Source Of New Nitrogen Research Articles

Carbon type and quantity regulate soil free-living nitrogen fixation through restructuring diazotrophic community

Free-living nitrogen fixation (FLNF) is a ubiquitous and significant source of new nitrogen in ecosystems. This process requires a substantial amount of energy, and as a result, the input of labile carbon to soils may enhance FLNF. Here, we aim to explore the precise influence of different types and quantities of carbon on FLNF. We conducted experiments where sugars, carboxylic acids, and amino acids (common labile carbon inputs via root exudates) were added to cropland and forest soils. We monitored the changes in FLNF rate and diazotrophic community in response to these additions. Our findings revealed that sugars and carboxylic acids significantly stimulated FLNF rate, with an increase of up to three orders of magnitude, while amino acids either restricted FLNF or had no effect. When a mixture of sugars and carboxylic acids was added, the stimulation of FLNF was not observed until a threshold quantity of carbon was reached, indicating competition between diazotrophs and other microbes. Furthermore, the enhanced FLNF rate following carbon addition was attributable to shifts in diazotrophic community composition (e.g., increased abundance of Paenibacillus, a genus frequently used as biofertilizers), rather than to changes in the overall abundance of diazotrophs (reflected by nifH gene copy number). Our findings highlight the significant role of carbon type and quantity in regulating soil FLNF, partly through restructuring the diazotrophic community. The results have implications for predicting the response of rhizosphere FLNF to shifts in root exudation under environmental changes and can contribute to the application of diazotrophic biofertilizers in sustainable agriculture.

Assessment of moss-associated biological nitrogen fixation of dominant moss species in mature black spruce forests in Eastern Canada

Biological nitrogen fixation (BNF) by bryophytes is an essential source of new nitrogen in nitrogen-limited boreal ecosystems. This assessment is, however, primarily based on reports from northern Europe and, to a lesser extent, Western North America. A more systematic and extensive evaluation of nitrogen fixation by moss is required to close current gaps in records of moss BNF and better assess the importance of bryophytes on N input in boreal ecosystems. Here, we evaluated nitrogen fixation activity and contribution to N input of dominant bryophyte species in black spruce forests from Eastern Canada. We assessed the relative abundance, BNF activity, and relative contribution to nitrogen input on five sites along a 600 km transect. We report that several moss species contribute to nitrogen input but that Ptilium crista-castrensis is the main contributor. Estimated nitrogen input by mosses (≪ 1 kg·ha−1·year−1) is in the low range of N inputs reported in the literature and is consistent with activities reported elsewhere for similar species, N deposition, and forest age. Results show that atmospheric deposition, while low, remains a more significant source of exogenous N to mature black spruce forests than bryophytes.

Rhizobia–diatom symbiosis fixes missing nitrogen in the ocean

Nitrogen (N2) fixation in oligotrophic surface waters is the main source of new nitrogen to the ocean1 and has a key role in fuelling the biological carbon pump2. Oceanic N2 fixation has been attributed almost exclusively to cyanobacteria, even though genes encoding nitrogenase, the enzyme that fixes N2 into ammonia, are widespread among marine bacteria and archaea3–5. Little is known about these non-cyanobacterial N2 fixers, and direct proof that they can fix nitrogen in the ocean has so far been lacking. Here we report the discovery of a non-cyanobacterial N2-fixing symbiont, ‘Candidatus Tectiglobus diatomicola’, which provides its diatom host with fixed nitrogen in return for photosynthetic carbon. The N2-fixing symbiont belongs to the order Rhizobiales and its association with a unicellular diatom expands the known hosts for this order beyond the well-known N2-fixing rhizobia–legume symbioses on land6. Our results show that the rhizobia–diatom symbioses can contribute as much fixed nitrogen as can cyanobacterial N2 fixers in the tropical North Atlantic, and that they might be responsible for N2 fixation in the vast regions of the ocean in which cyanobacteria are too rare to account for the measured rates.

