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It has been estimated that more than 48% of global methane emissions from lakes and reservoirs occur at low latitudes (<24°). To improve this estimate, knowledge regarding underexplored ecosystems, particularly deep lakes and reservoirs in Asian monsoon regions, is needed because the magnitude of methane emissions is influenced by lake bathymetry and climatic conditions. We conducted long-term studies beginning in 2004 at Feitsui Reservoir (FTR) in Taiwan, a subtropical monomictic system with a maximum depth of 120 m to monitor seasonal and interannual variations of three key characteristics and to understand the mechanisms underlying these variations. Key characteristics investigated were as follows: (1) the balance of primary production and heterotrophic respiration as a determinant of vertical oxygen distribution, (2) methane production at the bottom of the reservoir, oxidation in the water column, and emissions from the lake surface, and (3) the contribution of methane-originated carbon to the pelagic food web through methane-oxidizing bacteria (MOB). This review highlights major achievements from FTR studies integrating isotopic, microbial, and modeling approaches. Based on our findings, we proposed two conceptual models: (1) a model of methane dynamics, which addresses the differences in methane emission mechanisms between deep and shallow lakes, and (2) a spatially explicit model linking benthic methane production to the pelagic food web, which addresses the diversity of MOB metabolisms and their dependence on oxygen availability. Finally, we address why long-term studies of subtropical lakes and reservoirs are important for better understanding the effects of climate on low- to mid-latitude ecosystems. [ABSTRACT FROM AUTHOR]

• NH 4 + was mainly produced by mineralization, with < 2% produced by DNRA. • Mineralization created serial reactions to modulate the NH 4 + pool. • NH 4 + sink process shifted from nitrification (upstream) to anammox (downstream). • NH 4 + would accumulate and be retained continuously in strong reducing groundwater. Ammonium (NH 4 +) retention/removal processes in groundwater are of great interest because of the continuous increase in nitrogenous compound loading due to anthropogenic activities. However, the transition of multiple co-occurring transformation processes that determine the fate of NH 4 + in groundwater along a redox gradient remains underexplored. We selected a high nitrogen (N) groundwater system in the western Hetao Basin, China, to identify and quantify NH 4 + source and sink processes, including mineralization, dissimilatory nitrate reduction to ammonium (DNRA), nitrification, and anammox, to better understand the dynamics of NH 4 +. Based on redox-sensitive parameters, that is, the oxidation-reduction potential (ORP) and NH 4 + and nitrate (NO 3 −) contents, etc., the groundwater system was classified into three zones from upstream to downstream: zone I (oxidizing), zone II (moderately reducing), and zone III (strongly reducing). Using the 15N tracing technique, we found that NH 4 + was mainly produced by mineralization while < 2% was produced by DNRA throughout the study area. Mineralization increased downstream because the supply of biodegradable N-containing compounds was augmented, which created a strong redox gradient to host a serial reaction chain. In zone I, NH 4 + was mainly transferred to NO 3 − via nitrification, whereas in zones II and III, NH 4 + was mainly transferred to N 2 via anammox. The average NH 4 + production/consumption ratios (P/C) in zones I, II, and III were 0.7, 6.9, and 51.1, respectively. Obviously, the NH 4 + purification ability can only exceed the supply under aerobic conditions, thus suggesting that NH 4 + will accumulate without limitation and be retained in strongly reducing groundwater. The situation of NH 4 + accumulation would deteriorate over space and time in groundwater as human activities increase without an additional artificial supply of oxidants. The results provide mechanistic insights for quantitatively comprehending the dynamics and fate of NH 4 + in groundwater, shedding light on groundwater NH 4 + mitigation techniques. [Display omitted] [ABSTRACT FROM AUTHOR]

The sediments from a piston core ECS12A recovered from the middle Okinawa Trough in the East China Sea were measured for total organic carbon (TOC), total nitrogen (TN), and other biogenic elements to provide constraints on the sources and burial rates of depositional organic matter (OM) and on the changes in primary productivity since 19 ka. The last glacial sediments (ca. 17–19 ka) are characterized by low contents of biogenic elements and well-developed turbidite layers, suggesting low primary productivity but a high component of terrigenous sediment. With rising sea level and enhanced monsoons during the deglacial period, the proportion of marine OM gradually increased. The least negative δ 13 C org values and the smallest grain size of sediments deposited ca. 10–14.5 ka indicate high primary productivity and a sedimentary environment dominated by the marine component. The source and burial rates of OM in the Holocene sediments (ca. 5.4–10 ka) were largely controlled by the intensification of the Kuroshio Current, which caused a slight decrease in primary productivity, but strengthened the oceanic circulation in the East China Sea. Overall, the source-to-sink process of OM in the Okinawa Trough is governed by complex interactions between sea level, climate and ocean circulation. [ABSTRACT FROM AUTHOR]

