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This study conducted sensitive and precise analyses of dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) concentrations and trace concentrations of nitrate and nitrite (N+N) and soluble reactive phosphorus (SRP) in seawater. The methods were applied to investigate the distributions and controls of N+N, SRP, DON, and DOP in the oligotrophic Kuroshio Current (KC) area off eastern Taiwan during warm seasons. The water in the studied area was classified into four major types: typical Kuroshio water (KW), KW influenced by the East China Sea water, KC influenced by the South China Sea (SCS) water, and KW influenced by the SCS water and river plumes, which is confined to the coastal zone. Nutrient distributions in KC revealed very low N+N (0.002-0.40 mM) and SRP (0.015-0.125 mM) concentrations but high DON (<8 mM) and DOP (<0.3 mM) concentrations above the nutricline depth, which accounted for >80% of TDN and TDP, respectively; theses concentrations can primarily be attributed to strong, permanent surface stratification. Among the water types, KW had the lowest N+N, SRP, DON, and DOP concentrations but greatest chlorophyll maximum depth and nutricline depth, except for in locations influenced by island-induced upwelling. The concentrations of all nutrients increased by various degrees in the other water types, which was attributed to the exchange and mixing of different water masses and coastal uplift of subsurface waters. KW was not only highly oligotrophic but also N+N-limited reflected from very low [N+N]/[SRP] ratio (0.02-0.15) in the mixed layer (ML). Overall, the N+N limitation and high nitrate anomaly value (N*: 2.47 ± 0.16 mM) above the nutricline depth strongly indicate prevailing N2 fixation at the surface of KW. Very high DON/DOP ratio in KW (16.9-69.1) probably resulted from the release of N-rich organic nutrients from phytoplankton including N2 fixers at the surface and faster recycling of DOP than DON in deep waters. Persistent coastal uplift of subsurface water occurs everywhere over the shore-side region of the KC, resulting in increasing surface concentrations of nutrients and chlorophyll a. Overall, the aforementioned physical and biogeochemical processes determined the upper-ocean distributions of nutrient species in warm seasons. [ABSTRACT FROM AUTHOR]

The spatial distribution of marine di-nitrogen (N2) fixation informs our understanding of the sensitivities of this process as well as the potential for this new nitrogen (N) source to drive export production, influencing the global carbon (C) cycle and climate. Using geochemically-derived δ15N budgets, we quantified rates of N2 fixation and its importance for supporting export production at stations sampled near the southwest Pacific Tonga-Kermadec Arc. Recent observations indicate that shallow (

A synoptic gulf-wide isoscape of carbon (δ13C) and nitrogen (δ15N) in the Gulf of Mexico based on mesozooplankton (335-1000 um) was used as a proxy for the isotopic baseline and for calculating regional contributions of dissolved inorganic nitrogen sources. Mesozooplankton were sampled at 0-200 m (depth permitting) during the XIXIMI-06 and GOMECC-3 cruises held during the summer of 2017. A striking latitudinal gradient was found in δ15N values of zooplankton, with the highest values (10.4 ± 1.2‰) found over the northern shelf, and lowest values in the central, oligotrophic gulf (1.9 ± 0.5‰). To estimate the fractional contribution of potential nitrogen sources, the gulf was divided into six regions based on the spatial distribution of surface Chl-a, SST from remote sensing products and likely region-specific source contributions. A literature survey of (δ13C and δ15N values of particulate organic matter was used to characterize region-specific endpoint isotope ratios for use in a Bayesian isotope mixing model. Regional differences in δ15N values and the results of mixing models indicated nitrogen fixation is most likely an important source (45-74% contribution) of new nitrogen in the oceanic regions of the Gulf, the Loop Current and the Yucatan Shelf. In the oligotrophic gulf, the potential input of relatively light nitrate that reflects remineralization of surface layer POM or the excretion of light nitrogen by heterotrophs was insufficient to explain the low δ15N values found in the central Gulf, although it could account for about 40% of the N supporting secondary production. The high nitrogen isotope ratios found in the northern shelf were attributed to denitrified N (60%) and the inflow of heavy nitrogen from the Mississippi-Atchafalaya river system. Our results support the potential importance of fixed nitrogen in the deep waters of the Gulf of Mexico during the summer, characteristic for its highly stratified surface waters.

