The underestimated role of manganese in modulating the nutrient structure in a eutrophic seasonally-stratified reservoir.

Accumulation and subsequent release of nutrients have great potential to trigger algal blooms in lakes and reservoirs. We conducted high vertical resolution (2 m interval) monitoring at ∼monthly intervals over a year for hydrological parameters, Chl-a, ammonium (NH4+), nitrate (NO3-) and different species of phosphorus (P) and manganese (Mn) in a 40-meter-deep subtropical reservoir (Shanmei Reservoir) in Fujian, southern China. In this seasonally stratified reservoir featured with high nutrient loading, the consistent trend in the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorus (DIP) between the euphotic zone and the hypolimnion, coupled with its mirrored correlation with Chl-a concentration indicates that upward flux from the hypolimnion affects phytoplankton growth in the euphotic zone. The monthly variation of the depth-integrated multiple species of N and P indicates that during the stratification period in the hypoxic hypolimnion, approximately 80% of the DIP is removed, leading to a remarkable decoupling phenomenon between NH4+ and DIP. This process effectively increases the ratio of DIN to DIP in the hypolimnion, thereby significantly reducing the potential of algal blooms caused by the upward flux. A robust positive linear correlation between iron-manganese bound phosphorus (CBD-P) and particulate Mn was observed during stratification period implying that DIP was scavenged by sediment-released Mn throughout the water column. Vertical profiles during stratification showed that upward diffusion of Mn2+ facilitated the formation of Mn oxide zones near the oxycline. The most significant decrease in DIP inventory occurs when Mn oxide zones migrate either upwards from the bottom or downwards from the oxycline, indicating that the migration of Mn oxides on the vertical profile is a key factor in the decoupling of NH4+and DIP. Our findings underscore the importance of Mn cycling as an underappreciated DIP self-immobilization process in the water column of reservoirs characterized by high nutrient loading. Furthermore, we propose that denitrification and Mn cycling establish a consecutive feedback mechanism, preventing excessive nutrient accumulation in low oxygen bottom water. In the context of global changes, we anticipate a heightened prominence of this feedback mechanism.

Seamounts generate efficient active transport loops to nourish the twilight ecosystem.

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.

Assessment of Secondary Sulfate Aqueous-Phase Formation Pathways in the Tropical Island City of Haikou: A Chemical Kinetic Perspective.

Sulfate (SO42-) is an essential chemical species in atmospheric aerosols and plays an influential role in their physical-chemical characteristics. The mechanisms of secondary SO42- aerosol have been intensively studied in air-polluted cities. However, few studies have focused on cities with good air quality. One-year PM2.5 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 SO42- (nss-SO42-) was the dominant species of water-soluble inorganic ions, accounting for 40-57% of the total water-soluble inorganic ions in PM2.5 in Haikou. The S(IV)+H2O2 pathway was the main formation pathway for secondary SO42- in wintertime in Haikou, contributing to 57% of secondary SO42- formation. By contrast, 54% of secondary SO42- was produced by the S(IV)+Fe×Mn pathway in summer. In spring and autumn, the S(IV)+H2O2, S(IV)+Fe×Mn, and S(IV)+NO2 pathways contributed equally to secondary SO42- formation. The ionic strength was the controlling parameter for the S(IV)+NO2 pathway, while pH was identified as a key factor that mediates the S(IV)+H2O2 and S(IV)+Fe×Mn pathways to produce secondary SO42-. This study contributes to our understanding of secondary SO42- production under low PM2.5 concentrations but high SO42- percentages.

Gene inversion led to the emergence of brackish archaeal heterotrophs in the aftermath of the Cryogenian Snowball Earth.

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 external/internal sources and sinks of greenhouse gases (CO2, CH4, N2O) in the Pearl River Estuary and adjacent coastal waters in summer.

