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The bioavailable nitrogen fixed by diazotrophs is critical for sustaining productivity in the oligotrophic ocean. Despite this, understanding how diazotroph-derived nitrogen integrates into the nitrogen cycle within the euphotic zone remains unknown. Here, we investigated nitrogen fixation rates in the particulate and dissolved fractions within the euphotic zone of the North Pacific Subtropical Gyre. Our findings reveal the proportion of nitrogen fixation rates in the dissolved fraction increases with depth. Light manipulation experiments uncover that reduced light levels can stimulate the net release of diazotroph-derived nitrogen, aligning with our depth-related observations. Furthermore, we identify two distinct transfer pathways vertically associated with light-driven ecological niches. Specifically, the released diazotroph-derived nitrogen is transferred to non-diazotrophic plankton in the upper layers. Meanwhile, in the lower layers, it contributes to the nitrification process. Our results underscore the high bioavailability of diazotroph-derived nitrogen and its rapid integration into the nitrogen cycle through multiple pathways within the well-lit ocean. New study finds diazotroph-derived nitrogen is utilized by phytoplankton in upper euphotic zone, while below, it is used by nitrifiers. The newly identified pathways reveal the marine nitrogen cycle may be more interconnected than previously recognized. [ABSTRACT FROM AUTHOR]

Ammonia-oxidizing archaea (AOA) are ubiquitously found in diverse habitats and play pivotal roles in the nitrogen and carbon cycle, especially in estuarine and coastal environments. Despite the fact that the diversity and distribution of AOA are thought to be tightly linked to habitats, little is known about the relationship that underpins their genomic traits, adaptive potentials, and ecological niches. Here, we have characterized and compared the AOA community in three estuaries of China using metagenomics. AOA were the dominant ammonia oxidizers in the three estuaries. Through phylogenetic analyses, five major AOA groups were identified, including the Nitrosomarinus-like, Nitrosopumilus-like, Aestuariumsis-like, Nitrosarchaeum-like, and Nitrosopelagicus-like groups. Statistical analyses showed that the aquatic and sedimentary AOA communities were mainly influenced by spatial factors (latitude and water depth) and environmental factors (salinity, pH, and dissolved oxygen) in estuaries, respectively. Compared to AOA dwelling in terrestrial and marine habitats, estuarine AOA encoded more genes involved in glucose and amino acid metabolism, transport systems, osmotic control, and cell motility. The low proteome isoelectric points (pI), high content of acidic amino acids, and the presence of potassium ion and mechanosensitive channels suggest a "salt-in" strategy for estuarine AOA to counteract high osmolarity in their surroundings. Our findings have indicated potential adaptation strategies and highlighted their importance in the estuarine nitrogen and carbon cycles. Key points: • Spatial and environmental factors influence water and sediment AOA respectively. • Estuarine AOA share low proteome isoelectric value and high acid amino acids content. • AOA adaptation to estuaries is likely resulted from their unique genomic features. [ABSTRACT FROM AUTHOR]

The heavily human-perturbed coastal oceans are hotspots of nitrous oxide (N2O) emission to the atmosphere. The processes underpinning the N2O flux, however, remain poorly understood, leading to large uncertainties in assessing global N2O budgets. Using a suite of nitrogen isotope labeling experiments, we show that multiple processes contribute to N2O production throughout the estuarine-coastal gradient, sustaining intensive N2O flux to the atmosphere. Unexpectedly, denitrification, rather than ammonia oxidation as previously assumed, constitutes the major source of N2O in well-oxygenated coastal waters. Size-fractionated manipulation experiments with gene analysis further reveal niche partitioning of ammonia oxidizers and denitrifiers across the particle size spectrum; denitrification dominated on large particles and ammonia oxidizers on small particles. Total N2O production rate increases with substrate and particle concentrations, suggesting a crucial interplay between nutrients and particles in controlling N2O production. The controlling factors identified here may help understand climate feedback mechanisms between human activity and coastal oceans.Incomplete denitrification associated with the micro-niche of marine particles, instead of nitrification as previously assumed, is a major source of N2O in the turbid coastal waters, even though the water is well-oxygenated. [ABSTRACT FROM AUTHOR]