Partitioning of carbon export in the euphotic zone of the oligotrophic South China Sea

Abstract. We conducted samplings of total and particulate 234Th, along with particulate organic carbon (POC), in the summer of 2017 to examine nutrient-dependent structures of export productivity within the euphotic zone (Ez) of the oligotrophic basin of the South China Sea (SCS). Nitrate concentrations throughout the study area were below detection limits in the nutrient-depleted layer (NDL) above the nutricline, while they sharply increased with depth in the nutrient-replete layer (NRL) across the nutricline until the base of the Ez. Based on our vertical profilings of 234Th–238U disequilibria, this study estimated for the first time POC export fluxes both out of the NDL and at the horizon of the Ez base. The total 234Th deficit relative to 238U was determined in the NDL at all study sites. By contrast, 234Th was mostly in equilibrium with 238U in the NRL, except at the northernmost station, SEATS (SouthEast Asian Time-series Study; 18∘ N, 116∘ E), where the 234Th deficit was also observed in the NRL. By combining 1D steady-state 234Th fluxes and POC/234Th ratios, we derived vertical patterns of POC export fluxes. The POC export fluxes at station SEATS were 1.6 ± 0.6 mmol C m−2 d−1 at the NDL base, accounting for approximately half of that at the base of the Ez. For the rest of the sampling sites, the POC export fluxes at the NDL base (averaged at 2.3 ± 1.1 mmol C m−2 d−1) were comparable with those at the base of the Ez (1.9 ± 0.5 mmol C m−2 d−1), suggesting rapid export of POC out of the NDL. This finding fundamentally changes our traditional view that the NDL, being depleted in nutrients, would not be a net exporter of POC. Furthermore, our results revealed a significant positive correlation between POC export fluxes at the NDL base and the potential of subsurface nutrient supplies, indicated by nutricline depth and nutrient concentrations obtained from both in situ measurements and numerical modeling. POC export fluxes (averaged at 3.4 ± 1.2 mmol C m−2 d−1) at the NDL base at stations with shallow nutriclines and high levels of subsurface nutrients approximately doubled those (averaged at 1.6 ± 0.5 mmol C m−2 d−1) at other stations. We subsequently used a two-endmember mixing model based on the mass and 15N isotopic balances to evaluate the relative contribution of different sources of new nitrogen that support the observed particle export at stations SEATS and SS1, located respectively in the northern and southern basin of the SCS with different hydrological features. We showed that more than 50 % of the particle flux out of the NDL was supported by nitrate sources likely supplied from depth and associated with episodic intrusions other than atmospheric deposition and nitrogen fixation. However, the exact mechanisms and pathways for subsurface nutrients to support the export production from the NDL merit additional careful and dedicated studies.

An efficient diazotroph‐derived nitrogen transfer pathway in coral reef system

AbstractThe underlying mechanism for sustaining the high productivity of coral reefs in nutrient deplete water, known as the Darwin Paradox, has long been debated. The input of new nitrogen, for example, derived from N2 fixation, and efficient nitrogen recycling are hypothesized as two critical factors in resolving the paradox. However, the source of new nitrogen and its connection to nitrogen recycling in the coral reef system remain poorly understood. We measured N2 fixation rates and investigated the diazotroph community in different components of a tropical coral reef system. Our results show that the reef‐building crustose coralline algae demonstrate a significantly higher N2 fixation rate than the overlying water or the coral holobiont, representing a large source of N2 fixation to the system. The associated diazotroph community was dominated by the non‐cyanobacterial Rhizobiales. 15N2‐based pulse‐chase incubation experiments revealed ~ 50% of the diazotroph‐derived nitrogen was released from the crustose coralline algae holobiont to the overlying water and coral holobiont within 48 h, which was five fold higher than the fraction of nitrogen released from nitrate‐based assimilation by crustose coralline algae. Furthermore, the released diazotroph‐derived nitrogen was rapidly transferred to the coral holobiont through grazing and was subsequently released back to the overlying water as dissolved organic nitrogen, revealing an efficient pathway that connects the new nitrogen input with the nitrogen recycling in the coral reef ecosystem. Our results provide new insights into the sources and fates of new nitrogen in the coral reefs, helping to resolve the Darwin Paradox in this unique ecosystem.