• Denitrification dominate nitrogen removal in sediments from subtropical estuary. • Particulate organic matter links nitrogen transform between water and sediments. • Estuarine sediments are weak in eliminating riverine nitrogen due to limited area. • Sedimentary denitrification directs a more than half of daily sea-air N 2 O flux. Sedimentary denitrification and anaerobic ammonium oxidation (anammox) are two microbially-mediated nitrogen removal pathways with distinct climatic feedbacks. Estuaries receive large fluxes of anthropogenic nitrogen and serve as hotspots for nitrogen loss. Applying 15N isotope pairing technique and sediment intact core incubation in two subtropical estuaries, the Yangtze River Estuary (YRE) and Jiulong River Estuary (JRE), we show that denitrification predominates the sedimentary nitrogen loss with a minor contribution (8.6 ± 7.5%) from anammox. Particulate organic matter degradation sustains the sedimentary nitrogen removal linking the nitrogen transformations between water column and sediment. Our results indicate that estuarine sediments exhibit high areal nitrogen removal rate, but play a relatively weak role in eliminating the nitrogen inputted from river basin due to the limited area. The riverine excess nitrogen will eventually enter into the adjacent continental shelf and be removed via phytoplankton assimilation-sedimentation-degradation-coupled nitrification-denitrification. In addition, sedimentary denitrification causes 1.8 ± 2.2% of nitrogen flow towards nitrous oxide (N 2 O) production and the derived N 2 O release flux accounts for 59% and 65% of the daily sea-air N 2 O emission in the YRE and JRE, respectively. These findings contribute to a better understanding of estuarine sedimentary nitrogen removal and associated climate feedbacks, and to the parameterization of Earth system models. [Display omitted] [ABSTRACT FROM AUTHOR]

The particulate organic carbon (POC) to 234 Th ratio, or POC/ 234 Th, is crucial to constrain the 234 Th-derived downward export flux. Marine particles for 234 Th based export studies are typically collected via two sampling modes: filtration of Niskin or GO-FLO bottle water samples and filtration of in situ pumped samples. Large discrepancies have been frequently observed between these bottle and pump filtration methods and attributed to the difference in adsorption of dissolved material. We noted however, that simultaneous measurements of carbon and 234 Th have not been made to evaluate such a putative effect on the POC/ 234 Th ratio. In this study, we adopted a “two filters in-line” approach method with one stacked on another and we measured both carbon and 234 Th to compare their adsorption behavior in bottle and pump sampling modes. We proposed that the 234 Th and carbon on the second filter (filter-B) might be attributable to direct adsorption from the dissolved phase (including the partial capture from the submicron particles) and/or breakdown of particles initially collected on the first filter (filter-A). The relative importance of these two processes differed between the two filtration modes. For bottle sampling, we observed significantly positive correlation (R 2 = 0.54) between the 234 Th recorded on the second filter and dissolved 234 Th activities, suggesting that adsorption was the dominant contributor to the second filter. The overestimation due to adsorption (filter-B/filter-A) is estimated to be 22 ± 7% for POC and 25 ± 17% for particulate 234 Th. The effect of adsorption on the POC/ 234 Th, on the other hand, would be negligible since the adsorption-corrected and -uncorrected POC/ 234 Th on filter-A were similar. For the large volume pumping mode, the absorbed 234 Th on the filters appeared to reach the saturation state as evidenced by the fact that the amount of particulate 234 Th (dpm cm − 2 ) on filter-B was independent of the filtered sample volume. We justified that the saturation amount should be > 0.21 dpm cm − 2 which was the highest unsaturated value from the bottle sample, but < 0.31 dpm cm − 2 which was the upper value on filter-B taken from pump samples. The calculated adsorption still dominated for 234 Th on filter-B and particle breakdown might be important in the euphotic zone. The collected POC on filter-B tended to increase with the filtration volume which implied that DOC adsorption remained under-saturated. Similar to bottle filtration, the corrected POC/ 234 Th was comparable to the uncorrected POC/ 234 Th on filter-A. We thus conclude that adsorption did not appear to influence the POC/ 234 Th ratio in either sampling mode. We suggested that the positive offset in POC/ 234 Th between bottle and pump sampling, if any, cannot be caused by adsorption. Other mechanisms such as zooplankton invasion and/or artificial particle formation need to be examined in order to resolve the discrepancy. [ABSTRACT FROM AUTHOR]

