Acidification Offset Warming-Induced Increase in N2O Production in Estuarine and Coastal Sediments.

Global warming and acidification, induced by a substantial increase in anthropogenic CO2 emissions, are expected to have profound impacts on biogeochemical cycles. However, underlying mechanisms of nitrous oxide (N2O) production in estuarine and coastal sediments remain rarely constrained under warming and acidification. Here, the responses of sediment N2O production pathways to warming and acidification were examined using a series of anoxic incubation experiments. Denitrification and N2O production were largely stimulated by the warming, while N2O production decreased under the acidification as well as the denitrification rate and electron transfer efficiency. Compared to warming alone, the combination of warming and acidification decreased N2O production by 26 ± 4%, which was mainly attributed to the decline of the N2O yield by fungal denitrification. Fungal denitrification was mainly responsible for N2O production under the warming condition, while bacterial denitrification predominated N2O production under the acidification condition. The reduced site preference of N2O under acidification reflects that the dominant pathways of N2O production were likely shifted from fungal to bacterial denitrification. In addition, acidification decreased the diversity and abundance of nirS-type denitrifiers, which were the keystone taxa mediating the low N2O production. Collectively, acidification can decrease sediment N2O yield through shifting the responsible production pathways, partly counteracting the warming-induced increase in N2O emissions, further reducing the positive climate warming feedback loop.

Role of n-DAMO in Mitigating Methane Emissions from Intertidal Wetlands Is Regulated by Saltmarsh Vegetations.

Coastal wetlands are hotspots for methane (CH4) production, reducing their potential for global warming mitigation. Nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) plays a crucial role in bridging carbon and nitrogen cycles, contributing significantly to CH4 consumption. However, the role of n-DAMO in reducing CH4 emissions in coastal wetlands is poorly understood. Here, the ecological functions of the n-DAMO process in different saltmarsh vegetation habitats as well as bare mudflats were quantified, and the underlying microbial mechanisms were explored. Results showed that n-DAMO rates were significantly higher in vegetated habitats (Scirpus mariqueter and Spartina alterniflora) than those in bare mudflats (P < 0.05), leading to an enhanced contribution to CH4 consumption. Compared with other habitats, the contribution of n-DAMO to the total anaerobic CH4 oxidation was significantly lower in the Phragmites australis wetland (15.0%), where the anaerobic CH4 oxidation was primarily driven by ferric iron (Fe3+). Genetic and statistical analyses suggested that the different roles of n-DAMO in various saltmarsh wetlands may be related to divergent n-DAMO microbial communities as well as environmental parameters such as sediment pH and total organic carbon. This study provides an important scientific basis for a more accurate estimation of the role of coastal wetlands in mitigating climate change.

Potential response of dark carbon fixation to global warming in estuarine and coastal waters.

Dark carbon fixation (DCF), through which chemoautotrophs convert inorganic carbon to organic carbon, is recognized as a vital process of global carbon biogeochemical cycle. However, little is known about the response of DCF processes in estuarine and coastal waters to global warming. Using radiocarbon labelling method, the effects of temperature on the activity of chemoautotrophs were investigated in benthic water of the Yangtze estuarine and coastal areas. A dome-shaped thermal response pattern was observed for DCF rates (i.e., reduced rates at lower or higher temperatures), with the optimum temperature (Topt ) varying from about 21.9 to 32.0°C. Offshore sites showed lower Topt values and were more vulnerable to global warming compared with nearshore sites. Based on temperature seasonality of the study area, it was estimated that warming would accelerate DCF rate in winter and spring but inhibit DCF activity in summer and fall. However, at an annual scale, warming showed an overall promoting effect on DCF rates. Metagenomic analysis revealed that the dominant chemoautotrophic carbon fixation pathways in the nearshore area were Calvin-Benson-Bassham (CBB) cycle, while the offshore sites were co-dominated by CBB and 3-hydroxypropionate/4-hydroxybutyrate cycles, which may explain the differential temperature response of DCF along the estuarine and coastal gradients. Our findings highlight the importance of incorporating DCF thermal response into biogeochemical models to accurately estimate the carbon sink potential of estuarine and coastal ecosystems in the context of global warming.

