and greatly outnumbered other ammonia-oxidizing prokaryotes in both DWTPs and WWTPs, except in Aug samples from DWTPs, suggesting the potential competitive advantage of AOA in summer. The nitrificans-like comammox and nitrosa-like comammox comprised the majority of the comammox community in DWTPs and WWTPs, respectively, and COD and NH₄⁺ concentrations significantly contributed to the distinct comammox phylotype distribution between DWTPs and WWTPs. The temperature-related distribution pattern of the comammox community was observed at each site. Moreover, the network complex of comammox communities was highest in Dec at all the sites, possibly contributing to the survival of comammox community in low temperature conditions.

Water diversion induces more changes in bacterial and archaeal communities of river sediments than seasonality

Previous studies have demonstrated that seasonal variation is often the most important factor affecting aquatic bacterial assemblages. Whether anthropogenic activities can dominate community dynamics remains unknown. Based on 16S rRNA high-throughput sequencing technology, this study revealed and compared the relative influence of water diversions and seasonality on bacterial and archaeal communities in river sediments from a region with obvious seasonality. The results indicate that the influence of water diversion on bacteria and archaea in water-receiving river sediments exceeded the influence of seasonal variation. Water diversion affected microbes by increasing EC, salinity, water flow rate, and organic matter carbon and nitrogen contents. Seasonal variations affected microbes by altering water temperature. Diversion responders but no season responders were classified by statistical methods in the microbial community. Diversion responder numbers were related to nitrogen concentrations, complex organic carbon contents and EC values, which were mainly affected by water diversion. With the joint impact of water diversion and seasonality, the correlations of bacterial and archaeal numbers with environmental factors were obviously weakened due to the increases in the ecological niche breadths of microorganisms. Natural seasonal changes in bacterial and archaeal communities were totally altered by changes in salinity, nutrients, and hydrological conditions induced by anthropogenic water diversions. These results highlight that human activity may be a stronger driver than natural seasonality in the alteration of bacterial and archaeal communities.

Nitrogen-cycling gene pool shrunk by species interactions among denser bacterial and archaeal community stimulated by excess organic matter and total nitrogen in a eutrophic bay

Microbial densities, functional genes, and their responses to environment factors have been studied for years, but still a lot remains unknown about their interactions with each other. In this study, the abundances of 7 nitrogen cycling genes in the sediments from Hangzhou Bay were analyzed along with bacterial and archaeal 16S rRNA abundances as the biomarkers of their densities. The amount of organic matter (OM) and total nitrogen (TN) strongly positively correlated with each other and microbial densities, while total phosphate (TP) and ammonia-nitrogen (NH₃-N) did not. Most studied genes were density suppressed, while nirS was density stable, and nosZ and hzo were density irrelevant. This suggests eutrophication could limit inorganic nitrogen cycle pathways and the removal of nitrogen in the sediment and emit more greenhouse gases. This study provides a new insight of microbial community structures, functions and their interactions in the sediments of eutrophic bays.

Abundance and niche specificity of different types of complete ammonia oxidizers (comammox) in salt marshes covered by different plants

The recently discovered complete ammonia oxidizers (comammox), which are ubiquitous in various natural and artificial ecosystems, have led to a paradigm shift in our understanding of aerobic nitrification. The coastal salt marsh covered by various plant species is an important ecosystem to link nitrogen cycles of terrestrial and marine environments; however, the distribution and structure of comammox in such ecosystems have not been clearly investigated. Here, we applied quantitative PCR and partial nested-PCR to investigate the abundance and community composition of comammox in salt marsh sediment samples covered by three plant types along the southern coastline of China. Our results showed a predominance of comammox clade A in majority of the samples, suggesting their ubiquity and the important role they play in nitrification in salt marsh ecosystems. However, variations by the sites were found when comparing the abundance of subclades of comammox clade A. Redundancy analysis demonstrated a coexistence pattern by comammox clade A.1 with ammonia-oxidizing archaea and comammox clade A.2 with canonical ammonia-oxidizing bacteria, indicating their differences in potential niche preference. However, the abundance of comammox clade B was lower than that of comammox clade A and other ammonia oxidizers in most samples. Moreover, pH and salinity were found to be the most significant factors affecting comammox community structures, suggesting their roles in driving niche partitioning of comammox, whereas plant types did not show a significant effect on the comammox community structure. Our study provided insights into the abundance, community diversity, and niche partitions of comammox, broadening the current understanding of the relationship of comammox with other ammonia oxidizers in salt marsh ecosystems.

The long-term effects of using nitrite and urea on the enrichment of comammox bacteria

The discovery of complete ammonia oxidizer (comammox) was a breakthrough in the study of nitrification. However, slow growth of comammox bacteria makes it challenging to distinguish them from traditional ammonia oxidizing microorganisms. Genomic data indicated that comammox bacteria encoded genes that can metabolize urea and had higher nitrite tolerance, which could only be found in several ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). This implies that using nitrite and urea as nitrogen sources may accelerate comammox bacteria's enrichment efficiency. In this study, two reactors using nitrite and urea as substrates, respectively, were operated for 390 days. At the end of cultivation, the reactor fed with urea exhibited higher nitrification potential than the reactor fed with nitrite. Comammox bacteria outcompeted AOA and AOB, regardless of whether they were cultured with nitrite or urea. Using nitrite can improve the proportion of comammox amoA to total amoA of 92%, while using urea may increase the proportion of comammox bacteria among total bacteria to 14.2%. Metagenomic results implied that nitrite was converted to ammonia by nitrate reduction and absorbed by comammox bacteria. On the other hand, urea may be directly utilized as substrate. These results demonstrated that using different nitrogen sources caused niche differentiation of comammox bacteria, AOA, and AOB. Using nitrite can increase the relative abundance of comammox amoA to total amoA, while using urea can increase the quantity of comammox amoA. Comammox bacteria were dominant among ammonia oxidizing microorganisms for both nitrite and urea cultures.

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

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