Nutrient stoichiometry (e.g., nitrate + nitrite to soluble reactive phosphorus, refer to N + N/SRP, N/P hereafter) governs growth, competition and niche partitioning of phytoplankton in the illuminated oceans. The N/P, however, varies widely across the ocean and the underlying mechanisms remain unclear. Here, we report direct observations of significant variations in N/P in response to different life stages of two cyclonic eddies observed in the western South China Sea. High N/P (19.1 ± 6.9) values were observed around the nitracline in a mature-stage eddy, whereas a decay-stage eddy was characterized with low N/P (14.4 ± 4.1). The elevated N/P ratios accompanied by enriched fucoxanthin (pigment for diatom) and biogenic silica around the nitracline suggest that eddy pumping enhanced the growth of diatom which preferentially uptakes P relative to N in the mature stage of the eddy. Such high N/P ratios in the upper ocean could be reproduced if diatom uptake ratio was set between 10 and 16 in a data constrained numerical model. The preferential P uptake by enhanced diatom growth might reduce the P supply to the surface ocean, which is critical for N2-fixers. The transient changes in nutrient stoichiometry associated within the life cycle of cyclonic eddies also challenges the parameterization of physical–biogeochemical models with fixed phytoplankton uptake stoichiometry ratios, which could lead to bias of the model output for phytoplankton dynamics in oligotrophic ocean, where eddies frequently occur. [ABSTRACT FROM AUTHOR]

Dissolved black carbon (DBC) is the largest known refractory organic carbon pool in the ocean. However, processes in estuaries connecting rivers (a significant DBC source) and oceans are underexplored. We carried out two sampling campaigns in the Jiulong River estuary (JRE) and conducted a dissolved organic matter (DOM) mixing experiment. DBC was quantified by the benzenepolycarboxylic acid (noted as DBC-BPCA) method and characterized with Fourier transform ion cyclotron resonance mass spectrometry (noted as DBC-FT). The molecular formulae of DBC-FT in the JRE were also compared with that in soil and atmospheric deposition. Overall, DBC-BPCA was removed during seaward dispersion. Soil- and deposition-derived molecules were identified in the DBC-FT of the JRE, confirming deposition inputs to estuarine DBC for the first time. The salinity-dependent decrease in the intensity normalized modified aromatic index (AImod) of DBC-FT and in DBC-BPCA revealed that DBC composition and concentration were primarily controlled by mixing. However, photodegradation and particle adsorption/flocculation may play a role in removing DBC depending on the concentration of total suspended matter (TSM). The two campaigns showed that high discharge increased both riverine input and estuarine output of DBC. However, the removal patterns of DBC-BPCA were minimally influenced by discharge. We further estimated that approximately 20% DBC-BPCA was lost in this estuary annually. Future studies should investigate the transformation of DBC in coastal bays and the open ocean where the residence time is longer and the transparency of bulk water is higher. [ABSTRACT FROM AUTHOR]

The response of phytoplankton and its satellite bacteria to various concentrations (0.01%–10% v/v) of ethanol is studied. To elucidate the effect of ethanol, single-strains of phytoplankton (SSP) culture, pure strains of satellite bacteria isolated from nonaxenic SSP cultures, and Escherichia coli were screened. Results indicate that ethanol could promote the growth and photosynthetic efficiency (Fv/Fm) of SSP at 0.01% and the growth of satellite bacteria at 0.01%–1%. Nevertheless, ethanol inhibited the growth and Fv/Fm of SSP at 0.1%–1%, and killed bacteria and SSP at 10% concentration. Further investigation on a satellite bacterium (Mameliella alba) revealed that ethanol promotes growth by serving as a growth stimulant rather than a metabolic carbon source. The 16S rRNA gene amplicon indicated that all nonaxenic SSP cultures harbor distinct satellite bacteria communities where the SSP culture of Skeletonema costatum, Phaeodactylum tricornutum, and Dunaliella bardawil were dominated by bacteria genera of Marivita (∼80%), Dinoroseobacter (∼47%), and Halomonas (∼87%), respectively, indicating that every SSP cultures have their own distinct satellite bacterial community. The bacteria family Rhodobacteraceae was dominant in the two marine diatoms, whereas Halomonadaceae was dominant in the saline green microalga. Compared to their respective controls, the supply of 0.5% ethanol to SSP cultures promoted the growth of the satellite bacteria but did not cause a significant difference in species composition of satellite bacteria. Therefore, a low concentration of ethanol can promote the growth of bacteria in a non-selective way. This study enriched our knowledge about the effect of ethanol on aquatic microbes and provided a baseline for basic and applied biotechnological research in the aquatic environment in the future. [ABSTRACT FROM AUTHOR]