The apportionment of riverine dissolved inorganic nitrogen (DIN) load to individual land use on a watershed scale demands the support of accurate DIN load estimation and differentiation of point and non-point sources, but both of them are rarely quantitatively determined in small montane watersheds. We introduced the Danshui River watershed of Taiwan, a mountainous urbanized watershed, to determine the export coefficients via a reverse Monte Carlo approach from riverine DIN load. The results showed that the dynamics of N fluctuation determines the load estimation method and sampling frequency. On a monthly sampling frequency basis, the average load estimation of the methods (GM, FW, and LI) outperformed that of individual method. Export coefficient analysis showed that the forest DIN yield of 521.5 kg-N km −2 yr −1 was ~ 2.7-fold higher than the global riverine DIN yield (mainly from temperate large rivers with various land use compositions). Such a high yield was attributable to high rainfall and atmospheric N deposition. The export coefficient of agriculture was disproportionately larger than forest suggesting that a small replacement of forest to agriculture could lead to considerable change of DIN load. The analysis of differentiation between point and non-point sources showed that the untreated wastewater (non-point source), accounting for ~ 93% of the total human-associated wastewater, resulted in a high export coefficient of urban. The inclusion of the treated and untreated wastewater completes the N budget of wastewater. The export coefficient approach serves well to assess the riverine DIN load and to improve the understanding of N cascade. [ABSTRACT FROM AUTHOR]

Situated at the border between the Southeast Asian continent and the northwestern Pacific, Taiwan's climate is primarily influenced by the East Asian Summer Monsoon (EASM) and episodic tropical cyclones formed in the western Pacific. The lake sediments in Taiwan thus potentially archive the information of typhoon events and past monsoon variability. A high-resolution multi-proxy (total carbon, mass accumulation rate of total carbon, weight percent of wood fragments, δ 13 C of the organic carbon, magnetic susceptibility, chemical weathering index) and well-dated lacustrine record from Dongyuan Lake (22°10′N, 120°50′E; 360 m above sea level) in southern Taiwan was used to reconstruct centennial to millennial timescale EASM oscillations specifically for the transition from the last deglaciation to the Early Holocene (17–9 ka BP). The temporal patterns of proxies on both timescales in our records broadly agree with those in the Greenland ice core, which shows enhanced EASM during the warm period and weakened monsoon during the cold period. Different from northern high-latitude climates, we found two phases in the Mystery Interval, opposite Bølling–Allerød (BA) trend with maximum monsoonal rainfall in the Allerød period and more gradual transition to Younger Dryas event in our record. Likely, other climate forcings beyond the North Atlantic climate jointly modulated the EASM. Changes of sea surface temperature in the western tropical Pacific also might have exerted synergistic control on EASM precipitation in Taiwan on the centennial timescale during the last deglaciation. In addition, several high sedimentation events accompanied by significant amounts of wood fragments were observed and coincide with the paleo-record of mass wasting events identified on various downstream floodplains in Taiwan. Since mass wasting processes were rainfall-driven and threshold-triggered, such coherence possibly indicated an intensification of typhoon activity during the early Holocene. More high-resolution lacustrine records are required to decipher the hydrological variation and climate dynamics that were co-influenced by the Intertropical Convergence Zone (ITCZ) and typhoon activity in Southeast Asia. [ABSTRACT FROM AUTHOR]

• NH 4 +-consuming hotpots and NO 3 −-consuming hotpots decoupled in space in estuary. • NO 3 −/NO 2 − behave conservatively due to the high preference on NH 4 + for phytoplankton. • Upstream NH 4 + turned into organics or NO 3 − in 2 days to affect microbial niches offshore. • Counterintuitively, high NH 4 + input may lead to net O 2 production through diel N-dynamics. The reactive nitrogen (N) emitted from continents significantly perturbs the pristine N cycle around the land-ocean boundary resulting in eutrophication and hypoxia. As nutrients are transported downstream through an estuary, various types of biological processes co-occur to modulate nitrogen speciation to influence the biogeochemical habitats for downstream microorganisms. We surveyed the Pearl River Estuary to examine the N transfer dynamics among nitrogen species with considering process-specific oxygen production and consumption. By using 15N pulse-tracing techniques, we measured ammonia oxidation and uptakes of ammonium, nitrite, and nitrate simultaneously under dark and light conditions in parallel. Light strongly inhibited nitrification but enhanced N uptake, and such light effect was further considered in the calculation for nitrogen transformation rates over a diel cycle. We found both oxidation and uptake of ammonium decreased seaward as substrate decreased. The nitrifier and phytoplankton work in antiphase to draw down incoming ammonium rapidly. Contrary to ammonium uptake, uptake of nitrite and nitrate showed a seaward increasing pattern. Such an inverse spatial pattern implies a shift in N preference for phytoplankton. Such high ammonium preference inhibits nitrate/nitrite uptake allowing them to behave conservatively in the estuary and to travel farther to outer estuary. By integrating oxygen consumption and production induced by N transformation processes over the diel cycle, oxygen was produced although allochthonous ammonium input is high (∼250 μM). For most stations, ammonium was completely consumed within 2 days, some stations even less than 0.5 days, implying that although the water residence time is short (2-15 days), tremendous input of ammonium N from upstream was transformed into particulate organic or nitrate forms during traveling to modulate the biogeochemical niche, including substrate, organics and oxygen, of coastal microbes in water column and sediments. [ABSTRACT FROM AUTHOR]