Widespread herbivory cost in tropical nitrogen-fixing tree species.

Recent observations suggest that the large carbon sink in mature and recovering forests may bestrongly limited by nitrogen1-3. Nitrogen-fixing trees (fixers) in symbiosis with bacteria provide the main natural source of new nitrogen to tropical forests3,4. However, abundances of fixers are tightly constrained5-7, highlighting the fundamental unanswered question of what limits new nitrogen entering tropical ecosystems. Here we examine whether herbivory by animals is responsible for limiting symbiotic nitrogen fixation in tropical forests. We evaluate whether nitrogen-fixing trees experience more herbivory than other trees, whether herbivory carries a substantial carbon cost, and whether high herbivory is a result of herbivores targeting the nitrogen-rich leaves of fixers8,9. We analysed 1,626 leaves from 350 seedlings of 43 tropical tree species in Panama and found that: (1) although herbivory reduces the growth and survival ofall seedlings, nitrogen-fixing trees undergo 26% more herbivory than non-fixers; (2) fixers have 34% higher carbon opportunity costs owing to herbivory than non-fixers, exceeding the metabolic cost of fixing nitrogen; and (3) the high herbivory of fixers is not driven by high leaf nitrogen. Our findings reveal that herbivory may be sufficient to limit tropical symbiotic nitrogen fixation and could constrain its role in alleviating nitrogen limitation on the tropical carbon sink.

Zinc- and magnesium-doped hydroxyapatite-urea nanohybrids enhance wheat growth and nitrogen uptake

The ongoing and unrestrained application of nitrogen fertilizer to agricultural lands has been directly linked to climate change and reductions in biodiversity. The agricultural sector needs a technological upgrade to adopt sustainable methods for maintaining high yield. We report synthesis of zinc and magnesium doped and undoped hydroxyapatite nanoparticles, and their urea nanohybrids, to sustainably deliver nitrogen to wheat. The urea nanohybrids loaded with up to 42% nitrogen were used as a new source of nitrogen and compared with a conventional urea-based fertilizer for efficient and sufficient nitrogen delivery to pot-grown wheat. Doping with zinc and magnesium manipulated the hydroxyapatite crystallinity for smaller size and higher nitrogen loading capacity. Interestingly, 50% and 25% doses of urea nanohybrids significantly boosted the wheat growth and yield compared with 100% doses of urea fertilizer. In addition, the nutritional elements uptake and grain protein and phospholipid levels were significantly enhanced in wheat treated with nanohybrids. These results demonstrate the potential of the multi-nutrient complexes, the zinc and magnesium doped and undoped hydroxyapatite-urea nanoparticles, as nitrogen delivery agents that reduce nitrogen inputs by at least 50% while maintaining wheat plant growth and nitrogen uptake to the same level as full-dose urea treatments.

Integrating the impact of global change on the niche and physiology of marine nitrogen-fixing cyanobacteria.