Molybdenum plays an important role in marine biological activity, especially in nitrogen cycling as a cofactor for N2 fixation and nitrate reductase. However, the dissolved Mo (dMo) behavior and its interaction with N cycling in the coastal waters is still unclear. In this study, the dMo concentrations and parameters related to Mo distribution and N cycling in surface and bottom seawaters of the Bohai (BS) and Yellow Seas (YS) were examined. The results showed that dMo concentrations ranged from 36.4 nmol L-1 to 125.0 nmol L-1, most of which deviated significantly from the conservative line, indicating nonconservative behavior of Mo relative to salinity. The highest dMo concentrations occurring in 36°N section of north of the South YS (SYS), were close to conservative value (105 nmol L-1). Significant depletion up to 40-50 nmol L-1 of dMo mainly appeared in the BS, NYS and south of the SYS, suggesting the possible removal of dMo by biological utilization and particle adsorption. Particularly, the increasing dMo concentrations away the Yellow River estuary indicated that freshwater dilution was one of reasons for dMo distributions in the BS. The similar spatial distribution of dMo and dissolved Mn concentrations suggested the possible scavenging by MnOx phases for Mo removal. The negative correlation between dMo and chlorophyll-a (Chl-a) concentrations in surface seawaters suggested that biological uptake was involved in dMo removal. The depleted dMo in most of sites corresponded with the higher nitrite concentrations, implying the possible involvement of nitrate reduction process. Although the highest N2 fixation rates and relative abundances of cyanobacteria appeared in 36°N section, corresponding with the conservative dMo, suggesting that Mo may play a minor role in N2 fixation process there. The ten-folds of relative abundance of bacteria with nitrate reduction function than that with N2 fixation function suggested that dMo seems to play more important role in nitration reduction than nitrogen fixation in the BS and YS.

The apparently obligate symbiosis between the diazotroph Candidatus Atelocyanobacterium thalassa (UCYN-A) and its haptophyte host, Braarudosphaera bigelowii, has recently been found to fix dinitrogen (N2) in polar waters at rates (per cell) comparable to those observed in the tropical/subtropical oligotrophic ocean basins. This study presents the novel observation that this symbiosis increased in abundance during a wind-driven upwelling event along the Alaskan Beaufort shelfbreak. As upwelling relaxed, the relative abundance of B. bigelowii among eukaryotic phytoplankton increased most significantly in waters over the upper slope. As the host’s nitrogen demands are believed to be supplied primarily by UCYN-A, this response suggests that upwelling may enhance N2 fixation as displaced coastal waters are advected offshore, potentially extending the duration of upwelling-induced phytoplankton blooms. Given that such events are projected to increase in intensity and number with ocean warming, upwelling-driven N2 fixation as a feedback on climate merits investigation.

The increasing occurrence of heat waves and water turbidity are threats to the persistence of seagrass meadows. Their effects on the productivity of seagrasses and the functioning of their associated microorganisms have not been studied extensively. The purpose of this study is to assess the effects of different light levels and temperatures on Posidonia oceanica; the endemic seagrass species in the Mediterranean Sea, and their N2-fixing community, which contributes importantly to the nitrogen requirements and high productivity of the plants. Aquarium experiments were conducted in winter, when the plants are more vulnerable to changes in temperature, subjecting them to short-term exposures to an ambient (15.5°C) and elevated temperatures (ambient+5.5°C), and at limited (13 μmol photons m−2 s−1) and saturating light conditions (124 μmol photons m−2 s−1). Primary production, chlorophyll content, reactive oxygen species production, polyphenols content, the nifH gene expression, N2 fixation, and alkaline phosphatase activities were measured in different plant tissues. Plants incubated at ambient temperature and high light exhibited an enhanced total chlorophyll production. Significantly higher gross and net primary production rates were also recorded under ambient temperature and high light conditions, which were approximately two-fold compared to the rest of the treatments. The oxidative stress analyses revealed an increased production of reactive oxygen species in young leaves incubated at ambient temperature and saturating light, while the polyphenols content in top leaves was considerably higher under elevated temperatures. In contrast, the N2 fixation and alkaline phosphatase rates were significantly higher under elevated temperature and low light levels. The presence of the N2-fixing phylotypes UCYN-A, -B, and -C was detected through genetic analyses, with UCYN-B demonstrating the highest nifH gene transcription levels at elevated temperatures. These findings emphasize the significant role of irradiance on the productivity of P. oceanica and the temperature dependence of the N2 fixation process in winter.

