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Global warming and acidification, induced by a substantial increase in anthropogenic CO2emissions, 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.

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 CH4consumption. However, the role of n-DAMO in reducing CH4emissions 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 mariqueterand Spartina alterniflora) than those in bare mudflats (P< 0.05), leading to an enhanced contribution to CH4consumption. Compared with other habitats, the contribution of n-DAMO to the total anaerobic CH4oxidation was significantly lower in the Phragmites australiswetland (15.0%), where the anaerobic CH4oxidation 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.

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.

Dark carbon fixation (DCF), conducted mainly by chemoautotrophs, contributes greatly to primary production and the global carbon budget. Understanding the response of DCF process to climate warming in coastal wetlands is of great significance for model optimization and climate change prediction. Here, based on a 4-yr field warming experiment (average annual temperature increase of 1.5°C), DCF rates were observed to be significantly inhibited by warming in coastal wetlands (average annual DCF decline of 21.6%, and estimated annual loss of 0.08–1.5 Tg C yr−1in global coastal marshes), thus causing a positive climate feedback. Under climate warming, chemoautotrophic microbial abundance and biodiversity, which were jointly affected by environmental changes such as soil organic carbon and water content, were recognized as significant drivers directly affecting DCF rates. Metagenomic analysis further revealed that climate warming may alter the pattern of DCF carbon sequestration pathways in coastal wetlands, increasing the relative importance of the 3-hydroxypropionate/4-hydroxybutyrate cycle, whereas the relative importance of the dominant chemoautotrophic carbon fixation pathways (Calvin–Benson–Bassham cycle and W-L pathway) may decrease due to warming stress. Collectively, our work uncovers the feedback mechanism of microbially mediated DCF to climate warming in coastal wetlands, and emphasizes a decrease in carbon sequestration through DCF activities in this globally important ecosystem under a warming climate.Graphical Abstract

Ammonia oxidation, the first and rate‐limiting step in nitrification, plays a critical role in the nitrogen cycle. However, the links between the dynamics of ammonia‐oxidizing communities and ecosystem processes along the estuarine salinity gradient remain uncertain. In this study, we examined the diversity, abundance, and community structure of ammonia‐oxidizing prokaryotes, and the potential nitrification rates along the Yangtze estuarine salinity gradient. Phylogenetic analysis showed that the predominant ammonia‐oxidizing bacteria (AOB) and ammonia‐oxidizing archaea (AOA) fell within the Nitrosospiraand Nitrosopumilusclusters, respectively. The AOB amoAgene abundance (4.67 × 105to 3.90 × 107copies per gram of dry sediment) outnumbered AOA (5.14 × 104to 8.88 × 106copies per gram of dry sediment). The potential nitrification rates varied between 0.13 and 0.63 μg N·g−1·day−1and related only to AOA amoAgene abundance. Salinity had significant effects on AOA amoAgene abundance, nitrification rates, and the community structure of ammonia‐oxidizing prokaryotes. Principal coordinate analysis showed that the AOB amoAgene clones derived from the middle‐ and high‐salinity regions behaved as a cohesive group, while all the low‐salinity clone libraries were grouped together. Moreover, the distribution of AOA communities showed a distinct salinity differentiation. Overall, this study improves the understanding of the dynamic shifts in ammonia‐oxidizing microorganisms in the Yangtze Estuary. Significant shifts in ammonia‐oxidizing community structure were found along the salinity gradientBoth of AOA amoAgene abundance and potential nitrification rates were significantly and negatively correlated with salinityPotential nitrification rates were significantly related to AOA amoAgene abundance, but not with AOB