Spatiotemporal Dynamics of Bacterial Taxonomic and Functional Profiles in Estuarine Intertidal Soils of China Coastal Zone.

Bacteria play an important role in regulating carbon (C), nitrogen (N), and sulfur (S) in estuarine intertidal wetlands. To gain insights into the ecological and metabolic modes possessed by bacteria in estuarine intertidal wetlands, a total of 78 surface soil samples were collected from China's coastal intertidal wetlands to examine the spatial and seasonal variations of bacterial taxonomic composition, assembly processes, and ecological system functions through shotgun metagenomic and 16S rRNA gene sequencing. Obvious spatiotemporal dynamic patterns in the bacterial community structure were identified, with more pronounced seasonal rather than spatial variations. Dispersion limitation was observed to act as a critical factor affecting community assembly, explaining approximately half of the total variation in the bacterial community. Functional bacterial community structure exhibited a more significant latitudinal change than seasonal variability, highlighting that functional stability of the bacterial communities differed with their taxonomic variability. Identification of biogeochemically related links between C, N, and S cycles in the soils showed the adaptive routed metabolism of the bacterial communities and the strong interactions between coupled metabolic pathways. Our study broadens the insights into the taxonomic and functional profiles of bacteria in China's estuarine intertidal soils and helps us understand the effects exerted by environmental factors on the ecological health and microbial diversity of estuarine intertidal flats.

Effects of Aquatic Acidification on Microbially Mediated Nitrogen Removal in Estuarine and Coastal Environments.

Acidification of estuarine and coastal waters is anticipated to influence nitrogen (N) removal processes, which are critical pathways for eliminating excess N from these ecosystems. We found that denitrification rates decreased significantly under acidified conditions (P < 0.05), which reduced by 41-53% in estuarine and coastal sediments under an approximately 0.3 pH reduction of the overlying water. However, the N removal rates through the anaerobic ammonium oxidation (anammox) process were concomitantly promoted under the same acidification conditions (increased by 47-109%, P < 0.05), whereas the total rates of N loss were significantly inhibited by aquatic acidification (P < 0.05), as denitrification remained the dominant N removal pathway. More importantly, the emission of nitrous oxide (N2O) from estuarine and coastal sediments was greatly stimulated by aquatic acidification (P < 0.05). Molecular analyses further demonstrated that aquatic acidification also altered the functional microbial communities in estuarine and coastal sediments; and the abundance of denitrifiers was significantly reduced (P < 0.05), while the abundance of anammox bacteria remained relatively stable. Collectively, this study reveals the effects of acidification on N removal processes and the underlying mechanisms and suggests that the intensifying acidification in estuarine and coastal waters might reduce the N removal function of these ecosystems, exacerbate eutrophication, and accelerate global climate change.

Nitrification Regulates the Spatiotemporal Variability of N2O Emissions in a Eutrophic Lake.

Nitrous oxide (N2O) emissions from lakes exhibit significant spatiotemporal heterogeneity, and quantitative identification of the different N2O production processes is greatly limited, causing the role of nitrification to be undervalued or ignored in models of a lake's N2O emissions. Here, the contributions of nitrification and denitrification to N2O production were quantitatively assessed in the eutrophic Lake Taihu using molecular biology and isotope mapping techniques. The N2O fluxes ranged from -41.48 to 28.84 µmol m-2 d-1 in the lake, with lower N2O concentrations being observed in spring and summer and significantly higher N2O emissions being observed in autumn and winter. The 15N site preference and relevant isotopic evidence demonstrated that denitrification contributed approximately 90% of the lake's gross N2O production during summer and autumn, 27-83% of which was simultaneously eliminated via N2O reduction. Surprisingly, nitrification seemed to act as a key process promoting N2O production and contributing to the lake as a source of N2O emissions. A combination of N2O isotopocule-based approaches and molecular techniques can be used to determine the precise characteristics of microbial N2O production and consumption in eutrophic lakes. The results of this study provide a basis for accurately assessing N2O emissions from lakes at the regional and global scales.

