Academic literature on the topic 'Planktonic diazotrophs'

Abstract. Biological dinitrogen (N2) fixation is the major source of new nitrogen (N) for the open ocean, and thus promotes marine productivity, in particular in the vast N-depleted regions of the surface ocean. Yet, the fate of the diazotroph-derived N (DDN) in marine ecosystems is poorly understood, and its transfer to auto- and heterotrophic surrounding plankton communities is rarely measured due to technical limitations. Moreover, the different diazotrophs involved in N2 fixation (Trichodesmium spp. vs. UCYN) exhibit distinct patterns of N2 fixation and inhabit different ecological niches, thus having potentially different fates in the marine food webs that remain to be explored. Here we used nanometer scale secondary ion mass spectrometry (nanoSIMS) coupled with 15N2 isotopic labelling and flow cytometry cell sorting to examine the DDN transfer to specific groups of natural phytoplankton and bacteria during artificially induced diazotroph blooms in New Caledonia (southwestern Pacific). The fate of the DDN was compared according to the three diazotrophs: the filamentous and colony-forming Trichodesmium erythraeum (IMS101), and the unicellular strains Crocosphaera watsonii WH8501 and Cyanothece ATCC51142. After 48 h, 7–17 % of the N2 fixed during the experiment was transferred to the dissolved pool and 6–12 % was transferred to non-diazotrophic plankton. The transfer was twice as high in the T. erythraeum bloom than in the C. watsonii and Cyanothece blooms, which shows that filamentous diazotrophs blooms are more efficient at promoting non-diazotrophic production in N-depleted areas. The amount of DDN released in the dissolved pool did not appear to be a good indicator of the DDN transfer efficiency towards the non-diazotrophic plankton. In contrast, the 15N-enrichment of the extracellular ammonium (NH4+) pool was a good indicator of the DDN transfer efficiency: it was significantly higher in the T. erythraeum than in unicellular diazotroph blooms, leading to a DDN transfer twice as efficient. This suggests that NH4+ was the main pathway of the DDN transfer from diazotrophs to non-diazotrophs. The three simulated diazotroph blooms led to significant increases in non-diazotrophic plankton biomass. This increase in biomass was first associated with heterotrophic bacteria followed by phytoplankton, indicating that heterotrophs took the most advantage of the DDN in this oligotrophic ecosystem.

Over the past decade, coral bleaching events have continued to recur and intensify. During bleaching, corals expel millions of their symbionts, depriving the host from its main food source. One mechanism used by corals to resist bleaching consists in exploiting food sources other than autotrophy. Among the food sources available in the reefs, dinitrogen (N2)-fixing prokaryotes or planktonic diazotrophs (hereafter called ‘PD’) have the particularity to reduce atmospheric dinitrogen (N2) and release part of this nitrogen (diazotroph-derived nitrogen or DDN) in bioavailable form. Here, we submitted coral colonies of Stylophora pistillata, fed or not with planktonic diazotrophs, to a temperature stress of up to 31 ± 0.5 °C and measured their physiological responses (photosynthetic efficiency, symbiont density, and growth rates). Heat-unfed colonies died 8 days after the heat stress while heat-PD-fed corals remained alive after 10 days of heat stress. The supply of PD allowed corals to maintain minimal chlorophyll concentration and symbiont density, sustaining photosynthetic efficiency and stimulating coral growth of up to 48% compared to unfed ones. By providing an alternative source of bioavailable nitrogen and carbon, this specific planktonic diazotroph feeding may have a profound potential for coral bleaching recovery.