Nitrogen fixation is a microbial‐mediated process converting atmospheric dinitrogen gas to biologically available ammonia or other molecules, and it plays an important role in regulating nitrogen budgets in coastal marine ecosystems. In this study, nitrogen fixation in the intertidal sediments of the Yangtze Estuary was investigated using nitrogen isotope tracing technique. The abundance of nitrogen fixation functional gene (nifH) was also quantified. The measured rates of sediment nitrogen fixation ranged from 0.37 to 7.91 nmol N g−1hr−1, while the abundance of nifHgene varied from 2.28 × 106to 1.28 × 108copies g−1in the study area. The benthic nitrogen fixation was correlated closely to the abundance of nifHgene and was affected significantly by salinity, pH, and availability of sediment organic carbon and ammonium. It is estimated that sediment nitrogen fixation contributed approximately 9.3% of the total terrigenous inorganic nitrogen transported annually into the Yangtze estuarine and coastal environment. This result implies that the occurrence of benthic nitrogen fixation acts as an important internal source of reactive nitrogen and to some extent exacerbates nitrogen pollution in this aquatic ecosystem. Nitrogen fixation in the intertidal sediments is associated tightly with the abundance of nifHgeneNitrogen fixation is greatly enhanced by high availability of organic matter, but inhibited by high ammonium concentrationsNitrogen fixation may be coupled to activity of sulfate‐reducing bacteria

Dissimilatory nitrate reduction to ammonium (DNRA) plays an important role in regulating the fate of reactive nitrogen in estuarine and coastal ecosystems. In this work, intertidal sediments of the Yangtze Estuary were collected in January and August 2015. Potential rates of DNRA and associated functional gene were investigated with nitrogen isotope‐tracing and molecular techniques. The measured DNRA rates ranged from 0.14 to 5.57 μmol 15N kg−1h−1in the intertidal sediments. DNRA rates were tightly related to abundance of nrfAgene (p< 0.001), demonstrating that fermentation reaction may be the dominant pathway of DNRA in the study area. Redundancy analysis (RDA) showed a relationship between DNRA and organic matter and NO2−, suggesting that these substrates stimulated the metabolism of DNRA microorganisms. On the other hand, the correlation between abundances of nrfAgene and Fe2+and sulfide in the RDA analysis implied that oxidation of both Fe2+and sulfide can enhance fermentative DNRA by providing extra free energy. DNRA converted approximately 2.29 × 105t of nitrate to ammonia annually in the sampling area of the Yangtze Estuary, and most of the converted ammonium was retained in the estuarine ecosystem. DNRA may further contribute to eutrophication in the Yangtze Estuary and also in the other hypereutrophic estuaries. Fermentation reaction is the dominant pathway of DNRA in the Yangtze EstuaryOrganic matter and NO2−stimulated the metabolism of DNRA microorganismsOxidation of Fe2+and sulfide can enhance fermentative DNRA by providing extra free energy

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–2d–1in 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.

Background: Connective tissue growth factor (CTGF)-induced angiogenesis is a crucial factor in rheumatoid arthritis (RA), but CTGF-interacting protein and related molecular mechanism of their interaction have not been fully elucidated. Methods: CTGF-interacting proteins were identified through the LC-MS/MS analysis of the Co-IP products from fibroblast-like synoviocyte (FLS) lysates, and the interaction between CTGF and annexin A2 (ANXA2) was further confirmed through Co-IP and BiFC assay. The binding domain, mutant, mechanism, and angiogenesis function were assessed by homology modeling, molecular docking, MTT, cell scratch, tube formation, and chick chorioallantoic membrane (CAM) assays. Additionally, severe combined immunodeficiency (SCID) mouse co-implantation model was constructed to confirm the effect of ANXA2/CTGF-TSP1 in the process of RA in vivo. Results: ANXA2 was identified and verified as an interaction partner of CTGF for the first time by Co-IP and LC-MS/MS analysis. Co-localization of CTGF and ANXA2 was observed in RA-FLS, and direct interaction of the TSP-1 domain of CTGF and ANXA2 was determined in HEK293T cells. The spatial conformation and stable combination of the ANXA2/CTGF-TSP1 complex were assessed by homology modeling in the biomimetic environment. The function of the ANXA2/CTGF-TSP1 complex was proved on promoting FLS proliferation, migration, and angiogenesis in vitro and deteriorating FLS invasion and joint damage in SCID mice. Conclusions: TSP-1 is the essential domain in CTGF/ANXA2 interaction and contributes to FLS migration and pannus formation, inducing the process of RA.