N2O emission dynamics along an intertidal elevation gradient in a subtropical estuary: Importance of N2O consumption.

Studying nitrous oxide (N2O) production and consumption processes along an intertidal elevation gradient can improve the understanding of N2O dynamics among coastal wetlands. A natural-abundance isotope technique was applied to characterize the processes responsible for N2O emission in high, middle and low intertidal zones in the Yangtze Estuary. The results showed that N2O emission rates in high tidal zones (0.84 ± 0.35 nmol g-1 h-1) were significantly higher than those in middle (0.21 ± 0.04 nmol g-1 h-1) and low tidal zones (0.26 ± 0.05 nmol g-1 h-1). Gross N2O production and consumption rates were greater in high and low tidal zones than in middle tidal zones, whereas N2O consumption proportions generally increased from high to low tidal zones. N2O consumption was quite pronounced, implying that N2O emission in estuarine wetlands accounts for only a small fraction of the total production. Higher degrees of N2O consumption were the pivotal driver of less N2O emission in low tidal zones. Bacterial denitrification (>84%) was the dominant pathway, although hydroxylamine (NH2OH) oxidation/fungal denitrification contributed substantially to N2O production in high tidal flats. The contribution to N2O production exhibited a decrease in NH2OH oxidation/fungal denitrification and an increase in bacterial denitrification with decreasing elevation. Changes in N2O dynamics along the elevation gradient were affected by carbon and nitrogen substrate availabilities as well as the redox environments. Overall, our findings highlight the importance of N2O consumption in controlling N2O emission in intertidal wetlands, especially with higher inundation frequencies and durations.

Marine aquaculture regulates dissimilatory nitrate reduction processes in a typical semi-enclosed bay of southeastern China.

Marine aquaculture in semi-enclosed bays can significantly influence nutrient cycling in coastal ecosystems. However, the impact of marine aquaculture on the dynamics of dissimilatory nitrate reduction processes (DNRPs) and the fate of reactive nitrogen remain poorly understood. In this study, the rates of DNRPs and the abundances of related functional genes were investigated in aquaculture and non-aquaculture areas. The results showed that marine aquaculture significantly increased the denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) rates and decreased the rate of anaerobic ammonium oxidation (ANA), as compared with non-aquaculture sites. DNF was the dominant pathway contributing to the total nitrate reduction, and its contribution to the total nitrate reduction significantly increased from 66.72% at non-aquaculture sites to 78.50% at aquaculture sites. Marine aquaculture can significantly affect the physicochemical characteristics of sediment and the abundances of related functional genes, leading to variations in the nitrate reduction rates. Although nitrate removal rates increased in the marine aquaculture area, ammonification rates and the nitrogen retention index in the aquaculture areas were 2.19 and 1.24 times, respectively, higher than those at non-aquaculture sites. Net reactive nitrogen retention exceeded nitrogen removal in the aquaculture area, and the retained reactive nitrogen could diffuse with the tidal current to the entire bay, thereby aggravating N pollution in the entire study area. These results show that marine aquaculture is the dominant source of nitrogen pollution in semi-enclosed bays. This study can provide insights into nitrogen pollution control in semi-enclosed bays with well-developed marine aquaculture.

In situ nitrogen removal processes in intertidal wetlands of the Yangtze Estuary.

Estuarine and intertidal wetlands are important sites for nitrogen transformation and elimination. However, the factors controlling nitrogen removal processes remain largely uncertain in the highly dynamic environments. In this study, continuous-flow experiment combined with 15N isotope pairing technique was used to investigate in situ rates of denitrification and anaerobic ammonium oxidation (anammox) and their coupling with nitrification in intertidal wetlands of the Yangtze Estuary. The measured rates varied from below the detection limit to 152.39 µmol N/(m2·hr) for denitrification and from below the detection limit to 43.06 µmol N/(m2·hr) for anammox. The coupling links of nitrogen removal processes with nitrification were mainly dependent on nitrate, organic carbon, sulfide, dissolved oxygen and ferric iron in the estuarine and intertidal wetlands. Additionally, it was estimated that the actual nitrogen removal processes annually removed approximately 5% of the terrigenous inorganic nitrogen discharged into the Yangtze Estuary. This study gives new insights into nitrogen transformation and fate in the estuarine and intertidal wetlands.