Abstract: Little information exists about centennial-scale climate variability on oceanic islands in the western Pacific where the East Asian monsoon (EAM) strongly influences the climate, mountain ecosystem and the society. In this study, we investigate a 168 cm long sediment core recovered from Emerald Peak Lake in subalpine NE Taiwan for the contents of grain size, total organic carbon (TOC), C/N ratio, and stable isotopes (δ13C and δ15N) to reconstruct the monsoon climate and vegetation density during the late Holocene. Six radiocarbon (14C) ages obtained on plant remains used for the chronology indicate that the sediment core has been accumulated since ∼3770 cal BP with a mean sedimentation rate of 44.6 cm/ka. The sub-centennial resolution of our proxy records reveals strong fluctuations of the EAM and vegetation density for the past ∼3770 cal BP. The greater contents of coarse and medium sediments with overall decreasing trends from 3770 to 2000 cal BP suggest an increasing fine sediment influx from the catchment likely due to an increasing lake water level. Although low TOC content, C/N ratio, and enriched δ13C values in bulk and fine sediments during this interval suggest a sparsely vegetated catchment, increasing trends of TOC content and C/N ratio together with decreasing trends of δ13C and δ15N values indicate a strengthening pattern of summer monsoon. This is in contrast to a decreasing monsoon strength inferred from Dongge Cave δ18O record at that time, supporting the idea of anti-phasing of summer EAM and Indian summer monsoon. Since 2000 cal BP, higher content of fine sediments with high TOC content and C/N ratio but relatively depleted δ13C and low δ15N values suggest a high but stable lake water level and dense C3 plants, consistent with a stronger summer monsoon in a wet climate. Within this general trend, we interpret a prominent change of proxy parameters in sediments from ∼560 to 150 cal BP, as subtropical evidence for the Little Ice Age in NE Taiwan. By comparing our proxy records with other diverse land and marine records from southern China and adjoining marine realm, we demonstrate that the centennial to millennial-scale fluctuations of the summer EAM over the northeastern Taiwan during the late Holocene have been largely modulated by the tropical Pacific forcing through El Niño along with solar forcing. [Copyright &y& Elsevier]

Seamounts are ecological oases nurturing abundant fisheries resources and epibenthic megafauna in the vast oligotrophic ocean. Despite their significance, the formation mechanisms underlying these seamount ecological oases remain uncertain. To shed light on this phenomenon, this study conducted interdisciplinary in situ observations focusing on a shallow seamount in the oligotrophic ocean. The findings show that the seamount's topography interferes with the oceanic current to generate lee waves, effectively enhancing the nutrient supply to the euphotic layer downstream of the seamount. This continuous supply enhances phytoplankton biomass and subsequently the grazing and diurnal vertical migration of zooplankton, rapidly transporting the augmented phytoplankton biomass to the aphotic layer. Unlike the cyclonic eddies that move in the upper ocean, seamounts stand at fixed locations creating a more efficient and steady active transport loop. This active transport loop connects the euphotic and twilight zones, potentially conveying nourishment to benthic ecosystems to create stereoscopic oases in the oligotrophic ocean.

Land-ocean interactions greatly impact the evolution of coastal life on earth. However, the ancient geological forces and genetic mechanisms that shaped evolutionary adaptations and allowed microorganisms to inhabit coastal brackish waters remain largely unexplored. In this study, we infer the evolutionary trajectory of the ubiquitous heterotrophic archaea Poseidoniales (Marine Group II archaea) presently occurring across global aquatic habitats. Our results show that their brackish subgroups had a single origination, dated to over 600 million years ago, through the inversion of the magnesium transport gene corA that conferred osmotic-stress tolerance. The subsequent loss and gain of corA were followed by genome-wide adjustment, characterized by a general two-step mode of selection in microbial speciation. The coastal family of Poseidoniales showed a rapid increase in the evolutionary rate during and in the aftermath of the Cryogenian Snowball Earth (∼700 million years ago), possibly in response to the enhanced phosphorus supply and the rise of algae. Our study highlights the close interplay between genetic changes and ecosystem evolution that boosted microbial diversification in the Neoproterozoic continental margins, where the Cambrian explosion of animals soon followed., (© The Author(s) 2024. Published by Oxford University Press on behalf of National Academy of Sciences.)

