Editorial: Marine N2 Fixation: Recent Discoveries and Future Challenges

International audienceUnicellular nitrogen fixing cyanobacteria (UCYN) are abundant members of phytoplankton communities in a wide range of marine environments, including those with rapidly changing nitrogen (N) concentrations. We hypothesized that differences in N availability (N 2 vs. combined N) would cause UCYN to shift strategies of intracellular N and C allocation. We used transmission electron microscopy and nanoscale secondary ion mass spectrometry imaging to track assimilation and intracellular allocation of 13 C-labeled CO 2 and 15 N-labeled N 2 or NO 3 at different periods across a diel cycle in Cyanothece sp. ATCC 51142. We present new ideas on interpreting these imaging data, including the influences of pre-incubation cellular C and N contents and turnover rates of inclusion bodies. Within cultures growing diazotrophically, distinct subpopulations were detected that fixed N 2 at night or in the morning. Additional significant within-population heterogeneity was likely caused by differences in the relative amounts of N assimilated into cyanophycin from sources external and internal to the cells. Whether growing on N 2 or NO 3 , cells prioritized cyanophycin synthesis when N assimilation rates were highest. N assimilation in cells growing on NO 3 switched from cyanophycin synthesis to protein synthesis, suggesting that once a cyanophycin quota is met, it is bypassed in favor of protein synthesis. Growth on NO 3 also revealed that at night, there is a very low level of CO 2 assimilation into polysaccharides simultaneous with their catabolism for protein synthesis. This study revealed multiple, detailed mechanisms underlying C and N management in Cyanothece that facilitate its success in dynamic aquatic environments

Abundance and distribution of invertebrate larvae in the Bellingshausen Sea (West Antarctica)

The central part of the Bellingshausen Sea has been poorly studied, partly because of the presence of ice during most of the year. The main aim of this study was to analyse the abundance and distribution of meroplankton, and the influence of oceanographic properties were investigated in the Bellingshausen Sea (West Antarctica) during the BENTART-06 cruise carried out in January–February 2006. Zooplankton samples were collected with a 80-μm mesh plankton net hauled vertically from a depth of 200 m to the surface at fifteen stations across the Bellingshausen Sea. Fifteen types of larval benthic invertebrates were found, with echinospira and nudibranch veligers being the most abundant. Hierarchical analysis and non-metric multidimensional scaling revealed a high degree of spatial variability in both larval abundance and larval types across the Bellingshausen Sea. The variability was significantly correlated with total chlorophyll-a and the contribution of large (>5 μm) phytoplankton to total chlorophyll, indicating the availability of food as an important factor determining the larval distribution observed. Nudibranch veligers, nemertean pilidia, echinoderm and planula larvae were more abundant at stations in the central Bellingshausen Sea, which was characterized by low phytoplankton biomass and production. Higher abundances of echinospira veligers and polychaete larvae were found at the more productive stations close to Peter I Island and the Antarctic Peninsula. The abundance and diversity of larval types found in the Bellingshausen Sea during the BENTART-06 cruise support the hypothesis that indirect development through larval swimming stages plays a key role in benthic recruitment in polar areas.Ministerio de Ciencia y Tecnología; CGL2004-0185

Nutrient limitation suppresses the temperature dependence of phytoplankton metabolic rates

Climate warming has the potential to alter ecosystem function through temperature-dependent changes in individual metabolic rates. The temperature sensitivity of phytoplankton metabolism is especially relevant, since these microorganisms sustain marine food webs and are major drivers of biogeochemical cycling. Phytoplankton metabolic rates increase with temperature when nutrients are abundant, but it is unknown if the same pattern applies under nutrient-limited growth conditions, which prevail over most of the ocean. Here we use continuous cultures of three cosmopolitan and biogeochemically relevant species (Synechococcus sp., Skeletonema costatum and Emiliania huxleyi) to determine the temperature dependence (activation energy, Ea) of metabolism under different degrees of nitrogen (N) limitation. We show that both CO2 fixation and respiration rates increase with N supply but are largely insensitive to temperature. Ea of photosynthesis (0.11 ± 0.06 eV, mean ± SE) and respiration (0.04 ± 0.17 eV) under N-limited growth is significantly smaller than Ea of growth rate under nutrient-replete conditions (0.77 ± 0.06 eV). The reduced temperature dependence of metabolic rates under nutrient limitation can be explained in terms of enzyme kinetics, because both maximum reaction rates and half-saturation constants increase with temperature. Our results suggest that the direct, stimulating effect of rising temperatures upon phytoplankton metabolic rates will be circumscribed to ecosystems with high-nutrient availabilityMinisterio de Economía y Competitividad | Ref. CTM2014-53582-