Evidence for complete nitrification in enrichment culture of tidal sediments and diversity analysis of clade a comammox Nitrospira in natural environments.

Complete ammonia oxidizers (comammox), as novel microbial communities, are predicted to play an important role in the nitrogen cycle. Here we reported the presence of complete nitrification in tidal sediments and examined the diversity and abundance of comammox in natural ecosystems. Metagenome and metatranscriptome of the enrichment culture from tidal sediments harbored the genes of comammox. Near-complete comammox AmoA/B/C- and Hao-like sequences showed close relationships to the known comammox (with sequence identity from 79 to 99%) rather than classical betaproteobacterial ammonia-oxidizing bacteria (ß-AOB) (57 to 66%) and ammonia-oxidizing archaea (AOA) (24 to 38%). To analyze the diversity of comammox in natural environments, a new primer set targeting clade A comammox Nitrospira (COM-A) amoA genes was designed based on sequences obtained in this study and sequences from published database. In silico evaluation of the primers showed the high coverage of 89 and 100% in the COM-A amoA database. Application of the primers in six different ecosystems proved their strong availability. Community composition of COM-A suggested a relatively higher diversity than ß-AOB in similar environments. Quantification results showed that COM-A amoA genes accounted for about 0.4-5.6% in total amoA genes. These results provide novel insight into our perception of the enigmatic comammox and have significant implications for profound understanding of complex nitrification process.

Community composition and activity of anaerobic ammonium oxidation bacteria in the rhizosphere of salt-marsh grass Spartina alterniflora.

Anaerobic ammonium oxidation (anammox) as an important nitrogen removal pathway has been investigated in intertidal marshes. However, the rhizosphere-driven anammox process in these ecosystems is largely overlooked so far. In this study, the community dynamics and activities of anammox bacteria in the rhizosphere and non-rhizosphere sediments of salt-marsh grass Spartina alterniflora (a widely distributed plant in estuaries and intertidal ecosystems) were investigated using clone library analysis, quantitative PCR assay, and isotope-tracing technique. Phylogenetic analysis showed that anammox bacterial diversity was higher in the non-rhizosphere sediments (Scalindua and Kuenenia) compared with the rhizosphere zone (only Scalindua genus). Higher abundance of anammox bacteria was detected in the rhizosphere (6.46 × 10(6)-1.56 × 10(7) copies g(-1)), which was about 1.5-fold higher in comparison with that in the non-rhizosphere zone (4.22 × 10(6)-1.12 × 10(7) copies g(-1)). Nitrogen isotope-tracing experiments indicated that the anammox process in the rhizosphere contributed to 12-14 % N2 generation with rates of 0.43-1.58 nmol N g(-1) h(-1), while anammox activity in the non-rhizosphere zone contributed to only 4-7 % N2 production with significantly lower activities (0.28-0.83 nmol N g(-1) h(-1)). Overall, we propose that the rhizosphere microenvironment in intertidal marshes might provide a favorable niche for anammox bacteria and thus plays an important role in nitrogen cycling.

nirS-Encoding denitrifier community composition, distribution, and abundance along the coastal wetlands of China.

For the past few decades, human activities have intensively increased the reactive nitrogen enrichment in China's coastal wetlands. Although denitrification is a critical pathway of nitrogen removal, the understanding of denitrifier community dynamics driving denitrification remains limited in the coastal wetlands. In this study, the diversity, abundance, and community composition of nirS-encoding denitrifiers were analyzed to reveal their variations in China's coastal wetlands. Diverse nirS sequences were obtained and more than 98 % of them shared considerable phylogenetic similarity with sequences obtained from aquatic systems (marine/estuarine/coastal sediments and hypoxia sea water). Clone library analysis revealed that the distribution and composition of nirS-harboring denitrifiers had a significant latitudinal differentiation, but without a seasonal shift. Canonical correspondence analysis showed that the community structure of nirS-encoding denitrifiers was significantly related to temperature and ammonium concentration. The nirS gene abundance ranged from 4.3 × 10(5) to 3.7 × 10(7) copies g(-1) dry sediment, with a significant spatial heterogeneity. Among all detected environmental factors, temperature was a key factor affecting not only the nirS gene abundance but also the community structure of nirS-type denitrifiers. Overall, this study significantly enhances our understanding of the structure and dynamics of denitrifying communities in the coastal wetlands of China.