Sulfate (SO 4 2- ) is an essential chemical species in atmospheric aerosols and plays an influential role in their physical-chemical characteristics. The mechanisms of secondary SO 4 2- aerosol have been intensively studied in air-polluted cities. However, few studies have focused on cities with good air quality. One-year PM 2.5 samples were collected in the tropical island city of Haikou, and water-soluble inorganic ions, as well as water-soluble Fe and Mn, were analyzed. The results showed that non-sea-salt SO 4 2- (nss-SO 4 2- ) was the dominant species of water-soluble inorganic ions, accounting for 40-57% of the total water-soluble inorganic ions in PM 2.5 in Haikou. The S(IV)+H 2 O 2 pathway was the main formation pathway for secondary SO 4 2- in wintertime in Haikou, contributing to 57% of secondary SO 4 2- formation. By contrast, 54% of secondary SO 4 2- was produced by the S(IV)+Fe×Mn pathway in summer. In spring and autumn, the S(IV)+H 2 O 2 , S(IV)+Fe×Mn, and S(IV)+NO 2 pathways contributed equally to secondary SO 4 2- formation. The ionic strength was the controlling parameter for the S(IV)+NO 2 pathway, while pH was identified as a key factor that mediates the S(IV)+H 2 O 2 and S(IV)+Fe×Mn pathways to produce secondary SO 4 2- . This study contributes to our understanding of secondary SO 4 2- production under low PM 2.5 concentrations but high SO 4 2- percentages.

The heavily human-perturbed coastal oceans are hotspots of nitrous oxide (N 2 O) emission to the atmosphere. The processes underpinning the N 2 O flux, however, remain poorly understood, leading to large uncertainties in assessing global N 2 O budgets. Using a suite of nitrogen isotope labeling experiments, we show that multiple processes contribute to N 2 O production throughout the estuarine-coastal gradient, sustaining intensive N 2 O flux to the atmosphere. Unexpectedly, denitrification, rather than ammonia oxidation as previously assumed, constitutes the major source of N 2 O 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 N 2 O production rate increases with substrate and particle concentrations, suggesting a crucial interplay between nutrients and particles in controlling N 2 O production. The controlling factors identified here may help understand climate feedback mechanisms between human activity and coastal oceans., (© 2023. The Author(s).)

Biological nitrogen fixation (BNF), the conversion of N 2 into bioavailable nitrogen (N), is the main process for replenishing N loss in the biosphere. However, BNF in groundwater systems remains poorly understood. In this study, we examined the activity, abundance, and community composition of diazotrophs in groundwater in the Hetao Plain of Inner Mongolia using 15 N tracing methods, reverse transcription qPCR (RT-qPCR), and metagenomic/metatranscriptomic analyses. 15 N 2 tracing incubation of near in situ groundwater (9.5-585.4 nmol N L -1 h -1 ) and N 2 -fixer enrichment and isolates (13.2-1728.4 nmol N g -1 h -1 , as directly verified by single-cell resonance Raman spectroscopy), suggested that BNF is a non-negligible source of N in groundwater in this region. The expression of nifH genes ranged from 3.4 × 10 3 to 1.2 × 10 6 copies L -1 and was tightly correlated with dissolved oxygen (DO), Fe(II), and NH 4 + . Diazotrophs in groundwater were chiefly aerobes or facultative anaerobes, dominated by Stutzerimonas, Pseudomonas, Paraburkholderia, Klebsiella, Rhodopseudomonas, Azoarcus, and additional uncultured populations. Active diazotrophs, which prefer reducing conditions, were more metabolically diverse and potentially associated with nitrification, sulfur/arsenic mobilization, Fe(II) transport, and CH 4 oxidation. Our results highlight the importance of diazotrophs in subsurface geochemical cycles., (© 2023. The Author(s), under exclusive licence to International Society for Microbial Ecology.)