Estuary acts as a hotspot of greenhouse gases (GHGs, including CO2, CH4 and N2O) to the atmosphere. However, the GHGs budgets, including input/output fluxes through interfaces and biogeochemical source/sink processes in water columns, of the estuarine systems are still not well constrained due to the lacking of comprehensive observational data. Here, we presented the spatial distributions of GHGs of surface/bottom water and sediment porewater along the Pearl River Estuary (PRE) and adjacent region during summertime. The incorporation of the monitoring for the sediment-water interface (SWI) with these of the water-air interface (WAI) allows us to close the budget revealing additional information of internal consumption/production processes of the three GHGs. The oversaturated CO2 (481-7573 µatm), CH4 (289-16,990 %) and N2O (108-649 %) in surface water suggested PRE is a significant GHGs source to the atmosphere, in which CO2 is the major contributor accounting for 90 % of total global warming potential (GWP), leaving 2.8 % from CH4, and 7.2 % from N2O. Addition to the river input, the SWI releases GHGs to the overlying water with fluxes of 3.5 × 107, 10.8 × 104 and 0.7 × 104 mol d-1 for CO2, CH4 and N2O, respectively. Although all three GHGs exhibited emission to the atmosphere, our mass balance calculation showed that 16.9× 107 mol d-1 of CO2 and 1.0 × 104 mol d-1 of N2O were consumed, respectively, inside the estuary water body, while extra-production (13.8 × 104 mol d-1) of CH4 was demanded in the water body to support its output flux. This is the first experiment quantitatively assessing the importance of internal carbon and nitrogen biogeochemical processes in the PRE. Our finding is of guiding significance to constrain the GHGs budget and draw up realistic pathways for modeling works of GHGs prediction.

Particle-associated denitrification is the primary source of N2O in oxic coastal waters.

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.

Active degradation-nitrification microbial assemblages in the hypoxic zone in a subtropical estuary.

In 2017 summer, we observed widespread bottom hypoxia at the lower estuary of the Pearl River estuary (PRE). Our previous study noticed that AOA and bacteria were highly abundant and clustered within the hypoxia zone. Moreover, nitrification and respiration rates were also evidently higher in these hypoxic waters. These observations prompt us to investigate whether these two oxygen-consuming microorganisms have symbiotic relationships and whether specific groups consistently coexist and form ecological-meaningful associations. In this study, we use network analysis to investigate the presence and active communities (DNA-RNA) based on bacterial and AOA communities sequencing (inferred from the 16S rRNA and amoA gene, respectively) to gain more insight into ecological-meaningful associations. We observed a highly diverse and active bacterial community in the hypoxia zone. The RNA networks were more modulized than the corresponding DNA networks, indicating that the active communities were better parsed into functional microbial assemblages. The network topology revealed that Gammaproteobacteria, Bacteroidetes (Flavobacteriales), Alphaproteobacteria (Rhodobacterales and Rhodospirillales), Marinimicrobia, Cyanobacteria (Synechococcales), and AOA sublineages were module hubs and connectors, indicating that they were the keystone taxa of the microbial communities. The hub-subnetwork further showed robust co-occurrence between Gammaproteobacteria, Bacteroidetes (Flavobacteriales), Alphaproteobacteria (Rhodobacterales and Rhodospirillales), Marinimicrobia with AOA sublineages, and Nitrospinae (presumably NOB) reflecting the formation of Degradation-Nitrification (sequential oxidation of Organic matter degradation to ammonia, then nitrate) microbial assemblage in the hypoxia zone. The subnetworks revealed AOA ecotype-specific modularization and niche partitioning of different AOA sublineages. Interestingly, the recurring co-occurrence of nitrifiers assemblage in the RNA subnetworks (SCM1-like-II (AOA) and Nitrospinae OTUs (NOB) suggests an active interaction via nitrite exchange. The Degradation-Nitrification microbial assemblage may contribute substantially to the oxygen consumption in the hypoxia formation in PRE. Our results provide new insight into the functional microbial assemblages, which is worth further investigation on their ecological implication in estuarine waters.

Nitrogen fixation and diazotroph diversity in groundwater systems.

Biological nitrogen fixation (BNF), the conversion of N2 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 15N tracing methods, reverse transcription qPCR (RT-qPCR), and metagenomic/metatranscriptomic analyses. 15N2 tracing incubation of near in situ groundwater (9.5-585.4 nmol N L-1 h-1) and N2-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 × 103 to 1.2 × 106 copies L-1 and was tightly correlated with dissolved oxygen (DO), Fe(II), and NH4+. 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 CH4 oxidation. Our results highlight the importance of diazotrophs in subsurface geochemical cycles.

Diversity and salinity adaptations of ammonia oxidizing archaea in three estuaries of China.