Marine nitrogen fixation is a major source of new nitrogen to the ocean, which interacts with climate driven changes to physical nutrient supply to regulate the response of ocean primary production in the oligotrophic tropical ocean. Warming and changes in nutrient supply may alter the ecological niche of nitrogen-fixing organisms, or 'diazotrophs', however, impacts of warming on diazotroph physiology may also be important. Lab-based studies reveal that warming increases the nitrogen fixation-specific elemental use efficiency (EUE) of two prevalent marine diazotrophs, Crocosphaera and Trichodesmium, thus reducing their requirements for the limiting nutrients iron and phosphorus. Here, we coupled a new diazotroph model based upon observed diazotroph energetics of growth and resource limitation to a state-of-the-art global model of phytoplankton physiology and ocean biogeochemistry. Our model is able to address the integrated response of nitrogen fixation by Trichodesmium and Crocosphaera to warming under the IPCC high emission RCP8.5 scenario for the first time. Our results project a global decline in nitrogen fixation over the coming century. However, the regional response of nitrogen fixation to climate change is modulated by the diazotroph-specific thermal performance curves and EUE, particularly in the Pacific Ocean, which shapes global trends. Spatially, the response of both diazotrophs is similar with expansion towards higher latitudes and reduced rates of nitrogen fixation in the lower latitudes. Overall, 95%-97% of the nitrogen fixation climate signal can be attributed to the combined effect of temperature on the niche and physiology of marine diazotrophs, with decreases being associated with a reduced niche and increases resulting due to a combination of expanding niche and temperature driven changes to EUE. Climate change impacts on both the niche and physiology of marine diazotrophs interact to shape patterns of marine nitrogen fixation, which will have important implications for ocean productivity in the future.

Assessment of the impacts of biological nitrogen fixation structural uncertainty in CMIP6 earth system models

Abstract. Biological nitrogen fixation is the main source of new nitrogen into natural terrestrial ecosystems and consequently in the nitrogen cycle in many earth system models. Representation of biological nitrogen fixation varies, and because of the tight coupling between the carbon and nitrogen cycles, previous studies have shown that this affects projected changes in net primary productivity. Here we present the first assessment of the performance of biological nitrogen fixation in models contributing to CMIP6 compared to observed and observation-constrained estimates of biological nitrogen fixation. We find that 9 out of 10 models represent global total biological nitrogen fixation within the uncertainty in recent global estimates. However, 6 out of 10 models overestimate the amount of fixation in the tropics and therefore the extent of the latitudinal gradient in the global distribution. For the SSP3-7.0 scenario of future climate change, models project increases in fixation over the 21st century of up to 80 %. However, while the historical range of biological nitrogen fixation amongst models is large (up to 140 kg N ha−1 yr−1 at the grid cell level and 43–208 Tg N yr−1 globally) this does not have explanatory power for variations within the model ensemble of net primary productivity or the coupled nitrogen–carbon cycle. Models with shared structures can have significant variations in both biological nitrogen fixation and other parts of the nitrogen cycle without differing in their net primary productivity. This points to systematic challenges in the representation of carbon–nitrogen model structures and the severe limitations of models using net primary productivity or evapotranspiration to project the biological nitrogen fixation response to elevated atmospheric carbon dioxide or other environmental changes.

The Angola Gyre is a hotspot of dinitrogen fixation in the South Atlantic Ocean

Biological dinitrogen fixation is the major source of new nitrogen to marine systems and thus essential to the ocean’s biological pump. Constraining the distribution and global rate of dinitrogen fixation has proven challenging owing largely to uncertainty surrounding the controls thereon. Existing South Atlantic dinitrogen fixation rate estimates vary five-fold, with models attributing most dinitrogen fixation to the western basin. From hydrographic properties and nitrate isotope ratios, we show that the Angola Gyre in the eastern tropical South Atlantic supports the fixation of 1.4–5.4 Tg N.a−1, 28-108% of the existing (highly uncertain) estimates for the basin. Our observations contradict model diagnoses, revealing a substantial input of newly-fixed nitrogen to the tropical eastern basin and no dinitrogen fixation west of 7.5˚W. We propose that dinitrogen fixation in the South Atlantic occurs in hotspots controlled by the overlapping biogeography of excess phosphorus relative to nitrogen and bioavailable iron from margin sediments. Similar conditions may promote dinitrogen fixation in analogous ocean regions. Our analysis suggests that local iron availability causes the phosphorus-driven coupling of oceanic dinitrogen fixation to nitrogen loss to vary on a regional basis.