Diversity, Abundance, and Distribution of nirS-Harboring Denitrifiers in Intertidal Sediments of the Yangtze Estuary.

Denitrification plays a critical role in nitrogen removal in estuarine and coastal ecosystems. In this study, the community composition, diversity, abundance, and distribution of cytochrome cd1-type nitrite reductase gene (nirS)-harboring denitrifiers in intertidal sediments of the Yangtze Estuary were analyzed using polymerase chain reaction (PCR)-based clone libraries and quantitative PCR techniques. Clone library analysis showed that the nirS-encoding bacterial biodiversity was significantly higher at the lower salinity sites than at the higher salinity sites. However, there was no significant seasonal difference in the nirS gene diversity between summer and winter. Phylogenetic analysis revealed that the nirS-harboring denitrifier communities at the study area had distinctive spatial heterogeneity along the estuary. At the lower salinity sites, the nirS-harboring bacterial community was co-dominated by clusters III and VII; while at the higher salinity sites, it was dominated by cluster I. Canonical correspondence analysis indicated that the community compositions of nirS-type denitrifiers were significantly correlated with salinity, ammonium, and nitrate. Quantitative PCR results showed that the nirS gene abundance was in the range of 1.01 × 10(6) to 9.00 × 10(7) copies per gram dry sediment, without significant seasonal variation. Among all the environmental factors, the nirS gene abundance was only significantly related to the change of salinity. These results can extend our current knowledge about the composition and dynamics of denitrification microbial community in the estuarine ecosystem.

Evidence of Nitrogen Loss from Anaerobic Ammonium Oxidation Coupled with Ferric Iron Reduction in an Intertidal Wetland.

Anaerobic ammonium oxidation coupled with nitrite reduction is an important microbial pathway of nitrogen removal in intertidal wetlands. However, little is known about the role of anaerobic ammonium oxidation coupled with ferric iron reduction (termed Feammox) in intertidal nitrogen cycling. In this study, sediment slurry incubation experiments were combined with an isotope-tracing technique to examine the dynamics of Feammox and its association with tidal fluctuations in the intertidal wetland of the Yangtze Estuary. Feammox was detected in the intertidal wetland sediments, with potential rates of 0.24-0.36 mg N kg(-1) d(-1). The Feammox rates in the sediments were generally higher during spring tides than during neap tides. The tidal fluctuations affected the growth of iron-reducing bacteria and reduction of ferric iron, which mediated Feammox activity and the associated nitrogen loss from intertidal wetlands to the atmosphere. An estimated loss of 11.5-18 t N km(-2) year(-1) was linked to Feammox, accounting for approximately 3.1-4.9% of the total external inorganic nitrogen transported into the Yangtze Estuary wetland each year. Overall, the co-occurrence of ferric iron reduction and ammonium oxidation suggests that Feammox can act as an ammonium removal mechanism in intertidal wetlands.

Effects of sulfamethazine on denitrification and the associated N2O release in estuarine and coastal sediments.