Coastal and open ocean regions of the Western Tropical South Pacific ocean have been identified as a hotspot of N2 fixation. However, the environmental factors driving the temporal variability of abundance, composition, and activity of diazotrophs are still poorly understood, especially during the winter season. To address this, we quantified N2 fixation rates and the abundance of seven diazotroph phylotypes (UCYN-A1 symbiosis, UCYN-B, UCYN-C, Trichodesmium, Het-1, Het-2, and Het-3) on a monthly basis during two full years (2012 to 2014) at four stations along a coast to open ocean transect in the New Caledonian lagoon. The total nifH gene concentration (sum of all nifH gene copies) clearly decreased from the barrier reef to the shore. Apart from UCYN-B, which peaked at very high abundances (106–108nifH gene copies L–1) at two occasions at the coastal station, the UCYN-A1 symbiosis was the most abundant group at all stations, accounting for 79% of the total nifH gene copy counts along the transect (average abundance 4.2 ± 10.3 × 104nifH gene copies L–1). The next most abundant groups were in order Trichodesmium (accounting for 14% of the total nifH gene copies), Het-groups (6% of the total) and UCYN-C (1% of the total). Statistical analyses reveal that the UCYN-A1 symbiosis and Het groups were associated with cold (25°C) waters, low NOx and PO43– concentrations, strong and (mostly) easterly winds. Average N2 fixation rates over the survey were 6.5 ± 6.7 nmol N L–1 d–1 and did not differ significantly among seasons. The year to year variability was more pronounced with average integrated rates significantly higher in the second year of the survey (162 ± 122 μmol N m–2 d–1) than the first year (66 ± 91 μmol N m–2 d–1). This dataset suggests that seasonality is less pronounced than previously thought, and that relatively high N2 fixation rates are maintained in the New Caledonian lagoon all year long, despite seasonal changes in the diazotroph community composition.

The biological fixation of dinitrogen (N2) by marine microbes called “diazotrophs” sustains ∼50% new production in the ocean, boosting CO2 absorption by photoautotrophs and thus contributing to the mitigation of climate change. New environmental conditions sustaining N2 fixation have been revealed in recent years, enabling more accurate forecasting of future nitrogen inputs and localized hot spots. However, at present the paucity and biased geographical coverage of N2 fixation and diazotroph diversity measurements impede attempts to reconcile global nitrogen budgets with observed rates. Most studies have been conducted at disparate spatiotemporal scales, including: (i) discrete and short duration measurements in small seawater volumes isolated from the environment, and (ii) spatial extrapolations and global models of diazotrophy projected over decades to centuries. We argue that this knowledge gap lies at the fine scales: dynamic seawater structures < 200 km wide and < 2 months lifetime. However, the spatiotemporal resolution of conventional oceanographic cruises, with stations separated by tens to hundreds of kilometers, is too poor to resolve fine scale processes. Bridging this gap requires leveraging high spatiotemporal resolution measurements. Here we present and discuss the advantages and disadvantages of contemporary methods and equipment able to provide high-resolution measurements at sea. We also provide insights into high-resolution sampling approaches to be developed in the near future. Increasing the spatiotemporal resolution of diazotroph activity and diversity will provide more realistic quantifications of nitrogen fluxes in the dynamic ocean.

Nitrogen (N) is an essential element for life and controls the magnitude of primary productivity in the ocean. In order to describe the microorganisms that catalyze N transformations in surface waters in the South Pacific Ocean, we collected high-resolution biotic and abiotic data along a 7000 km transect, from the Antarctic ice edge to the equator. The transect, conducted between late Austral autumn and early winter 2016, covered major oceanographic features such as the polar front (PF), the subtropical front (STF) and the Pacific equatorial divergence (PED). We measured N2 fixation and nitrification rates and quantified the relative abundances of diazotrophs and nitrifiers in a region where few to no rate measurements are available. Even though N2 fixation rates are usually below detection limits in cold environments, we were able to measure this N pathway at 7/10 stations in the cold and nutrient rich waters near the PF. This result highlights that N2 fixation rates continue to be measured outside the well-known subtropical regions. The majority of the mid to high N2 fixation rates (>∼20 nmol L–1 d–1), however, still occurred in the expected tropical and subtropical regions. High throughput sequence analyses of the dinitrogenase reductase gene (nifH) revealed that the nifH Cluster I dominated the diazotroph diversity throughout the transect. nifH gene richness did not show a latitudinal trend, nor was it significantly correlated with N2 fixation rates. Nitrification rates above the mixed layer in the Southern Ocean ranged between 56 and 1440 nmol L–1 d–1. Our data showed a decoupling between carbon and N assimilation (NO3– and NH4+ assimilation rates) in winter in the South Pacific Ocean. Phytoplankton community structure showed clear changes across the PF, the STF and the PED, defining clear biomes. Overall, these findings provide a better understanding of the ecosystem functionality in the South Pacific Ocean across key oceanographic biomes.