Magnitude of nitrate turbulent diffusion in contrasting marine environments

Difficulties to quantify ocean turbulence have limited our knowledge about the magnitude and variability of nitrate turbulent diffusion, which constitutes one of the main processes responsible for the supply of nitrogen to phytoplankton inhabiting the euphotic zone. We use an extensive dataset of microturbulence observations collected in contrasting oceanic regions, to build a model for nitrate diffusion into the euphotic zone, and obtain the first global map for the distribution of this process. A model including two predictors (surface temperature and nitrate vertical gradient) explained 50% of the variance in the nitrate diffusive flux. This model was applied to climatological data to predict nitrate diffusion in oligotrophic mid and low latitude regions. Mean nitrate diffusion (~ 20 Tmol N y−1) was comparable to nitrate entrainment due to seasonal mixed-layer deepening between 40°N–40ºS, and to the sum of global estimates of nitrogen fixation, fluvial fluxes and atmospheric deposition. These results indicate that nitrate diffusion represents one of the major sources of new nitrogen into the surface ocean in these regions.

Toward resolving disparate accounts of the extent and magnitude of nitrogen fixation in the Eastern Tropical South Pacific oxygen deficient zone

AbstractExamination of dinitrogen (N2) fixation in the Eastern Tropical South Pacific oxygen deficient zone has raised questions about the range of diazotrophs in the deep sea and their quantitative importance as a source of new nitrogen globally. However, technical considerations in the deployment of stable isotopes in quantifying N2 fixation rates have complicated interpretation of this research. Here, we report the findings of a comprehensive survey of N2 fixation within, above and below the Eastern Tropical South Pacific oxygen deficient zone. N2 fixation rates were measured using a robust 15N tracer method (bubble removal) that accounts for the slow dissolution of N2 gas and calculated using a conservative approach. N2 fixation was only detected in a subset of samples (8 of 125 replicated measurements) collected within suboxic waters (< 20 μmol O2 kg−1) or at the oxycline. Most of these detectable rates were measured at nearshore stations, or where surface productivity was high. These findings support the hypothesis that low oxygen/high organic carbon conditions favor non‐cyanobacterial diazotrophs. Nevertheless, this study indicates that N2 fixation is neither widespread nor quantitatively important throughout this region.

Diversity and abundance of diazotrophic cyanobacteria in the central coastal area of the Gulf of Gabès (South-eastern Tunisia)

Gabès Gulf is limited by ultra-oligotrophic waters which are regularly fertilized with both phosphate and iron enriched atmospheric dust. Despite the lack of river run nitrogen source, an essential nutrient for the ecosystem productivity. Diazotrophic cyanobacteria are an important source of new nitrogen in oligotrophic oceans and seas. So far only the filamentous diazotrophic cyanobacteria Trichodesmium spp. has been investigated in Gabès gulf, while it is known that other diazotrophic cyanobacteria, such as the heterocysted Richelia intracellularis and unicellular cyanobacteria (UCYN), are major source of new nitrogen in oligotrophic ecosystems. In this study, we investigated the diversity and abundance of marine diazotrophic cyanobacteria within two size fractions (> 10 μm and 0.2–3​ μm) during July in the central coastal area of the Gulf of Gabès. Richelia intracellularis were present under free living and hosted in the diatoms Hemiaulus hauckii. Unicellular diazotrophic cyanobacteria were classified under tow size classes (small cells: 0.8–1.5μm and large cells: 1.6–3.2μm) with a predominance of the small cells (98 %). Richelia intracellularis reached concentrations that can be considered as a bloom phenomenon (up to 227.5 cells mL −1), while the concentration of UCYN (from 8.8 to 23.9 cells mL −1) were slightly higher than mean concentrations observed across the Mediterranean Sea. High temperature and dissolved nitrogen and orthophosphate concentrations in the central coastal area of the Gulf of Gabès seem to the main factors leading to such abundances. Contribution of each diazotrophic cyanobacteria to the N/P balance in the central coastal area of Gulf was also discussed.

