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Denitrification is an important pathway for reactive nitrogen removal from aquatic ecosystems. In this study, the biodiversity, abundance, and activity of cytochrome cd 1 -type nitrate reductase gene (nirS)-harboring denitrifiers in the sediments of the Indus River Estuary were examined by molecular and isotope-tracing techniques. Results showed that the nirS -harboring denitrifier communities showed significant geographical variations along the estuarine salinity gradient. Real-time quantitative PCR showed that the abundance of nirS -harboring denitrifiers ranged from 5.3 × 106 to 2.5 × 108 copies g−1, without significant spatiotemporal variation. The potential rates of denitrification varied from 0.01 to 6.27 μmol N kg−1 h−1 and correlated significantly to TOC and Fe(II) (P < 0.05). On the basis of 15N isotope-tracing experiments, the denitrification process contributed 18.4–99.4% to the total nitrogen loss in the sediments of the Indus River Estuary. This study provides novel insights into the microbial mechanism of nitrogen removal process in estuarine ecosystems. Unlabelled Image • nirS -type denitrifier community had significant spatial variation along the estuary. • Abundance of nirS -type denitrifier ranged from 5.3 × 106 to 2.5 × 108 copies g−1. • The potential rates of denitrification varied from 0.01 to 6.27 μmol N kg−1 h−1. • Denitrification contributed up to 99% to total N loss in the Indus River Estuary. [ABSTRACT FROM AUTHOR]

Denitrification is a dominant reactive nitrogen removal pathway in most estuarine and coastal ecosystems, and plays a significant role in regulating N2O release. Although multiple antibiotics residues are widely detected in aquatic environment, combined effects of antibiotics on denitrification remain indistinct. In this work, 5 classes of antibiotics (sulfonamides, chloramphenicols, tetracyclines, macrolides, and fluoroquinolones) were selected to conduct orthogonal experiments in order to explore their combined effects on denitrification. 15N-based denitrification and N2O release rates were determined in the orthogonal experiments, while denitrifying functional genes were examined to illustrate the microbial mechanism of the combined antibiotics effect. Denitrification rates were inhibited by antibiotics treatments, and synergistic inhibition effect was observed for multiple antibiotics exposure. Different classes of antibiotics had different influence on N2O release rates, but multiple antibiotics exposure mostly led to stimulatory effect. Abundances of denitrifying functional genes were inhibited by multiple antibiotics exposure due to the antimicrobial properties, and different inhibition on denitrifiers may be the major mechanism for the variations of N2O release rates. Combined effects of antibiotics on denitrification may lead to nitrate retention and N2O release in estuarine and coastal ecosystems, and consequently cause cascading environmental problems, such as greenhouse effects and hyper-eutrophication. [ABSTRACT FROM AUTHOR]

Anaerobic ammonium oxidation (anammox) is an important bioprocess for nitrogen removal and has been studied in estuarine environments. However, knowledge on anammox bacterial community dynamics and related controlling factors remains limited in these ecosystems. In this study, the community compositions, abundance, and activity of anammox bacteria in the surface sediments from the Indus Estuary were investigated along a salinity gradient, considering the links between the anammox bacterial community dynamics and environmental variables. The potential importance of anammox was also estimated for nitrogen removal. High anammox bacterial diversity was detected in the sediments of the Indus Estuary, including Kuenenia , Brocadia , Scalindua , Jettenia , and a novel anammox-like cluster. Kuenenia was identified as the dominant anammox bacteria in most samples. Anammox bacterial diversity was significantly correlated with sediment NO 3 −, while the distribution of anammox bacterial community was significantly related to temperature and sediment sulfide (P < 0.05). The anammox bacterial abundance based on the 16S rRNA gene varied between 1.64 × 106 copies g−1 and 8.21 × 108 copies g−1, and was significantly correlated with sediment Fe(II). Based on an 15N isotope-tracing technique, potential anammox rates were found in the range 0.01–0.32 μ mol N kg−1 h−1, and were controlled mainly by salinity, Fe(II), and TOC. It was estimated that the anammox bacteria contributed about 21.9% to the total nitrogen loss, on average. These results show the importance of anammox bacteria for nitrogen transformation and removal in estuarine and coastal environments. Image 1 • High diversity of anammox bacteria was detected in the estuarine environments. • Anammox bacterial diversity was associated with sediment NO 3 − • Distribution of anammox bacteria was related to temperature and sediment sulfide. • Anammox bacteria played an important role in nitrogen removal. [ABSTRACT FROM AUTHOR]

