Impact of diazotrophy on N stable isotope signatures of nitrate and particulate organic nitrogen: case studies in the north-eastern tropical Atlantic Ocean

During two independent cruises in the north-eastern tropical Atlantic Ocean, we applied two different approaches to investigate the impact of diazotrophy on nitrogen stable isotope signatures in nitrate and particulate organic nitrogen (PON) of the food-web constituents. The first approach, used during the Poseidon cruise 348 in the Mauritanian upwelling, investigated the long-term influence of diazotrophy on the natural abundance of 15N-NO-3 and PON. The second approach, adopted during the Cape Verde field cruise, applied stable isotope tracer addition experiments. These served to determine the instantaneous transfer of diazotrophic N to the higher trophic level. Both approaches showed that N2 fixation was compatible with the pattern and the magnitude of the isotopic depletion of dissolved NO-3 during the Mauritanian upwelling cruise, as well as PON in zooplankton and phytoplankton during the Cape Verde cruises. An N-budget using 15N incorporation rates and diazotrophic N2 fixation rates showed that 6 % of the daily N2 fixation was potentially taken up by the mesozooplankton community. Direct grazing accounted for 56 % of gross mesozooplanktonic N incorporation, while 46 % occurred due to channelling through the microbial loop

Incorporation of diazotrophic fixed N2 by mesozooplankton — Case studies in the southern Baltic Sea

During two simultaneous cruises in the Central Baltic Sea in July 2007 we applied a 15N tracer addition approach to assess the impact of cyanobacterial N2 fixation on mesozooplankton production in the Central Baltic Sea. We determined rates of diazotrophic 15N2 fixation, as well as uptake of diazotrophic derived 15N by mesozooplankton species. Diazotrophic 15N2 fixation rates were low representing pre-bloom situations. A first order estimate using a two source mixing model of natural δ15N-PON abundance revealed that diazotrophic fixed N contributed to 27 ± 8% to mesozooplankton biomass. Additionally, the application of stable isotope tracer showed that fixed 15N was detectable in the mesozooplankton fraction within 1 h after the onset of the incubation. On a daily basis, 5% up to 100% of newly fixed 15N and 14% of cyanobacteria standing stock were incorporated by mesozooplankton species in our experimental set-ups. By applying size fractionating experiments and the usage of different control treatments, we calculated that the majority of 15N transfer (67%) was mediated by the release of nitrogenous compounds and their channelling through the microbial loop towards the mesozooplankton community. Moreover, direct grazing on filamentous cyanobacteria accounted for 33% of gross 15N incorporation. Grazing in the experiments seemed to be largely influenced by cyanobacterial species dominating the community and by the abundance of Cladoceran species like Evadne. Overall, N2 fixing cyanobacteria are ecological more important as instantaneous sources of nitrogen for higher trophic levels of the Baltic Sea food web than previously assumed

Biotic and abiotic water-column characteristics (Chla, Phaeo, TC, TN, δ13C, δ15N) at stations in the Weddell Sea (POLARSTERN cruise PS 96, ANT-XXXI/2, December 2015–February 2016)

This dataset contains biotic and abiotic data from water-column samples taken with Niskin bottles mounted on the CTD rosette from 14 sites in the Weddell Sea (mostly South-Eastern). Data are provided for water-column pigments (chlorophyll a and phaeopigment content through fluorometry), total carbon (TC) and nitrogen (TN) and stable isotope values of carbon and nitrogen (δ13C, δ15N) from the chlorophyll maximum (Cmax, defined as the water depth with maximum fluorescence detected during in-situ profiles) and close to the sea bottom. Water was filtered onto glass fiber filters (GF/C for pigments, combusted GF/F for C and N analyses) and stored at -80°C prior to analysis. Detailed methods are described in Säring et al. (submitted) except for stable isotopes: Flash combustion in a Flash 2000 (Thermo) elemental analyser to a Delta V advantage (Thermo) isotope ratio masspectrometer. δ values are reported relative to atmospheric N₂ (δ15N) and Vienna PeeDee Belemnite (δ13C). Reference materials for stable isotope analysis: IAEA-N1, IAEA-N2, IAEA-N3, NBS 22, IAEA-CH-3 and IAEA-CH-6; calibration material: Acetanilide (Merck). The analytical precision for both stable isotope ratios was <±0.2‰. Samples were not acidified to avoid loss of material. We assumed low inorganic carbon content in the water column for our analyses. This data table is part of a larger study analysing the role of environmental parameters for meio- and macrofaunal community composition (see Related to below)

Sediment characteristics (Chla, Phaeo, TOC, TN, δ13C, δ15N, grain size) at stations in the Weddell Sea (POLARSTERN cruise PS 96, ANT-XXXI/2, December 2015–February 2016)