Nitrogen Fixation in a Changing Arctic Ocean: An Overlooked Source of Nitrogen?

The Arctic Ocean is the smallest ocean on Earth, yet estimated to play a substantial role as a global carbon sink. As climate change is rapidly changing fundamental components of the Arctic, it is of local and global importance to understand and predict consequences for its carbon dynamics. Primary production in the Arctic Ocean is often nitrogen-limited, and this is predicted to increase in some regions. It is therefore of critical interest that biological nitrogen fixation, a process where some bacteria and archaea termed diazotrophs convert nitrogen gas to bioavailable ammonia, has now been detected in the Arctic Ocean. Several studies report diverse and active diazotrophs on various temporal and spatial scales across the Arctic Ocean. Their ecology and biogeochemical impact remain poorly known, and nitrogen fixation is so far absent from models of primary production in the Arctic Ocean. The composition of the diazotroph community appears distinct from other oceans – challenging paradigms of function and regulation of nitrogen fixation. There is evidence of both symbiotic cyanobacterial nitrogen fixation and heterotrophic diazotrophy, but large regions are not yet sampled, and the sparse quantitative data hamper conclusive insights. Hence, it remains to be determined to what extent nitrogen fixation represents a hitherto overlooked source of new nitrogen to consider when predicting future productivity of the Arctic Ocean. Here, we discuss current knowledge on diazotroph distribution, composition, and activity in pelagic and sea ice-associated environments of the Arctic Ocean. Based on this, we identify gaps and outline pertinent research questions in the context of a climate change-influenced Arctic Ocean – with the aim of guiding and encouraging future research on nitrogen fixation in this region.

Physical Forcing Controls the Basin‐Scale Occurrence of Nitrogen‐Fixing Organisms in the North Pacific Ocean

AbstractBiological nitrogen fixation is increasingly recognized as an important source of new nitrogen in a warming ocean. However, the basin‐scale spatiotemporal distribution of nitrogen‐fixing organisms (diazotrophs) in the ocean and its controlling environmental factors remain unclear. Here we examined the basin‐scale seasonal distribution patterns of major diazotrophs (filamentous cyanobacterial Trichodesmium, unicellular cyanobacterial UCYN‐A1, and proteobacterial Gamma‐A) in surface waters of the North Pacific from 2014 to 2016 with unprecedented coverage and resolution. In general, UCYN‐A1, Trichodesmium, and Gamma‐A were abundant during spring‐autumn, summer‐autumn, and spring respectively. Regarding latitudinal patterns of abundance, UCYN‐A1 showed dome shape; Trichodesmium was gradually decreasing from low‐ to high‐latitude regions; and Gamma‐A did not show a clear pattern, which were coincident with the distinct correlations between the diazotrophs and temperature. All three diazotrophs were abundant (reached 106–107 nifH gene copy number L−1) in the North Pacific transition zone and subtropical gyre, where the cyanobacterial diazotrophs were more abundant in both the western and eastern North Pacific than in the central North Pacific. The diazotroph abundance in the western North Pacific was positively correlated with eddy kinetic energy and sea surface height anomaly, which implies an enhancement of diazotrophs in mesoscale eddies associated with the western boundary current Kuroshio and its extension. The cyanobacterial diazotrophs were positively correlated with wind stress curl, a measurable parameter of wind‐driven upwelling, in the eastern North Pacific. Our study refines the biogeography of three major diazotrophs and highlights the importance of physical forcing in mediating their dynamics.