Abstract. The VAHINE mesocosm experiment in the oligotrophic waters of the Nouméa lagoon (New Caledonia), where high N2 fixation rates and abundant diazotroph organisms were observed, aimed to assess the role of the nitrogen input through N2 fixation in carbon production and export and to study the fate of diazotroph-derived nitrogen (DDN) throughout the planktonic food web. A 1-D vertical biogeochemical mechanistic model was used in addition to the in situ experiment to enrich our understanding of the dynamics of the planktonic ecosystem and the main biogeochemical carbon (C), nitrogen (N) and phosphate (P) fluxes. The mesocosms were intentionally enriched with ∼ 0.8 µmol L−1 of inorganic P to trigger the development of diazotrophs and amplify biogeochemical fluxes. Two simulations were run, one with and the other without the phosphate enrichment. In the P-enriched simulation, N2 fixation, primary production (PP) and C export increased by 201, 208 and 87 %, respectively, consistent with the trends observed in the mesocosms (+124, +141 and +261 % for N2 fixation, PP and C export, respectively). In total, 5–10 days were necessary to obtain an increase in primary and export productions after the dissolved inorganic phosphate (DIP) enrichment, thereby suggesting that classical methods (short-term microcosms experiments) used to quantify nutrient limitations of primary production may not be relevant. The model enabled us to monitor the fate of fixed N2 by providing the proportion of DDN in each compartment (inorganic and organic) of the model over time. At the end of the simulation (25 days), 43 % of the DDN was found in the non-diazotroph organisms, 33 % in diazotrophs, 16 % in the dissolved organic nitrogen pool, 3 % in the particulate detrital organic pool and 5 % in traps, indicating that N2 fixation was of benefit to non-diazotrophic organisms and contributed to C export.

AbstractThe 15N2-tracer assay [Montoya et al. (1996) A simple, high-precision, high-sensitivity tracer assay for N2 fixation. Appl. Environ. Microbiol., 62, 986–993.] is the most used method for measuring biological N2 fixation in terrestrial and aquatic environments. The reliability of this technique depends on the purity of the commercial 15N2 gas stocks used. However, Dabundo et al. [(2014) PLoS One, 9, e110335.] reported the contamination of some of these stocks with labile 15N-labeled compounds (ammonium, nitrate and/or nitrite). The contamination of commercial 15N2 gas stocks with 15N-labeled nitrate and 142 ammonium and consequences for nitrogen fixation measurements. Considering that the tracer assay relies on the conversion of isotopically labeled 15N2 into organic nitrogen, this contamination may have led to overestimated N2 fixation rates. We conducted laboratory and field experiments in order to (i) test the susceptibility of 15N contaminants to assimilation by non-diazotroph organisms and (ii) determine the potential overestimation of the N2 fixation rates estimated in the field. Our findings indicate that the contaminant 15N-compounds are assimilated by non-diazotrophs organisms, leading to an overestimation of N2 fixation rates in the field up to 16-fold under hydrographic conditions of winter mixing.

Abstract. In marine ecosystems, biological N2 fixation provides the predominant external source of nitrogen (N; 140 ± 50 Tg N yr−1), contributing more than atmospheric and riverine inputs to the N supply. Yet the fate and magnitude of the newly fixed N, or diazotroph-derived N (hereafter named DDN) in marine ecosystems is poorly understood. Moreover, whether the DDN is preferentially and directly exported out of the photic zone, recycled by the microbial loop and/or transferred into larger organisms remains unclear. These questions were investigated in the framework of the VAHINE (VAriability of vertical and tropHIc transfer of diazotroph derived N in the south wEst Pacific) project. Triplicate large volume ( ∼ 50 m3) mesocosms were deployed in the tropical south-west Pacific coastal ocean (New Caledonia). The mesocosms were intentionally fertilized with ∼ 0.8 µM dissolved inorganic phosphorus (DIP) at the start of the experiment to stimulate diazotrophy. A total of 47 stocks, fluxes, enzymatic activities and diversity parameters were measured daily inside and outside the mesocosms by the 40 scientists involved in the project. The experiment lasted for 23 days and was characterized by two distinct and successive diazotroph blooms: a dominance of diatom-diazotroph associations (DDAs) during the first half of the experiment (days 2–14) followed by a bloom of unicellular cyanobacterial lineage C (UCYN-C during the second half of the experiment (days 15–23). These conditions provided a unique opportunity to compare the DDN transfer and export efficiency associated with different diazotrophs. Here we summarize the major experimental and modelling results obtained during the project and described in the VAHINE special issue, in particular those regarding the evolution of the main standing stocks, fluxes and biological characteristics over the 23-day experiment, the contribution of N2 fixation to export fluxes, the DDN released to dissolved pool and its transfer to the planktonic food web (bacteria, phytoplankton, zooplankton). We then apply our Eco3M modelling platform to further infer the fate of DDN in the ecosystem and the role of N2 fixation on productivity, food web structure and carbon export. Recommendations for future work are finally provided in the conclusion section.