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.

Salinity change induces distinct climate feedbacks of nitrogen removal in saline lakes.

Current estimations of nitrogen biogeochemical cycling and N2O emissions in global lakes as well as predictions of their future changes are overrepresented by freshwater datasets, while less consideration is given to widespread saline lakes with different salinity (representing salinization or desalinization). Here, we show that N2O production by denitrification is the main process of reactive nitrogen (Nr, the general abbreviations of NH4+-N, NO2--N and NO3--N) removal in hypersaline lake sediments (e.g. Lake Chaka). The integration of our field measurements and literature data shows that in response to natural salinity decrease, potential Nr removal increases while N2O production decreases. Furthermore, denitrification-induced N2 production exhibits higher salinity sensitivity than denitrification-induced N2O production, suggesting that the contribution of N2O to Nr removal decreases with decreasing salinity. This field-investigation-based salinity response model of Nr removal indicates that under global climate change, saline lakes in the process of salinization or desalination may have distinct Nr removal and climate feedback effects: salinized lakes tend to generate a positive climate feedback, while desalinated lakes show a negative feedback. Therefore, salinity change should be considered as an important factor in assessing future trend of N2O emissions from lakes under climate change.

Reduced nitrite accumulation at the primary nitrite maximum in the cyclonic eddies in the western North Pacific subtropical gyre.

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.

Permeability decides the effect of antibiotics on sedimentary nitrogen removal in Jiulong River Estuary.

Sedimentary denitrification takes place beneath the oxic layer at the sediment-water interface, where nitrate and antibiotics need to diffuse through the overlying water. However, the antibiotics' effect on sedimentary N removal and associated N2O production has not been adequately investigated under in situ conditions. Here, isotope pairing techniques, including slurry incubations (potential) and intact core incubations (in situ), combined with metagenomic analysis were applied to investigate the impacts of two protein-inhibiting antibiotics (oxytetracycline and thiamphenicol) on sediment nitrogen removal in a subtropical estuary. Slurry incubations showed that the two antibiotics significantly inhibited denitrification (67-98%) and anammox (49-99%), while intact core incubations presented no antibiotic effect at upstream but significant inhibition (23%-52%) at downstream. Meanwhile, N2O yields were stimulated up to 20 folds in slurry incubations yet showing insignificant response in intact cores. Such contrasting results between up- and down-stream and between slurry and intact core incubations strongly indicated that permeability, which determines diffusion of antibiotics to microbes, is the key to exert the effect of antibiotics on in situ sedimentary nitrogen removal processes regardless the existence of antibiotics resistance genes. This diffusive obstruction may mitigate the toxic effect of antibiotics on nitrogen removal related microbes in natural environments.

Comparative genomic insights into habitat adaptation of coral-associated Prosthecochloris.

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, CO2 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.

Pathways of N2O production by marine ammonia-oxidizing archaea determined from dual-isotope labeling.

The ocean is a net source of the greenhouse gas and ozone-depleting substance, nitrous oxide (N2O), to the atmosphere. Most of that N2O 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 N2O production and their kinetics, however, are not completely understood. Here, we use 15N and 18O isotopes to determine the kinetics of N2O production and trace the source of nitrogen (N) and oxygen (O) atoms in N2O produced by a model marine AOA species, Nitrosopumilus maritimus. We find that during ammonia oxidation, the apparent half saturation constants of nitrite and N2O production are comparable, suggesting that both processes are enzymatically controlled and tightly coupled at low ammonia concentrations. The constituent atoms in N2O are derived from ammonia, nitrite, O2, and H2O via multiple pathways. Ammonia is the primary source of N atoms in N2O, but its contribution varies with ammonia to nitrite ratio. The ratio of 45N2O to 46N2O (i.e., single or double labeled N) varies with substrate ratio, leading to widely varying isotopic signatures in the N2O pool. O2 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 N2O. Our study highlights the power of dual 15N-18O isotope labeling to disentangle N2O production pathways in microbes, with implications for interpretation of pathways and regulation of marine N2O sources.

Live tracking metabolic networks and physiological responses within microbial assemblages at single-cell level.

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 15N2 and 13CO2 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 N2 and CO2 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.

Synergistic/antagonistic effects of nitrate/ammonium enrichment on fatty acid biosynthesis and translocation in coral under heat stress.