Quantifying the Impact of Climate Change on Marine Diazotrophy: Insights From Earth System Models

Nitrogen fixation is a major source of new nitrogen to the ocean, supporting biological productivity in the large nitrogen-limited tropical oceans. In Earth System Models, the response of nitrogen fixation to climate change acts in concert with projected changes to physical nitrogen supply to regulate the response of primary productivity in nitrogen-limited regions. We examine the response of diazotrophy from nine Earth System Models and find large variability in the magnitude and spatial pattern of nitrogen fixation in both contemporary periods and future projections. Although Earth System Models tend to agree that nitrogen fixation will decrease over the next century, strong regional variations exist, especially in the tropical Pacific which may counteract the response of the Atlantic and Indian oceans. As the climate driven trend of nitrogen fixation emerges by mid-century in the RCP8.5 scenario, on regional scales it may modulate the broad climate trends in productivity that emerge later in the century. The generally poor skill and lack of agreement amongst Earth System Models indicates that the climate response of nitrogen fixation is a key uncertainty in projections of future ocean primary production in the tropical oceans. Overall, we find that the future evolution of nitrogen fixation plays an important role in shaping future trends in net primary production in the tropics, but the poor skill of models highlights significant uncertainty, especially considering the role of multiple concurrent drivers.

Transportation of Diazotroph Community From the Upstream to Downstream of the Kuroshio

AbstractThe Kuroshio is a strong western boundary current in the North Pacific, in which N2 fixation is an important source of new nitrogen. In order to understand the dynamics of diazotrophs in the Kuroshio, we used 454‐pyrosequencing of nifH gene amplicons to unravel and compare the diazotroph communities along the main path of the Kuroshio (23.44° N, 121.99° E–34.3° N, 140.1° E) with its adjacent waters. The diazotroph communities in the geographically distanced stations along the Kuroshio mainstream were highly similar, while those in the Kuroshio adjacent waters were highly varied, indicated that diazotroph communities were actively transported in the Kuroshio. Along the Kuroshio, the UCYN‐A1, UCYN‐C, and Gamma‐A were rather evenly distributed along the Kuroshio, whereas the UCYN‐A2 and Trichodesmium were mainly distributed in the Kuroshio upstream. The diazotroph communities shifted from cyanobacteria dominated in the Kuroshio upstream to proteobacteria (mainly Bradyrhizobium and sulfate‐reducing bacteria) dominated in the downstream. Among all cyanobacterial diazotrophs detected, UCYN‐A1 was the most sustained group in the Kuroshio with >103 gene copies per L and that may explain its extended distribution to higher latitude regions as compared with other cyanobacterial diazotrophs. Considering that the Kuroshio is a hot spot of unicellular cyanobacterial diazotrophs, we have adopted DNA‐directed RNA polymerase subunit beta (rpoC1) gene as a biomarker to further explore their phylogeny. Comparing the data set of nifH and rpoC1 gene fragments, we provided new insights into the phylogeny of the recently defined UCYN‐B2 and poorly understood UCYN‐C groups.

Atmospheric deposition of anthropogenic inorganic nitrogen in airborne particles and precipitation in the East Sea in the northwestern Pacific Ocean

The atmospheric deposition of anthropogenic nitrogen is an increasingly important new source of nitrogen to the ocean. Coastal areas east of the Korean Peninsula are suitable for the investigation of the effects of atmospheric anthropogenic nitrogen on the ocean nutrient system because of the low riverine discharge rates and the prevailing influence of the East Asian outflow. Thus, we measured the concentrations of nitrate (NO3−) and ammonium (NH4+) in airborne particles and in precipitation from March 2014 to February 2016 at a coastal site (37.08°N, 129.41°E) on the east coast of Korea. The dry deposition of NO3− (27–30 mmol N m−2 yr−1) was far greater than that of NH4+ (6–8 mmol N m−2 yr−1). The greater rate of dry NO3− deposition was associated with air masses traveling over northeastern China and central Korea. In contrast, the rates of wet deposition of NO3− (17–24 mmol N m−2 yr−1) and NH4+ (14–27 mmol N m−2 yr−1) were comparable and were probably associated with in-cloud scavenging of these ions. The results indicate that the total deposition of NO3− and NH4+ combined could contribute to ~2.4% and ~1.9% of the primary production in the coastal areas east of the Korean Peninsula and in the East Asian marginal seas, respectively, which would be a lower bound because the dry deposition of reactive nitrogen gas was not included. Our study shows that the atmospheric input of anthropogenic NO3− and NH4+ may substantially increase phytoplankton biomass in the coastal waters of the East Sea near the Korean Peninsula.