Mangrove species are undergoing environmental changes from nutrient-poor to enrichment due to the large nutrient input. The potential difference in adaptive strategies between the slow- and fast-growing species may lead to great changes in species interaction and ecosystem stability. This study aims to test whether the slow-growing species Aegiceras corniculatum (L.) Blanco and Kandelia obovata Sheue, Liu & Yong sp. nov. are distinctly different from a fast-growing species Laguncularia racemosa (L.) Gaertn. f. in response to soil nutrient enrichment. With the increase of soil nutrients, L. racemosa shifted from a more conservative to a more acquisitive strategy. The potential causes included the increases in specific leaf area, nutrient resorption efficiency, and photosynthetic capacity as indicated by the increase of leaf δ13C and unchanged leaf succulence, as well as the relocation of photosynthetic products as indicated by the shift toward fast-growing at the cost of constructive and defense compounds. In contrast, A. corniculatum and K. obovata maintained conservative strategies at any soil nutrient levels with only a slight increase in growth. These findings implied that L. racemosa will be more competitive over the slow-growing species in nutrient-rich soils through altering adaptive strategies. [ABSTRACT FROM AUTHOR]

In the context of continuously increasing anthropogenic nitrogen inputs, knowledge of how ammonia oxidation (AO) in the ocean responds to warming is crucial to predicting future changes in marine nitrogen biogeochemistry. Here, we show divergent thermal response patterns for marine AO across a wide onshore/offshore trophic gradient. We find ammonia oxidizer community and ambient substrate co-regulate optimum temperatures (Topt), generating distinct thermal response patterns with Topt varying from ≤14 °C to ≥34 °C. Substrate addition elevates Topt when ambient substrate is unsaturated. The thermal sensitivity of kinetic parameters allows us to predict responses of both AO rate and Topt at varying substrate and temperature below the critical temperature. A warming ocean promotes nearshore AO, while suppressing offshore AO. Our findings reconcile field inconsistencies of temperature effects on AO, suggesting that predictive biogeochemical models need to include such differential warming mechanisms on this key nitrogen cycle process. Microbial ammonia oxidation is important in marine nutrient cycling and greenhouse gas dynamics, but the responses to ocean warming are unclear. Here coast to open ocean incubations show that projected year 2100 temperatures might be too hot for these microbes in oligotrophic regions to handle, but may facilitate oxidation rates in coastal waters. [ABSTRACT FROM AUTHOR]

Methane, a major greenhouse gas, plays an important role in global carbon cycling and climate change. Methanogenesis is identified as an important process for methane formation in estuarine sediments. However, the metabolism of methane in the water columns of estuaries is not well understood. The goal of this research was to examine the dynamics in abundance and community composition of methanogens and methanotrophs, and to examine whether and how they take part in methane metabolism in the water columns from the lower Pearl River (freshwater) to the coastal South China Sea (seawater). Quantitative PCR (qPCR) and high-throughput sequencing results showed that the abundance of methanogens decreased with increasing salinity, suggesting that growth of these methanogens in the Pearl River Estuary may be influenced by high salinity. Also, the methane concentration in surface waters was lower than that in near-bottom waters at most sites, suggesting sediment methanogens are a likely source of methane. In the estuarine mixing zone, significantly high methane concentrations existed with the presence of salt-tolerant methanogens (e.g., Methanomicrobiaceae, Methanocella, Methanosaeta and Methanobacterium) and methanotrophs (e.g., Methylocystis and Methylococcaceae), which were found in brackish habitats. Furthermore, a number of methanotrophic OTUs (from pmoA gene sequence data) had specific positive correlations with methanogenic OTUs (from mcrA gene sequence data), and some of these methanogenic OTUs were correlated with concentrations of particulate organic carbon (POC). The results indicate that methanotrophs and methanogens may be intimately linked in methane metabolism attached with particles in estuarine waters. [ABSTRACT FROM AUTHOR]