Though nitrogen fixation by epiphytic diazotrophs on pelagic Sargassum has been recognized for decades, it has been assumed to contribute insignificantly to the overall marine nitrogen budget. This six-year study reframes this concept through long-term measurements of Sargassum community nitrogen fixation rates, and by extrapolating mass-specific rates to a theoretical square meter portion of Sargassum mat allowing for comparison of these rates to those of other marine and coastal diazotrophs. On 24 occasions from 2015 to 2021, rates of nitrogen fixation were measured using whole fronds of Sargassum collected from the western edge of the Gulf Stream off Cape Hatteras, North Carolina. Across all dates, mass-specific rates ranged from 0 to 37.77 μmol N g-1 h-1 with a mean of 4.156 μmol N g-1 h-1. Extrapolating using a mat-specific density of Sargassum, these rates scale to a range of 0 to 30,916 μmol N m-2 d-1 and a mean of 3,697 μmol N m-2 d-1. Quantifying this community’s rates of nitrogen fixation over several years captured the sometimes-extreme variability in rates, characteristic of marine diazotrophs, which has not been reported in the literature to date. When these measurements are considered alongside estimates of the density of pelagic Sargassum, rates of nitrogen fixation by Sargassum’s epiphytic diazotrophs rival that of their coastal macrophyte and planktonic counterparts. Given Sargassum’s wide and expanding geographic range, the results of this study suggest this community may contribute reactive nitrogen on a meaningful, basin-wide scale, which merits further study.

ABSTRACT Dinitrogen (N2)-fixing microorganisms (diazotrophs) play important roles in ocean biogeochemistry and plankton productivity. In this study, we examined the presence and expression of specific planktonic nitrogenase genes (nifH) in the upper ocean (0 to 175 m) at Station ALOHA in the oligotrophic North Pacific Ocean. Clone libraries constructed from reverse-transcribed PCR-amplified mRNA revealed six unique phylotypes. Five of the nifH phylotypes grouped with sequences from unicellular and filamentous cyanobacteria, and one of the phylotypes clustered with γ-proteobacteria. The cyanobacterial nifH phylotypes retrieved included two sequence types that phylogenetically grouped with unicellular cyanobacteria (termed groups A and B), several sequences closely related (97 to 99%) to Trichodesmium spp. and Katagnymene spiralis, and two previously unreported phylotypes clustering with heterocyst-forming nifH cyanobacteria. Temporal patterns of nifH expression were evaluated using reverse-transcribed quantitative PCR amplification of nifH gene transcripts. The filamentous and presumed unicellular group A cyanobacterial phylotypes exhibited elevated nifH transcription during the day, while members of the group B (closely related to Crocosphaera watsonii) unicellular phylotype displayed greater nifH transcription at night. In situ nifH expression by all of the cyanobacterial phylotypes exhibited pronounced diel periodicity. The γ-proteobacterial phylotype had low transcript abundance and did not exhibit a clear diurnal periodicity in nifH expression. The temporal separation of nifH expression by the various phylotypes suggests that open ocean diazotrophic cyanobacteria have unique in situ physiological responses to daily fluctuations of light in the upper ocean.