This dataset contains abiotic and biotic data from sediment samples from nine sites in the Weddell Sea (mostly South-Eastern). Data are provided for sediment pigments (chlorophyll a and phaeopigment content through fluorometry), total organic carbon (TOC), total nitrogen (TN), stable isotope values of carbon and nitrogen (δ13C, δ15N) and grain size (silt&clay 1000 µm). Before the TOC and carbon isotope analysis the sediment samples were acidified to eliminate inorganic carbon. A minimum of three replicate samples (cores) were collected using a MUC10 multicorer or giant box corer. Sediment cores were subsampled with a 60-ml syringe (inner diameter 2.7 cm) for stations 017, 026, 061, 072, and with a 10-ml syringe (inner diameter 1 cm) for stations 001, 037, 048, 104, 115. Subsamples were sliced in 1-cm steps down to 5 cm depth. Detailed methods are described in Säring et al. (submitted) except for stable isotopes: Flash combustion in a Flash 2000 (Thermo) elemental analyser to a Delta V advantage (Thermo) isotope ratio masspectrometer. δ values are reported relative to atmospheric N2 (δ15N) and Vienna PeeDee Belemnite (δ13C). Reference materials for stable isotope analysis: IAEA-N1, IAEA-N2, IAEA-N3, NBS 22, IAEA-CH-3 and IAEA-CH-6; calibration material: Acetanilide (Merck). The analytical precision for both stable isotope ratios was <±0.2‰. Cores with the label -e (Environment) were only used to collect the above data. Environmental and fauna data were collected from cores with the label -i (Incubation). This data table is part of a larger study analysing the role of environmental parameters for meio- and macrofaunal community composition (see Related to below)

Sediment characteristics (Chla, Phaeo, TOC, TN, δ13C, δ15N, grain size) at four stations in the Weddell Sea (POLARSTERN cruise PS118, February - April 2019)

This dataset contains abiotic and biotic data from sediment samples from four sites in the northwestern Weddell Sea. Data are provided for sediment pigments (chlorophyll a and phaeopigment content through fluorometry), total organic carbon (TOC), total nitrogen (TN), stable isotope values of carbon and nitrogen (δ13C, δ15N) and grain size (silt&clay 1000 µm). A minimum of three replicate samples (cores) were collected using a MUC10 multicorer. All sediment cores were subsampled with a 10-ml syringe (inner diameter 1 cm). Subsamples were sliced in 1-cm steps down to 5 cm depth. The same sampling and measurements as for PS 96 were carried out (see below "Related to". Prior to TOC and δ13C analysis sediment samples were acidified to eliminate inorganic carbon. Detailed methods are described in Säring et al. (2022), except for stable isotopes: Flash combustion in a Flash 2000 (Thermo) elemental analyser to a Delta V advantage (Thermo) isotope ratio masspectrometer. δ values are reported relative to atmospheric N2 (δ15N) and Vienna PeeDee Belemnite (δ13C). Reference materials for stable isotope analysis: IAEA-N1, IAEA-N2, IAEA-N3, NBS 22, IAEA-CH-3 and IAEA-CH-6; calibration material: Acetanilide (Merck). The analytical precision for both stable isotope ratios was <±0.2‰. Cores with the label -e (Environment) were only used to collect the above data. Environmental, fauna and benthic boundary flux data were collected from cores with the label -i (Incubation). This data table is part of studies analyzing the role of environmental parameters for meio- and macrofaunal community compositions and polychaete community distribution patterns (see "Supplement to", "Related to" and "Further details" below)

Superposition of Individual Activities: Urea-mediated Suppression of Nitrate Uptake in the Dinoflagellate Prorocentrum minimum Revealed at the Population and Single-cell Levels

Dinoflagellates readily use diverse inorganic and organic compounds as nitrogen sources, which is advantageous in eutrophied coastal areas exposed to high loads of anthropogenic nutrients, e.g. urea, one of the most abundant organic nitrogen substrates in seawater. Cell-to-cell variability in nutritional physiology can further enhance the diversity of metabolic strategies among dinoflagellates of the same species, but it has not been studied in free-living microalgae. We applied stable isotope tracers, isotope ratio mass spectrometry and nanoscale secondary ion mass spectrometry (NanoSIMS) to investigate the response of cultured nitrate-acclimated dinoflagellates Prorocentrum minimum to a sudden input of urea and the effect of urea on the concurrent nitrate uptake at the population and single-cell levels. We demonstrate that inputs of urea lead to suppression of nitrate uptake by P. minimum, and urea uptake exceeds the concurrent uptake of nitrate. Individual dinoflagellate cells within a population display significant heterogeneity in the rates of nutrient uptake and extent of the urea-mediated inhibition of the nitrate uptake, thus forming several groups characterized by different modes of nutrition. We conclude that urea originating from sporadic sources is rapidly utilized by dinoflagellates and can be used in biosynthesis or stored intracellularly depending on the nutrient status; therefore, sudden urea inputs can represent one of the factors triggering or supporting harmful algal blooms. Significant physiological heterogeneity revealed at the single-cell level is likely to play a role in alleviation of intra-population competition for resources and can affect the dynamics of phytoplankton populations and their maintenance in natural environments