The land-to-ocean export and subsequent preservation of terrestrial particulate organic carbon (POC) in oceans represents a major carbon sink in the global carbon cycle. Small mountainous rivers (SMRs) are hotspots of this process; however, how the sources and fluxes of POC change during rain events in low-gradient SMRs and how they compare with those in steep SMRs are unknown. Here, we combined the bulk properties and lignin phenols to trace changes in POC sources and fluxes in a low-gradient SMR (stream gradient: 0.6%) in southeastern China. Our results were further compared with other SMRs to understand the geomorphological influences on the POC export from SMRs. The results indicated that POC sources changed from aquatic organisms and deep soils during low-flow periods to surface soils and plant debris during high-flow periods. The lignin concentration showed a significant positive relationship with the runoff, suggesting that hydrology played an important role in exporting terrigenous POC in low-gradient SMRs. We further estimated that approximately 22%, 56% and 79% of annual water, POC and particulate lignin were exported during high-flow periods, respectively. Furthermore, we found that with increasing water discharge, the magnitude of the lignin increase was greater in SMRs with smaller stream gradients. Our results suggest that the export of terrigenous POC from low-gradient SMRs is driven by hydrology and is more sensitive to rain events than that in steep SMRs. [ABSTRACT FROM AUTHOR]

The dynamics and behaviors of streamwater chemistry are rarely documented for subtropical small mountainous rivers. A 1-year detailed time series of streamwater chemistry, using non-typhoon and typhoon samples, was monitored in two watersheds, with and without cultivation, in central Taiwan. Rainwater, soil leachate, and well water were supplemented to explain the streamwater chemistry. The concentrations of fluoride, chloride, sulfate, magnesium, potassium, calcium, strontium, silicon, and barium of all the water samples were measured. Principal component analysis and residual analysis were applied to examine the mechanisms of solute transport and investigate possible sources contributing to the streamwater chemistry. In addition to the influence of well water and soil leachate on streamwater chemistry during non-typhoon period, overland flow and surface erosion affect streamwater chemistry during the typhoon period. The latter has not been discussed in previous studies. Surface erosion is likely to be an end member and non-conservatively mixed with other end members, resulting in a previously unobserved blank zone in the mixing space. This has a particularly great impact on small mountainous watersheds, which suffer from rapid erosion. Moreover, fertilizer contaminates agricultural soil, making soil water end members more identifiable. To our knowledge, this study is the first to clearly illustrate the dynamics and sources of streamwater chemistry of small mountainous rivers that are analogous to rivers in Oceania. [ABSTRACT FROM AUTHOR]

Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Correction to: Biogeochemistry https://doi.org/10.1007/s10533-022-00987-9 The percentage symbol in the equation was not correctly typed, it was changed from % to ‰. [Extracted from the article]

The Danshuei River flows through the heavily populated metropolitan area of Taipei and New Taipei cities, which causes remarkable additions of nutrient elements. In spite of the rather short residence time of water, the Danshuei estuary is distinctive for the very high ammonium concentration and extensive hypoxia in its lower reach. Because particulate organic matter (POM) is potentially the culprit of hypoxia, we investigate the isotopic characteristics of POM collected in February and July 2009 at a fixed station over four semidiurnal tidal cycles. By using nitrogen isotopic composition and C/N ratio of POM, we derive the relative contributions of POM from different sources. One potential source that combines dead and living phytoplankton, phytodetritus, has δN values that can be predicted by the δN of ammonium and the isotope effect during ammonium uptake; however, the isotope effect is concentration dependent. We employ a three-end-member mixing model based on δN and C/N ratio to calculate the fractional contributions from three major POM sources, i.e., phytodetritus, soil, and sediment. Sensitivity test was conducted for the derivations from both carbon and nitrogen basis. For February 2009 we found the three fractions (in terms of contribution to the particulate organic carbon) to be 45 ± 19, 10 ± 11 and 45 ± 13 %, respectively; for July 2009, 71 ± 18, 11 ± 10 and 18 ± 13 %, respectively. The results imply that phytodetritus is probably the major culprit for the hypoxic conditions in the estuary, especially, in summer. [ABSTRACT FROM AUTHOR]

This study analyzed the concentration and stable nitrogen (δN) and oxygen (δO) isotopic compositions of water NO, as well as NOconcentration and δN values of soils and manure-sourced fertilizers to assess pollution and variation in stream nitrate at the watershed of the Chi-Chia-Wan Stream (CCWS), a protected high-mountain stream in Central Taiwan. Results indicate a gully (G1) that contributes significantly high NO concentration water (up to 122 mg/L) to trunk water as the major pollution source of CCWS. The high NO concentration gully water has a close relationship with manure-sourced fertilizer with both having compatible enriched δN values. Results also indicate that water mixing over isotopic fractionation processes such as denitrification or assimilation is the major process accounting for variations in concentrations and isotopic values for stream NO. Incorporation of gully/tributary water of high NO concentration increases both the concentration and isotopic values of trunk water and vice versa for the incorporation of low NO concentration tributary water. Despite G1 contributing high NO concentration water to the trunk water of CCWS, the concentration of the trunk water is only slightly elevated and is still lower than the required water quality standard due to much lower drainage of the gully water compared to trunk water's runoff. In addition to gully or tributary water and rainwater, NO derived from soil is another important contributor to trunk water. The NO contribution of soil to trunk water is greater in summer than in winter. Additionally, NO concentrations in soil from ex-cultivated land are significantly lower than that in cultivated land. This means that NO contribution from ex-cultivated land soil to trunk water is small and demonstrates that the land-recovery plan that has been underway in the studied watershed for sometime is effective. [ABSTRACT FROM AUTHOR]