Denitrification is an important pathway of nitrogen removal and nitrous oxide (N2O) production in estuarine and coastal ecosystems, and plays a significant role in counteracting aquatic eutrophication induced by excessive nitrogen loads. Estuarine and coastal environments also suffer from increasing antibiotic contamination because of the growing production and usage of antibiotics. In this study, sediment slurry incubation experiments were conducted to determine the influence of sulfamethazine (SMT, a sulphonamide antibiotic) on denitrification and the associated N2O production. Genes important for denitrification and antibiotic resistance were quantified to investigate the microbial physiological mechanisms underlying SMT's effects on denitrification. SMT was observed to significantly inhibit denitrification rates, but increasing concentrations of SMT enhanced N2O release rates. The negative exponential relationships between denitrifying gene abundances and SMT concentrations showed that SMT reduced denitrification rates by restricting the growth of denitrifying bacteria, although the presence of the antibiotic resistance gene was detected during the incubation period. These results imply that the wide occurrence of residual antibiotics in estuarine and coastal ecosystems may influence eutrophication control, greenhouse effects, and atmospheric ozone depletion by inhibiting denitrification and stimulating the release of N2O.

A novel membrane inlet mass spectrometer method to measure ¹5NH44⁺ for isotope-enrichment experiments in aquatic ecosystems.

Nitrogen (N) pollution in aquatic ecosystems has attracted much attention over the past decades, but the dynamics of this bioreactive element are difficult to measure in aquatic oxygen-transition environments. Nitrogen-transformation experiments often require measurement of (15)N-ammonium ((15)NH4(+)) ratios in small-volume (15)N-enriched samples. Published methods to determine N isotope ratios of dissolved ammonium require large samples and/or costly equipment and effort. We present a novel ("OX/MIMS") method to determine N isotope ratios for (15)NH4(+) in experimental waters previously enriched with (15)N compounds. Dissolved reduced (15)N (dominated by (15)NH4(+)) is oxidized with hypobromite iodine to nitrogen gas ((29)N2 and/or (30)N2) and analyzed by membrane inlet mass spectrometry (MIMS) to quantify (15)NH4(+) concentrations. The N isotope ratios, obtained by comparing the (15)NH4(+) to total ammonium (via autoanalyzer) concentrations, are compared to the ratios of prepared standards. The OX/MIMS method requires only small sample volumes of water (ca. 12 mL) or sediment slurries and is rapid, convenient, accurate, and precise (R(2) = 0.9994, p < 0.0001) over a range of salinities and (15)N/(14)N ratios. It can provide data needed to quantify rates of ammonium regeneration, potential ammonium uptake, and dissimilatory nitrate reduction to ammonium (DNRA). Isotope ratio results agreed closely (R = 0.998, P = 0.001) with those determined independently by isotope ratio mass spectrometry for DNRA measurements or by ammonium isotope retention time shift liquid chromatography for water-column N-cycling experiments. Application of OX/MIMS should simplify experimental approaches and improve understanding of N-cycling rates and fate in a variety of freshwater and marine environments.

Nitrogen input modulates the effects of coastal acidification on nitrification and associated N2O emission.

Acidification of coastal waters, synergistically driven by increasing atmospheric carbon dioxide (CO2) and intensive land-derived nutrient inputs, exerts significant stresses on the biogeochemical cycles of coastal ecosystem. However, the combined effects of anthropogenic nitrogen (N) inputs and aquatic acidification on nitrification, a critical process of N cycling, remains unclear in estuarine and coastal ecosystems. Here, we showed that increased loading of ammonium (NH4+) in estuarine and coastal waters alleviated the inhibitory effect of acidification on nitrification rates but intensified the production of the potent greenhouse gas nitrous oxide (N2O), thus accelerating global climate change. Metatranscriptomes and natural N2O isotopic signatures further suggested that the enhanced emission of N2O may mainly source from hydroxylamine (NH2OH) oxidation rather than from nitrite (NO2-) reduction pathway of nitrifying microbes. This study elucidates how anthropogenic N inputs regulate the effects of coastal acidification on nitrification and associated N2O emissions, thereby enhancing our ability to predict the feedbacks of estuarine and coastal ecosystems to climate change and human perturbations.

Urban agglomerations as an environmental dimension of antibiotics transmission through the "One Health" lens.