Nitrite, an intermediate product of the oxidation of ammonia to nitrate (nitrification), accumulates in upper oceans, forming the primary nitrite maximum (PNM). Nitrite concentrations in the PNM are relatively low in the western North Pacific subtropical gyre (wNPSG), where eddies are frequent and intense. To explain these low nitrite concentrations, we investigated nitrification in cyclonic eddies in the wNPSG. We detected relatively low half-saturation constants (i.e., high substrate affinities) for ammonia and nitrite oxidation at 150 to 200 meter water depth. Eddy-induced displacement of high-affinity nitrifiers and increased substrate supply enhanced ammonia and nitrite oxidation, depleting ambient substrate concentrations in the euphotic zone. Nitrite oxidation is more strongly enhanced by the cyclonic eddies than ammonia oxidation, reducing concentrations and accelerating the turnover of nitrite in the PNM. These findings demonstrate a spatial decoupling of the two steps of nitrification in response to mesoscale processes and provide insights into physical-ecological controls on the PNM.

Green sulfur bacteria (GSB) are a distinct group of anoxygenic phototrophic bacteria that are found in many ecological niches. Prosthecochloris , a marine representative genus of GSB, was found to be dominant in some coral skeletons. However, how coral-associated Prosthecochloris (CAP) adapts to diurnal changing microenvironments in coral skeletons is still poorly understood. In this study, three Prosthecochloris genomes were obtained through enrichment culture from the skeleton of the stony coral Galaxea fascicularis . These divergent three genomes belonged to Prosthecochloris marina and two genomes were circular. Comparative genomic analysis showed that between the CAP and non-CAP clades, CAP genomes possess specialized metabolic capacities (CO oxidation, CO 2 hydration and sulfur oxidation), gas vesicles (vertical migration in coral skeletons), and cbb 3 -type cytochrome c oxidases (oxygen tolerance and gene regulation) to adapt to the microenvironments of coral skeletons. Within the CAP clade, variable polysaccharide synthesis gene clusters and phage defense systems may endow bacteria with differential cell surface structures and phage susceptibility, driving strain-level evolution. Furthermore, mobile genetic elements (MGEs) or evidence of horizontal gene transfer (HGT) were found in most of the genomic loci containing the above genes, suggesting that MGEs play an important role in the evolutionary diversification between CAP and non-CAP strains and within CAP clade strains. Our results provide insight into the adaptive strategy and population evolution of endolithic Prosthecochloris strains in coral skeletons., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Nie, Tang, Wang, Wang, Guo, Wang, Kao, Yin and Wang.)

The ocean is a net source of the greenhouse gas and ozone-depleting substance, nitrous oxide (N 2 O), to the atmosphere. Most of that N 2 O is produced as a trace side product during ammonia oxidation, primarily by ammonia-oxidizing archaea (AOA), which numerically dominate the ammonia-oxidizing community in most marine environments. The pathways to N 2 O production and their kinetics, however, are not completely understood. Here, we use 15 N and 18 O isotopes to determine the kinetics of N 2 O production and trace the source of nitrogen (N) and oxygen (O) atoms in N 2 O produced by a model marine AOA species, Nitrosopumilus maritimus . We find that during ammonia oxidation, the apparent half saturation constants of nitrite and N 2 O production are comparable, suggesting that both processes are enzymatically controlled and tightly coupled at low ammonia concentrations. The constituent atoms in N 2 O are derived from ammonia, nitrite, O 2 , and H 2 O via multiple pathways. Ammonia is the primary source of N atoms in N 2 O, but its contribution varies with ammonia to nitrite ratio. The ratio of 45 N 2 O to 46 N 2 O (i.e., single or double labeled N) varies with substrate ratio, leading to widely varying isotopic signatures in the N 2 O pool. O 2 is the primary source for O atoms. In addition to the previously demonstrated hybrid formation pathway, we found a substantial contribution by hydroxylamine oxidation, while nitrite reduction is an insignificant source of N 2 O. Our study highlights the power of dual 15 N- 18 O isotope labeling to disentangle N 2 O production pathways in microbes, with implications for interpretation of pathways and regulation of marine N 2 O sources.

The atmospheric wet deposition has been recognized as a significant allochthonous source of dissolved organic carbon (DOC) to the ocean. However, few studies have examined the biolability of rainwater dissolved organic matter (DOM) at the molecular level. Rainwater samples were collected and incubated with ambient microbes. DOC, UV-vis spectroscopy, formic acid (FA), acetic acid (AA), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS) were applied. Approximately 50 ± 16 % of rainwater DOC and ~90 % of FA and AA were bioconsumed within 28 days. The contribution of FA and AA to the total BDOC was ~30 %, which was the largest known biolabile fraction in rainwater DOC. In contrast, only approximately 15 % of formulae identified by FT-ICRMS were consumed, which were characterized by higher saturation, higher heteroatom content and lower modified aromaticity. Among the major high molecular weight secondary organic carbon (HWW-SOC)-like compounds, organosulfate contained the largest fraction of consumed formulae, while biogenic volatile organic-derived CHO compounds had the lowest. Our study for the first time provided both quantitative and qualitative understanding of the bioavailability of rainwater DOM, which is essential for understanding their effects on the biogeochemical cycles and the environmental health in the receiving waters., 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 © 2022 Elsevier B.V. All rights reserved.)