Due to anthropogenic disturbance such as mass road construction, the sediment yield in the Lanyang River, northeastern Taiwan has been reported to have increased substantially since 1976. Water samples were collected from the river on three occasions, and total suspended matter (TSM) was analyzed for aliphatic hydrocarbons and alkanols/sterols. Results show that the distribution of aliphatic hydrocarbons in the TSM is rather complicated during periods of normal flow, but is relatively simple during high flow and after a period of an extraordinarily high, daily mean discharge. Carbon preference index (CPI) values of n-alkanes (C25–C33) are comparatively low, ranging from 1.14 to 2.35, with an average of 1.62, when compared with a control station (CPI=5.88). Because there has been no anthropogenic fossil-fuel contamination in the study area in the past, the present results indicate a predominant hydrocarbon contribution from the old lipids derived from the bedrock (argillite-slate and meta-sandstone) due to massive road construction in 1975–1980, with a small contribution from modern vegetation. By using a two-end-member assumption, the percentage of old hydrocarbons in the TSM was estimated to be about 87%. The presence of a C22 n-alkanol maximum is attributed mainly to bacterial contribution. Higher plants are considered a major source of the campesterol, stigmasterol, and sitosterol in the TSM. [ABSTRACT FROM AUTHOR]

Lacustrine record of marine aerosol input has rarely been documented. Here, we present the sulfur geochemistry during the last deglaciation and early Holocene of a sediment core retrieved from the Dongyuan Lake in southern Taiwan. An unusually high sulfur peak accompanying pyrite presence is observed at 10.5 ka BP. Such high sulfur content in lacustrine record is unusual. The δ34S of sulfur varied from +9.5 to + 17.1‰ with two significant positive shifts at 10.5 and 9.4 ka BP. The sources of sulfur and potential processes involving the sulfur isotope variation including bacterial sulfate reduction, volcanic emissions, in-catchment sulfide oxidation and marine aerosol input are discussed. Enhanced marine aerosol input is the most likely explanation for such sulfur peaks and δ34S shifts. The positive δ34S shifts appeared concurrently with the maximum landslide events over Taiwan resulted from enhanced typhoon activities. The synchronicity among records suggests that increased typhoon activities promoted sea spray, and consequently enhanced the marine aerosol input with 34S-enriched sulfate. Our sulfur geochemistry data revealed sea spray history and marine influence onto terrestrial environment at coastal regions. Wider coverage of spatial-temporal lacustrine sulfur geochemistry record is needed to validate the applicability of sulfur proxy in paleoenvironmental research. [ABSTRACT FROM AUTHOR]

The land-to-ocean export of particulate organic carbon (POC) connects carbon flow from the atmosphere through land to the ocean, of which the contemporary fraction that reaches the deep sea for burial may effectively affect atmospheric CO2. In this regard, small mountainous rivers (SMRs) in Oceania, a global erosion hotspot driven by torrential typhoon rain and active earthquakes are potentially important. Here we measured typhoon lignin discharges for Taiwan SMRs. We found that the particulate lignin export in 96 hours by a single SMR amounting to ~20% of the annual export by Mississippi River. The yearly particulate lignin discharge from Taiwan Island (35,980 km2) is governed by the frequency and magnitude of typhoon; thus, the historical lignin export ranged widely from 1.5 to 99.7 Gg yr−1, which resulted in a 10-100 times higher areal yield relative to non-Oceanian rivers. The lignin-derived modern POC output from Oceania region is 37 ± 21 Tg C yr−1, account for approximately 20% of the annual modern POC export from global rivers. Coupled with the hyperpycnal pathway, the forested watersheds of SMRs in Oceania may serve as a giant factory to rapidly produce and efficiently convey modern POC into deep sea for sequestration. [ABSTRACT FROM AUTHOR]