Superimposed on ocean warming, nitrogen enrichment caused by human activity puts corals under even greater pressure. Biosynthesis of fatty acids (FA) is crucial for coral holobiont survival. However, the responses of FA biosynthesis pathways to nitrogen enrichment under heat stress in coral hosts and Symbiodiniaceae remain unknown, as do FA translocation mechanisms in corals. Herein, we used the thermosensitive coral species Acropora hyacinthus to investigate changes in FA biosynthesis pathways and polyunsaturated FA translocation of coral hosts and Symbiodiniaceae with respect to nitrate and ammonium enrichment under heat stress. Heat stress promoted pro-inflammatory FA biosynthesis in coral hosts and inhibited FA biosynthesis in Symbiodiniaceae. Nitrate enrichment inhibited anti-inflammatory FA biosynthesis in Symbiodiniaceae, and promoted pro-inflammatory FA biosynthesis in coral hosts and translocation to Symbiodiniaceae, leading to bleaching after 14 days of culture. Intriguingly, ammonium enrichment promoted anti-inflammatory FA biosynthesis in Symbiodiniaceae and translocation to hosts, allowing corals to better endure heat stress. We constructed schematic diagrams of the shift in FA biosynthesis and translocation in and between A. hyacinthus and its Symbiodiniaceae under heat stress, heat and nitrate co-stress, and heat and ammonium co-stress. The findings provide insight into the mechanisms of coral bleaching under environmental stress from a fatty acid perspective.

What controls aerosol δ15N-NO3-? NOx emission sources vs. nitrogen isotope fractionation.

Atmospheric δ15N-NO3- has been used to reveal NOx (NO + NO2) sources as NO3- is the ultimate sink of NOx. However, it remains questionable whether the nitrogen isotope fractionation among NOy (NO, NO2, NO3, N2O5, HNO3 and NO3-) engender the misjudgment of NOx emission sources by affecting δ15N-NOy. To explore this issue, we integrated the dataset of aerosol δ15N-NO3- values and ratios of fNO2 (fNO2 = NO2/(NO2 + NO)), calculated the nitrogen isotope fractionation factors (Δs) among NOy, compared the total energy consumption in Beijing-Tianjin-Hebei region (BTH) from 2013 to 2018. Results showed that, although the total energy consumption structure changed from 2013 to 2018 in BTH, there were fewer interannual variances of aerosol δ15N-NO3- values. Nitrogen isotope fractionation factors between NO and NO2 (Δ0), NO2 and NO3 (Δ2), NO2 and N2O5 (Δ3), NO2 and ClONO2 (Δ4) also displayed less interannual variations from 2013 to 2018 in BTH. But both aerosol δ15N-NO3- and Δs displayed significant seasonal patterns, and there was significant relationship between monthly aerosol δ15N-NO3- and Δs, which suggested that Δs have important influence on shaping aerosol δ15N-NO3- and further discriminating NOx emission sources. This study implies that we should refine the Δs when employing atmospheric δ15N-NO3- to quantify NOx source allocation.

Transition of source/sink processes and fate of ammonium in groundwater along with redox gradients.

Ammonium (NH4+) 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 NH4+ 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 NH4+ source and sink processes, including mineralization, dissimilatory nitrate reduction to ammonium (DNRA), nitrification, and anammox, to better understand the dynamics of NH4+. Based on redox-sensitive parameters, that is, the oxidation-reduction potential (ORP) and NH4+ and nitrate (NO3-) 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 NH4+ 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, NH4+ was mainly transferred to NO3- via nitrification, whereas in zones II and III, NH4+ was mainly transferred to N2 via anammox. The average NH4+ production/consumption ratios (P/C) in zones I, II, and III were 0.7, 6.9, and 51.1, respectively. Obviously, the NH4+ purification ability can only exceed the supply under aerobic conditions, thus suggesting that NH4+ will accumulate without limitation and be retained in strongly reducing groundwater. The situation of NH4+ 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 NH4+ in groundwater, shedding light on groundwater NH4+ mitigation techniques.

Molecular level characterization of the biolability of rainwater dissolved organic matter.

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.

Inherent tendency of Synechococcus and heterotrophic bacteria for mutualism on long-term coexistence despite environmental interference.

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 15N 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.