Fine scale physical-biological interactions during a shift from relaxation to upwelling with a focus on Dinophysis acuminata and its potential ciliate prey

Wind reversals and quick transitions from relaxation to upwelling in coastal areas cause major changes in water column structure, phytoplankton distribution and dominance, and rates of physiological processes. The cruise "ASIMUTH-Rías" (17–21 June 2013) was carried out in the Galician Rías and adjacent shelf, at the time of a DSP outbreak, to study small-scale physical processes associated with late spring blooms of "D. acuminata" and accompanying microzooplanktonic ciliates with the overall objective of improving predictive models of their occurrence. The cruise coincided with the initiation of an upwelling pulse following relaxation and deepening of a previously formed thin layer of diatoms. A 36-h cell cycle study carried on 18–20 June showed the vertical excursions of the thin layer, mainly delimited by the 13.5–14 °C isotherms and turbulence levels (ε) of 10−8–10−6 m2 s −3, as well as marked changes in phytoplankton composition (increased density and dominance of diatoms). There was no evidence of daily vertical migration of D. acuminata, which remained in the top layer during the cycle study, but the opposite was observed in the ciliate populations. Dinophysis and its potential prey (Mesodinium species) cell maxima overlapped after midday, when the ciliate moved to the surface, suggesting an “ambush” strategy of Dinophysis to catch prey. A remarkable decline (from 0.65 to 0.33 d−1) in division rates (µ) of D. acuminata was associated with increased turbulence (ε 2°C in about 8 h). In contrast, high division rates (µmin ∼ 0.69 d−1) persisted at a mid-shelf station where environmental conditions below the mixed layer were more stable. The onset of upwelling pulses appears to have a double negative effect on the net growth of Dinophysis populations: a direct physical effect due to advective dispersion and an indirect effect, decreased division rates. The latter would be caused by the rapid cooling of the mixed layer, and the increased turbulence at the surface resulting in shear stress to the cells. The short-term impact of upwelling pulses (and the winds promoting it) on the physiology of "Dinophysis" and its ciliate prey, and the role of mid-shelf populations of "Dinophysis" as a relatively undisturbed reservoir for the inoculation of subsequent blooms are discussed.European Commission | Ref. EC FP7-SPACE-2010-1, n. 261860Ministerio de Economía | Ref. CTM2016-75451-C2-2-RInterreg Atlantic Area | Ref. EAPA_182/2016Comisión Nacional de Ciencia y Tecnología Research (CONICYT), Chile | Ref. PAI79160065Comisión Nacional de Ciencia y Tecnología Research (CONICYT), Chile | Ref. REDES17010

High sensitivity of ultra-oligotrophic marine ecosystems to atmospheric nitrogen deposition

Using a model of plankton and organic-matter cycling we demonstrate that variable stoichiometric ratios can lead to a more than 5-fold higher sensitivity of simulated carbon export to atmospheric N deposition in the ultra-oligotrophic eastern part of the North Atlantic subtropical gyre compared to the westerly oligotrophic region near Bermuda, often used as a reference site for subtropical regions. Stronger nutrient limitation in the ultra-oligotrophic east causes higher phytoplankton C:N ratios and lower carbon assimilation efficiency of zooplankton in the model, which results in a higher export efficiency of carbon to the deep ocean compared to the less nutrient-limited western site. Our results indicate that previous estimates of oceanic carbon uptake associated with atmospheric nitrogen deposition may not be fully robust and that spatial variability in nutrient stress and ecological stoichiometry could significantly affect the biogeochemical impact of increasing atmospheric deposition of anthropogenic nitrogen

Temporal variability of diazotroph community composition in the upwelling region off NW Iberia.