Microbial interactions impact the functioning of both natural and engineered systems, yet our ability to directly monitor these highly dynamic and spatially resolved interactions in living cells is very limited. Here, we developed a synergistic approach coupling single-cell Raman microspectroscopy with 15 N 2 and 13 CO 2 stable isotope probing in a microfluidic culture system (RMCS-SIP) for live tracking of the occurrence, rate, and physiological shift of metabolic interactions in active microbial assemblages. Quantitative and robust Raman biomarkers specific for N 2 and CO 2 fixation in both model and bloom-forming diazotrophic cyanobacteria were established and cross-validated. By designing a prototype microfluidic chip allowing simultaneous microbial cultivation and single-cell Raman acquisition, we achieved temporal tracking of both intercellular (between heterocyst and vegetative cells of cyanobacteria) and interspecies N and C metabolite exchange (from diazotroph to heterotroph). Moreover, single-cell N and C fixation and bidirectional transfer rate in living cells were quantified via SIP-induced characteristic Raman shifts. Remarkably, RMCS captured physiological responses of metabolically active cells to nutrient stimuli through comprehensive metabolic profiling, providing multimodal information on the evolution of microbial interactions and functions under fluctuating conditions. This noninvasive RMCS-SIP is an advantageous approach for live-cell imaging and represents an important advancement in the single-cell microbiology field. This platform can be extended for real-time tracking of a wide range of microbial interactions with single-cell resolution and advances the understanding and manipulation of microbial interactions for societal benefit., (© The Author(s) 2023. Published by Oxford University Press on behalf of National Academy of Sciences.)

Mutualism between Synechococcus and heterotrophic bacteria has been found to support their prolonged survival in nutrient-depleted conditions. However, environmental interference on the fate of their mutualism is not understood. Here, we show that exogenous nutrients disrupt their established mutualism. Once the exogenous nutrients were exhausted, Synechococcus and heterotrophic bacteria gradually reestablished their metabolic mutualism during 450 days of culture, which revived unhealthy Synechococcus cells. Using metagenomics, metatranscriptomics, and the 15 N tracer method, we reveal that the associated bacterial nitrogen fixation triggered the reestablishment of the mutualism and revival of Synechococcus health. During this process, bacterial community structure and functions underwent tremendous adjustments to achieve the driving effect, and a cogeneration of nitrogen, phosphorus, iron, and vitamin by the heterotrophic bacteria sustained Synechococcus 's prolonged healthy growth. Our findings suggest that Synechococcus and heterotrophic bacteria may have an inherent tendency toward mutualism despite environmental interference. This may exhibit their coevolutionary adaptations in nutrient-deficient environments.

Marine phytoplankton and heterotrophic bacteria share a very close but usually changeable relationship. However, the ultimate fate of their unstable relationship on a long-term scale is unclear. Here, the relationship between Synechococcus and heterotrophic bacterial communities underwent a dramatic shift from antagonism to commensalism and eventually to mutualism during long-term cocultivation. The relationship change is attributed to the different (even opposite) effects of diverse bacterial members on Synechococcus and the ratio of beneficial to harmful bacteria. Different bacterial members also interact with each other (e.g., quorum-sensing communication, hostility, or mutual promotion) and drive a dynamic succession in the entire community structure that corresponds exactly to the shift in its relationship with Synechococcus . In the final mutualism stage, a self-sufficient nitrogen cycle, including nitrogen fixation, denitrification, and organic nitrogen degradation, contributed to the healthy survival of Synechococcus for 2 years without an exogenous nutrient supply. This natural selective trait of Synechococcus and heterotrophic bacteria toward mutualism under long-term coexistence provides a novel clue for understanding the ubiquity and competitive advantage of Synechococcus in global oceans. IMPORTANCE Phytoplankton and heterotrophic bacteria have a close but usually changeable relationship. Uncovering the dynamic changes and driving factors of their interrelationships is of great significance for an in-depth understanding of the ecological processes and functions of marine microorganisms. Here, we observed that Synechococcus and heterotrophic bacterial communities underwent a dramatic change in their relationship from antagonism to mutualism during a long-term cocultivation process. We revealed that the interactions between different members of the bacterial community and the combined effects of different bacterial individuals on Synechococcus promoted the dynamic changes of the Synechococcus -bacterium relationship. In the end, a self-sufficient nutrient cycle (especially nitrogen) established by Synechococcus and bacterial communities supported their long-term survival without any external nutrition supply. This study provides novel insight into the interaction between Synechococcus and heterotrophic bacteria in the ocean and provides a novel clue for understanding the ubiquity and competitive advantage of Synechococcus in global oceans.