Acidification of coastal waters, synergistically driven by increasing atmospheric carbon dioxide (CO 2 ) 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 (NH 4 + ) 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 (N 2 O), thus accelerating global climate change. Metatranscriptomes and natural N 2 O isotopic signatures further suggested that the enhanced emission of N 2 O may mainly source from hydroxylamine (NH 2 OH) oxidation rather than from nitrite (NO 2 - ) reduction pathway of nitrifying microbes. This study elucidates how anthropogenic N inputs regulate the effects of coastal acidification on nitrification and associated N 2 O emissions, thereby enhancing our ability to predict the feedbacks of estuarine and coastal ecosystems to climate change and human perturbations., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

The widespread spread of antibiotics in the environment poses a growing threat to human health. This study investigated the distribution and fate of antibiotics concerning land use characteristics, hydrological conditions, and spatial contiguity within a megacity river network. Temporally, the average concentrations of twenty antibiotics in water (354 ng/L), suspended particulate matter (SPM) (46 ng/L), and sediment (151 ng/g) during dry season were notably higher than that in the corresponding environment media (water: 127 ng/L, SPM: 2 ng/L, and sediment: 49 ng/g) during the wet season. Moreover, the inter-annual variation of antibiotics in water showed a decreasing trend. Spatially, substantial antibiotic contamination was observed in a human-intensive watershed, particularly in the upstream and central city sections. The macrolides in water were most affected by land use types and hydrological processes. Antibiotic contamination in water exhibited a stronger spatial autocorrelation compared to other media. Nevertheless, the interconnectedness of antibiotic contamination in sediments during the wet season warrants attention, and relevant authorities should enhance environmental monitoring in watersheds with pollution hotspots. Certain antibiotics, such as sulfamethoxazole, enrofloxacin, and florfenicol, were transported via urban rivers to the ocean, potentially posing environmental risks to coastal water quality. Local sources accounted for the predominant portion (>50 %) of most antibiotics in various media. The correlation distances of antibiotics in waters during the wet season could screen ecological risk prioritization in aquatic environments., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Global warming and acidification, induced by a substantial increase in anthropogenic CO 2 emissions, are expected to have profound impacts on biogeochemical cycles. However, underlying mechanisms of nitrous oxide (N 2 O) production in estuarine and coastal sediments remain rarely constrained under warming and acidification. Here, the responses of sediment N 2 O production pathways to warming and acidification were examined using a series of anoxic incubation experiments. Denitrification and N 2 O production were largely stimulated by the warming, while N 2 O 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 N 2 O production by 26 ± 4%, which was mainly attributed to the decline of the N 2 O yield by fungal denitrification. Fungal denitrification was mainly responsible for N 2 O production under the warming condition, while bacterial denitrification predominated N 2 O production under the acidification condition. The reduced site preference of N 2 O under acidification reflects that the dominant pathways of N 2 O 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 N 2 O production. Collectively, acidification can decrease sediment N 2 O yield through shifting the responsible production pathways, partly counteracting the warming-induced increase in N 2 O emissions, further reducing the positive climate warming feedback loop.

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-H 2 O (ETM-H 2 O), 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., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

In the context of an increasing atmospheric carbon dioxide (CO 2 ) level, acidification of estuarine and coastal waters is greatly exacerbated by land-derived nutrient inputs, coastal upwelling, and complex biogeochemical processes. A deeper understanding of how nitrifiers respond to intensifying acidification is thus crucial to predict the response of estuarine and coastal ecosystems and their contribution to global climate change. Here, we show that acidification can significantly decrease nitrification rate but stimulate generation of byproduct nitrous oxide (N 2 O) in estuarine and coastal waters. By varying CO 2 concentration and pH independently, an expected beneficial effect of elevated CO 2 on activity of nitrifiers ("CO 2 -fertilization" effect) is excluded under acidification. Metatranscriptome data further demonstrate that nitrifiers could significantly up-regulate gene expressions associated with intracellular pH homeostasis to cope with acidification stress. This study highlights the molecular underpinnings of acidification effects on nitrification and associated greenhouse gas N 2 O emission, and helps predict the response and evolution of estuarine and coastal ecosystems under climate change and human activities., (© 2023. The Author(s).)

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

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.

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.