Knowledge of the ecology of N2-fixing (diazotrophic) plankton is mainly limited to oligotrophic (sub)tropical oceans. However, diazotrophs are widely distributed and active throughout the global ocean. Likewise, relatively little is known about the temporal dynamics of diazotrophs in productive areas. Between February 2014 and December 2015, we carried out 9 one-day samplings in the temperate northwestern Iberian upwelling system to investigate the temporal and vertical variability of the diazotrophic community and its relationship with hydrodynamic forcing. In downwelling conditions, characterized by deeper mixed layers and a homogeneous water column, non-cyanobacterial diazotrophs belonging mainly to nifH clusters 1G (Gammaproteobacteria) and 3 (putative anaerobes) dominated the diazotrophic community. In upwelling and relaxation conditions, affected by enhanced vertical stratification and hydrographic variability, the community was more heterogeneous vertically but less diverse, with prevalence of UCYN-A (unicellular cyanobacteria, subcluster 1B) and non-cyanobacterial diazotrophs from clusters 1G and 3. Oligotyping analysis of UCYN-A phylotype showed that UCYN-A2 sublineage was the most abundant (74%), followed by UCYN-A1 (23%) and UCYN-A4 (2%). UCYN-A1 oligotypes exhibited relatively low frequencies during the three hydrographic conditions, whereas UCYN-A2 showed higher abundances during upwelling and relaxation. Our findings show the presence of a diverse and temporally variable diazotrophic community driven by hydrodynamic forcing in an upwelling system

Nitrogen inputs influence on biomass and trophic structure of ocean plankton: a study using biomass and stable isotope size-spectra.

Large scale patterns in planktonic food web structure were studied by applying continuous size-scaled models of biomass and δ15N to plankton samples, collected at 145 stations during the Malaspina-2010 Expedition across three ocean basins and including major biomes. Carbon biomass and δ15N were determined in size-fractionated samples (40 to 5000 μm) collected by vertical hauls (0-200 m). Biomass-normalized size-spectra were constructed to summarize food web structure and spatial patterns in spectral parameters were analyzed using geographically-weighted regression analysis. Except in the northwestern Atlantic, size-spectra showed low variability, reflecting a large homogeneity in nitrogen sources and food web structure for the central oceans. Estimated predator-to-prey mass ratios 20% (Trades and Westerlies biomes) suggested that oceanic plankton food webs could support a larger number of trophic levels than current estimates based on high efficiency values. The largest changes in spectral parameters and nitrogen sources were related to inputs of atmospheric nitrogen, either from diazotrophic organisms or dust deposition. These results suggest geographic homogeneity in the net transfer of nitrogen up the food web.CONSOLIDER-INGENIO 2010 (CSD2008-00077) ; EURO-BASIN (FP7-ENV-2010 264933)Preprint1,749

Vertical zonation of bacterial assemblages attributed to physical stratification during the summer relaxation of the coastal upwelling off Galicia (NW Spain)