The classical paradigm about marine N2 fixation establishes that this process is mainly constrained to nitrogen-poor tropical and subtropical regions, and sustained by the colonial cyanobacterium Trichodesmium spp. and diatom-diazotroph symbiosis. However, the application of molecular techniques allowed determining a high phylogenic diversity and wide distribution of marine diazotrophs, which extends the range of ocean environments where biological N2 fixation may be relevant. Between February 2014 and December 2015, we carried out 10 one-day samplings in the upwelling system off NW Iberia in order to: (1) investigate the seasonal variability in the magnitude of N2 fixation, (2) determine its biogeochemical role as a mechanism of new nitrogen supply, and (3) quantify the main diazotrophs in the region under contrasting hydrographic regimes. Our results indicate that the magnitude of N2 fixation in this region was relatively low (0.001 ± 0.002 – 0.095 ± 0.024 μmol N m−3 d−1), comparable to the lower-end of rates described for the subtropical NE Atlantic. Maximum rates were observed at the surface during both upwelling and relaxation conditions. The comparison with nitrate diffusive fluxes revealed the minor role of N2 fixation (

Large amounts of dust and atmospheric aerosols, originating from surrounding desert areas (e.g., Sahara and Middle East) are deposited annually on the surface of the Eastern Mediterranean Sea. These depositions can provide high amounts of micro (such as Fe, Zn, Co) and macro nutrients (such as P and N) to supplement nutrient-poor surface waters- that typically limit primary productivity and also dinitrogen (N2) fixation in many marine environments. Here, we studied the impact of the atmospheric deposition of dust and aerosols on N2 fixation in the Cretan Sea (Eastern Mediterranean Sea). Mixed polluted aerosols (hereafter A) and Saharan dust (hereafter SD) were added to nine mesocosms (3-m3 each) containing surface mixed layer seawater (~10 m), and N2 fixation was evaluated for 6 days during May 2012 (springtime). The addition of SD triggered a rapid (30 h) and robust (2-4 fold) increase in N2 fixation rates that remained high for 6 days and contributed 3-8% of the primary productivity. The A addition also resulted in higher N2 fixation rates compared to the unamended control mesocosms, although the responses were less profound (1.5-2 fold) and accounted for only 2-4% of the primary productivity. The microbial community responded differently to the two additions. Heterotrophic bacterial N2 fixers dominated the diazotroph community in A and the control mesocosms, while the non-filamentous cyanobacterial group Trichodesmium prevailed in the SD treatment (68% of all the operational taxonomic units, verified by qPCR analyses). Our results indicate that the aerosol source, its route prior to deposition, and its specific chemical composition, can alter the diazotrophic diversity and activity in the Eastern Mediterranean Sea and may thus impact both the N and C dynamics in this impoverished environment.

The dinitrogen (N2) released as dissolved nitrogen (DN) has been compared in batch cultures of four marine diazotrophic cyanobacteria: the colony forming Trichodesmium IMS101 and the unicellular strains Cyanothece ATCC51142, Crocosphaera watsonii WH8501 and WH0003. Two approaches were conducted for this purpose. The first approach consisted in the comparison of the total accumulation of fixed N2 in the culture (both in dissolved and particulate pools) with the net N2 fixation rates (i. e. the fixed N2 incorporated only in the particulate fraction after 15N2 incubation). The difference between the two measures accounted for the fixed N2 released as DN. The second approach consisted in the direct measure of the 15N-enrichment of ammonium (NH4+) and dissolved organic N (DON) following the 15N2 incubations. The N2 released as DN accounted for ~0 - 20 % and ~1 % of the N2 fixed after 24 h in the first and second approach, respectively. We show that the recent methodological improvements in the net N2 fixation determination applied in this study tend to reconcile the two approaches that formerly led to contrasted values. However, the large analytical uncertainties of the first approach limit its reliability. Thus, the direct determination of the 15N-enrichment of the dissolved pool remains the best tool to assess the fixed N2 released in the DN pool, in particular as it allows shorter incubation times. There were no clear patterns detected between the filamentous Trichodesmium and unicellular strains, neither in terms of the amount of N2 released as DN nor in terms of the proportion of NH4+ relative to DON. This suggests that N2 release processes are shared among the filamentous and free living diazotrophs.