Diazotrophic community and associated dinitrogen fixation within the temperate coral Oculina patagonica.

Dinitrogen (N2 ) fixing bacteria (diazotrophs) are an important source of new nitrogen in oligotrophic environments and represent stable members of the microbiome in tropical corals, while information on corals from temperate oligotrophic regions is lacking. Therefore, this study provides new insights into the diversity and activity of diazotrophs associated with the temperate coral Oculina patagonica from the Mediterranean Sea by combining metabarcoding sequencing of amplicons of both the 16S rRNA and nifH genes and 15 N2 stable isotope tracer analysis to assess diazotroph-derived nitrogen (DDN) assimilation by the coral. Results show that the diazotrophic community of O. patagonica is dominated by autotrophic bacteria (i.e. Cyanobacteria and Chlorobia). The majority of DDN was assimilated into the tissue and skeletal matrix, and DDN assimilation significantly increased in bleached corals. Thus, diazotrophs may constitute an additional nitrogen source for the coral host, when nutrient exchange with Symbiodinium is disrupted (e.g. bleaching) and external food supply is limited (e.g. oligotrophic summer season). Furthermore, we hypothesize that DDN can facilitate the fast proliferation of endolithic algae, which provide an alternative carbon source for bleached O. patagonica. Overall, O. patagonica could serve as a good model for investigating the importance of diazotrophs in coral recovery from bleaching.

Diazotrophy as the main driver of the oligotrophy gradient in the western tropical South Pacific Ocean: results from a one-dimensional biogeochemical–physical coupled model

Abstract. The Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise took place in the western tropical South Pacific (WTSP) during the austral summer (March–April 2015). The aim of the OUTPACE project was to investigate a longitudinal gradient of biological and biogeochemical features in the WTSP, and especially the role of N2 fixation in the C, N, and P cycles. Two contrasted regions were considered in this study: the Western Melanesian Archipelago (WMA), characterized by high N2 fixation rates, significant surface production and low dissolved inorganic phosphorus (DIP) concentrations, and the South Pacific Gyre (WGY), characterized by very low N2 fixation rates, surface production and high DIP concentrations. Since physical forcings and mixed layer dynamics in both regions were similar, it was considered that the gradient of oligotrophy observed in situ between the WMA and WGY was not explained by differences in physical processes, but rather by differences in biogeochemical processes. A one-dimensional physical–biogeochemical coupled model was used to investigate the role of N2 fixation in the WTSP by running two identical simulations, only differing by the presence (simWMA) or absence (simWGY) of diazotrophs. We showed that the nitracline and the phosphacline had to be, respectively, deeper and shallower than the mixed layer depth (MLD) to bring N-depleted and P-repleted waters to the surface during winter mixing, thereby creating favorable conditions for the development of diazotrophs. We also concluded that a preferential regeneration of the detrital phosphorus (P) matter was necessary to obtain this gap between the nitracline and phosphacline depths, as the nutricline depths significantly depend on the regeneration of organic matter in the water column. Moreover, the model enabled us to highlight the presence of seasonal variations in primary production and P availability in the upper surface waters in simWMA, where diazotrophs provided a new source of nitrogen (N) to the ecosystem, whereas no seasonal variations were obtained in simWGY, in the absence of diazotrophs. These main results emphasized the fact that surface production dynamics in the WTSP is based on a complex and sensitive system which depends on the one hand on physical processes (vertical mixing, sinking of detrital particles), and on the other hand on biogeochemical processes (N2 fixation, remineralization).