Research articleWe combined flow cytometry, CARD-FISH, and 16S rRNA gene tag pyrosequencing to investigate bacterioplankton dynamics along a transect in shelf waters off A Coruña (Galicia, NW Spain). Over five days (16-20th July 2012) we sampled during the relaxation of a summer upwelling pulse, providing an opportunity to examine the impact of pulses of cold nutrient-rich water into coastal microbial communities. The hydrographic conditions, characterized by intense density stratification of surface waters and the presence of a deep chlorophyll maximum (DCM) at 20–30 m, were relatively maintained over the sampling period. Indeed, bacterial abundance and composition displayed low day to day variation. Alpha diversity analysis suggested that species richness and diversity increased from coastal to shelf stations and from the surface down to the coastal DCM, which could be caused by the mixing of upwelled bacteria with the coastal surface waters. SAR11, SAR86, and Roseobacter were the most abundant bacteria detected in the samples by using CARD-FISH. The assemblages observed by pyrosequencing displayed a strong vertical zonation along the transect. Rhodobacteraceae (under class Alphaproteobacteria) and Bacteriodetes dominated the surface waters and decreased during the upwelling pulse, while SAR 86 (under class Gammaproteobacteria), Actinobacteria and SAR11 clade increased their relative abundance at the coastal DCM with upwelling relaxation, particularly at the shelf stations. Bacterial assemblages from surface waters were associated with higher temperature and light conditions, while coastal DCM assemblages were rather associated to salinity, inorganic nutrients and a diatom-bloom leading to high chlorophyll-a. Our findings suggest that the vertical variability in environmental conditions induced by the intense density stratification, the exportation of warmer and less saline surface water from the rias to the adjacent shelf, and the fertilizing effect of recently upwelled water at the deeper layer, determined the composition of distinct bacterial assemblages at the subsurface and DCM layers.XUNTA DE GALICIA (O9MMA 027604PR, 10MMA604024PR), IEO (PROYECTO RADIALES) y PLAN NACIONAL (MODUPLAN CTM2011-24008)Versión del editor2,24

Short term variability of bacterial communities in the shelf waters off Galicia (NW Iberian Peninsula): Patterns and drivers shaping the diversity

PósterEcological and biogeochemical processesEcological and biogeochemical processes in the ocean are dependent on a diverse assemblage of microbes, including members of Bacteria (Glöckner et al, 2012). These bacterial assemblages occurs in vast numbers and represent a huge genetic diversity, fulfilling a wide of ecological roles in the marine system such as carbon geochemical cycle and energy transfer into higher trophic levels.We combined flow cytometry, CARD-FISH and 16S rRNA gene tag pyrosequencing to investigate the short-term variability of the bacterial communities in samples collected along a transect in the shelf off NW Iberian Peninsula over one week

Temporal variability of diazotroph community composition in the upwelling region off NW Iberia

Knowledge of the ecology of N 2 -fixing (diazotrophic) plankton is mainly limited to oligotrophic (sub)tropical oceans. However, diazotrophs are widely distributed and active throughout the global ocean. Likewise, relatively little is known about the temporal dynamics of diazotrophs in productive areas. Between February 2014 and December 2015, we carried out 9 one-day samplings in the temperate northwestern Iberian upwelling system to investigate the temporal and vertical variability of the diazotrophic community and its relationship with hydrodynamic forcing. In downwelling conditions, characterized by deeper mixed layers and a homogeneous water column, non-cyanobacterial diazotrophs belonging mainly to nifH clusters 1G (Gammaproteobacteria) and 3 (putative anaerobes) dominated the diazotrophic community. In upwelling and relaxation conditions, affected by enhanced vertical stratification and hydrographic variability, the community was more heterogeneous vertically but less diverse, with prevalence of UCYN-A (unicellular cyanobacteria, subcluster 1B) and non-cyanobacterial diazotrophs from clusters 1G and 3. Oligotyping analysis of UCYN-A phylotype showed that UCYN-A2 sublineage was the most abundant (74%), followed by UCYN-A1 (23%) and UCYN-A4 (2%). UCYN-A1 oligotypes exhibited relatively low frequencies during the three hydrographic conditions, whereas UCYN-A2 showed higher abundances during upwelling and relaxation. Our findings show the presence of a diverse and temporally variable diazotrophic community driven by hydrodynamic forcing in an upwelling system.Xunta de Galicia | Ref. EM2013/021Ministerio de Economía, Industria y Competitividad | Ref. CTM2016-75451-C2-1-RMinisterio de Educación, Cultura y Deporte | Ref. FPU13/01674Ministerio de Educación, Cultura y Deporte | Ref. EST16/00142Universidad de Vigo | Ref. Axudas á investigación 201