The spatiotemporal distributions of antibiotics in different media have been widely reported; however, their occurrence in the environmental dimension of the Chinese urban agglomerations has received less attention, especially in bioaccumulation and health risks of antibiotics through the "One Health" lens. The review presents the current knowledge on the environmental occurrence, bioaccumulation, as well as health exposure risks in urban agglomerations through the "One Health" lens, and identifies current information gaps. The reviewed studies suggested antibiotic concentrations in water and soil were more sensitive to social indicators of urban agglomerations than those in sediment. The ecological risk and resistance risk of antibiotics in water were much higher than those of sediments, and the high-risk phenomenon occurred at a higher frequency in urban agglomerations. Erythromycin-H2O (ETM-H2O), amoxicillin (AMOX) and norfloxacin (NFC) were priority-controlled antibiotics in urban waters. Tetracyclines (TCs) posed medium to high risks to soil organisms in the soil of urban agglomerations. Health risk evaluation based on dietary intake showed that children had the highest dietary intake of antibiotics in urban agglomerations. The health risk of antibiotics was higher in children than in other age groups. Our results also demonstrated that dietary structure might impact health risks associated with target antibiotics in urban agglomerations to some extent.

Active dark carbon fixation evidenced by 14C isotope assimilation and metagenomic data across the estuarine-coastal continuum.

Estuaries, as important land-ocean transitional zones across the Earth's surface, are hotspots of microbially driven dark carbon fixation (DCF), yet understanding of DCF process remains limited across the estuarine-coastal continuum. This study explored DCF activities and associated chemoautotrophs along the estuarine and coastal environmental gradients, using radiocarbon labelling and molecular techniques. Significantly higher DCF rates were observed at middle- and high-salinity regions (0.65-2.31 and 0.66-2.82 mmol C m-2 d-1, respectively), compared to low-salinity zone (0.07-0.19 mmol C m-2 d-1). Metagenomic analysis revealed relatively stable DCF pathways along the estuarine-coastal continuum, primarily dominated by Calvin-Benson-Bassham (CBB) cycle and Wood-Ljungdahl (WL) pathway. Nevertheless, chemoautotrophic communities driving DCF exhibited significant spatial variations. It is worth noting that although CBB cycle played an important role in DCF in estuarine sediments, WL pathway might play a more significant role, which has not been previously recognized. Overall, this study highlights that DCF activities coincide with the genetic potential of chemoautotrophy and the availability of reductive substrates across the estuarine-coastal continuum, and provides an important scientific basis for accurate quantitative assessment of global estuarine carbon sink.

Diversity, abundance, and activity of ammonia-oxidizing bacteria and archaea in Chongming eastern intertidal sediments.

Ammonia oxidation plays a pivotal role in the cycling and removal of nitrogen in aquatic sediments. Certain bacterial groups and a novel group of archaea, which is affiliated with the novel phylum Thaumarchaeota, can perform this initial nitrification step. We examined the diversity and abundance of ammonia-oxidizing ß-Proteobacteria (ß-AOB) and ammonia-oxidizing archaea (AOA) in the sediments of Chongming eastern tidal flat using the ammonia monooxygenase-α subunit (amoA) gene as functional markers. Clone library analysis showed that AOA had a higher diversity of amoA gene than ß-AOB. The ß-Proteobacterial amoA community composition correlated significantly with water soluble salts in the sediments, whereas the archaeal amoA community composition was correlated more with nitrate concentrations. Quantitative PCR (qPCR) results indicated that the abundance of ß-AOB amoA gene (9.11 × 10(4)-6.47 × 10(5) copies g(-1) sediment) was always greater than that of AOA amoA gene (7.98 × 10(3)-3.51 × 10(5) copies g(-1) sediment) in all the samples analyzed in this study. The ß-Proteobacterial amoA gene abundance was closely related to organic carbon, while no significant correlations were observed between archaeal amoA gene abundance and the environmental factors. Potential nitrification rates were significantly greater in summer than in winter and correlated strongly with the abundance of amoA genes. Additionally, a greater contribution of single amoA gene to potential nitrification occurred in summer (1.03-5.39 pmol N copy(-1) day(-1)) compared with winter (0.16-0.38 pmol N copy(-1) day(-1)), suggesting a higher activity of ammonia-oxidizing prokaryotes in warm seasons.