Ammonia oxidation to nitrite and its subsequent oxidation to nitrate provides energy to the two populations of nitrifying chemoautotrophs in the energy-starved dark ocean, driving a coupling between reduced inorganic nitrogen (N) pools and production of new organic carbon (C) in the dark ocean. However, the relationship between the flux of new C production and the fluxes of N of the two steps of oxidation remains unclear. Here, we show that, despite orders-of-magnitude difference in cell abundances between ammonia oxidizers and nitrite oxidizers, the two populations sustain similar bulk N-oxidation rates throughout the deep waters with similarly high affinities for ammonia and nitrite under increasing substrate limitation, thus maintaining overall homeostasis in the oceanic nitrification pathway. Our observations confirm the theoretical predictions of a redox-informed ecosystem model. Using balances from this model, we suggest that consistently low ammonia and nitrite concentrations are maintained when the two populations have similarly high substrate affinities and their loss rates are proportional to their maximum growth rates. The stoichiometric relations between the fluxes of C and N indicate a threefold to fourfold higher C-fixation efficiency per mole of N oxidized by ammonia oxidizers compared to nitrite oxidizers due to nearly identical apparent energetic requirements for C fixation of the two populations. We estimate that the rate of chemoautotrophic C fixation amounts to ∼1 × 10 13 to ∼2 × 10 13 mol of C per year globally through the flux of ∼1 × 10 14 to ∼2 × 10 14 mol of N per year of the two steps of oxidation throughout the dark ocean., Competing Interests: The authors declare no competing interest., (Copyright © 2020 the Author(s). Published by PNAS.)

As a unique biomarker of terrigenous organic matter (OM), lignin has provided valuable information for tracing the sources of OM in land to ocean transfer. Oceanian small mountainous rivers (SMRs) are characterized by extremely high erosional rate and quick change in microclimate within watershed, which may potentially affect the distribution of soil OC and lignin concentrations and compositions. Bulk OC% and lignin were determined on surface soils and soil profiles from a Taiwanese SMR (Jhuoshuei River) and nearby region along a large altitudinal gradient (3-3176 m) to investigate the influence of microclimate on soil OC and lignin. Both surface soils OC% and lignin increased in higher altitude, suggesting higher preservation of OM in the cold region. Variations in lignin vegetation indices (S/V and C/V) in surface soils generally reflect the vegetation change in this river basin, and were more affected by precipitation seasonality than mean annual precipitation. Lignin concentration decreased with depth, along with a decrease in S/V and C/V and an increase in degradation indices ((Ad/Al)v and DHBA/V), reflecting a decreased input and/or biodegradation of lignin in subsoils. Our survey on soil lignin in Taiwan SMR provided the basis for utilizing lignin to trace the source of OC in land to ocean transfer as well as paleo-climate and paleo-vegetation reconstruction study in Taiwan SMRs.

Polychlorinated dibenzo-p-dioxin (PCDD), polychlorinated dibenzofuran (PCDF) and polychlorinated biphenyl (PCB) concentrations were analyzed at 1-2cm intervals in a sediment core collected from a reservoir in Northern Taiwan to evaluate the organic pollution history. The highest PCDD/F (14.4ng TEQ/kg d.w.) and PCB (0.261ng TEQ(WHO)/kg d.w.) concentrations were determined at 13-15cm (estimated year: 1992). The ages of the levels of sediment core were estimated from the sedimentation rate. Analysis results demonstrate that the PCDD/F concentration of the sediment core measured in the reservoir reached their peak when the municipal waste incinerators (MWIs) in the area started to operate. Furthermore, the decrease in sediment core PCDD/F concentration is related to the time of enforcement of the PCDD/F emission limit set by the Environmental Protection Administration (EPA) in Taiwan. Significant distribution of OCDD in homologue profiles was noted in archived soil samples in Taiwan in which the major input of PCDD/Fs was thought to be atmospheric. Major PCB congeners found in the sediment core were the major components of the commercial PCB products. Input fluxes of PCDD/Fs (5.75-158ng-I-TEQ/m(2)-yr) and PCBs (0.248-3.71ng TEQ(WHO)/m(2)yr) into the reservoir of interest are also calculated from the concentration and sedimentation rate of the sediment. The results reveal that considerable amounts of PCDD/Fs and PCBs were carried into the reservoir of interest in the flood stage but not during normal stage.