AbstractFernández, A., Graña, R., Mouriño-Carballido, B., Bode, A., Varela, M., Domínguez-Yanes, J. F., Escánez, J., de Armas, D., and Marañón, E. 2013. Community N2 fixation and Trichodesmium spp. abundance along longitudinal gradients in the eastern subtropical North Atlantic. – ICES Journal of Marine Science, 70:223–231. We have determined planktonic community N2 fixation, Trichodesmium abundance, the concentration and vertical diffusive flux of phosphate, and satellite-derived estimates of atmospheric concentration of dust along two longitudinal transects in the eastern subtropical North Atlantic during November 2007 and from April–May 2008. Trichodesmium abundance was particularly low (<3 trichome l−1) during the spring 2008 cruise, when low sea surface temperatures were recorded and vertical stratification was less marked. However, community N2 fixation was always measurable, albeit low compared with other regions of the tropical Atlantic. The average, vertically-integrated N2 fixation rate was 1.20 ± 0.48 µmol N m−2 d−1 in autumn 2007 and 8.31 ± 3.31 µmol N m−2 d−1 in spring 2008. The comparison of these rates of diazotrophy with the observed Trichodesmium abundances suggests that other, presumably unicellular, diazotrophs must have contributed significantly to community N2 fixation, at least during the spring 2008 cruise. Satellite data of atmospheric dust concentration suggested similar rates of atmospheric deposition during the two surveys. In contrast, vertical diffusive fluxes of phosphate were 5-fold higher in spring than in autumn (14.2 ± 12.1 µmol P m−2 d−1 and 2.8 ± 2.6 µmol P m−2 d−1, respectively), which may have stimulated N2 fixation. These findings agree with the growing view that N2 fixation is a more widespread process than the distribution of Trichodesmium alone may suggest. Our data also suggest a role for phosphorus supply in controlling the local variability of diazotrophic activity in a region subject to relatively high atmospheric inputs of iron.

Abstract. Nitrogen fixation by filamentous cyanobacteria supplies significant amounts of new nitrogen (N) to the Baltic Sea. This balances N loss processes such as denitrification and anammox and forms an important N source supporting primary and secondary production in N-limited post-spring bloom plankton communities. Laboratory studies suggest that filamentous diazotrophic cyanobacteria growth and N2-fixation rates are sensitive to ocean acidification with potential implications for new N supply to the Baltic Sea. In this study, our aim was to assess the effect of ocean acidification on diazotroph growth and activity as well as the contribution of diazotrophically-fixed N to N supply in a natural plankton assemblage. We enclosed a natural plankton community in a summer season in the Baltic Sea near the entrance to the Gulf of Finland in six large-scale mesocosms (volume ~ 55 m3) and manipulated fCO2 over a range relevant for projected ocean acidification by the end of this century (average treatment fCO2: 365–1231 μatm). The direct response of diazotroph growth and activity was followed in the mesocosms over a 47 day study period during N-limited growth in the summer plankton community. Diazotrophic filamentous cyanobacteria abundance throughout the study period and N2-fixation rates (determined only until day 21 due to subsequent use of contaminated commercial 15N-N2 gas stocks) remained low. Thus estimated new N inputs from diazotrophy were too low to relieve N limitation and stimulate a summer phytoplankton bloom. Instead regeneration of organic N sources likely sustained growth in the plankton community. We could not detect significant CO2-related differences in inorganic or organic N pools sizes, or particulate matter N : P stoichiometry. Additionally, no significant effect of elevated CO2 on diazotroph activity was observed. Therefore, ocean acidification had no observable impact on N cycling or biogeochemistry in this N-limited, post-spring bloom plankton assemblage in the Baltic Sea.

Abstract. The nutrient-rich waters of the Amazon River plume (ARP) support dense blooms of diatom-diazotroph assemblages (DDAs) that introduce large quantities of new nitrogen to the planktonic ecosystem and, unlike other nitrogen-fixers, are likely to directly fuel vertical carbon flux. To investigate the factors controlling DDA blooms, we develop a five phytoplankton (cyanobacteria, diatoms, unicellular microbial diazotrophs, DDAs, and Trichodesmium), two zooplankton model and embed it within a 1/6° resolution physical model of the tropical and subtropical Atlantic. The model generates realistic DDA blooms in the ARP and also exhibits basin-wide primary production, nitrogen fixation, and grazing rates consistent with observed values. By following ARP water parcels with synthetic Lagrangian drifters released at the river mouth we are able to assess the relative impacts of grazing, nutrient supply, and physical forcing on DDA bloom formation. DDA bloom formation is stimulated in the nitrogen-poor and silica-rich water of the ARP by decreases in grazing pressure when mesozooplankton (which co-occur in high densities with coastal diatom blooms) concentrations decrease. Bloom termination is driven primarily by silica limitation of the DDAs. In agreement with in situ data, this net growth niche for DDAs exists in a salinity range from ∼20–34 PSU, although this co-occurrence is coincidental rather than causative. Because net growth rates are relatively modest, bloom formation in ARP water parcels depends critically on the time spent in this ideal habitat, with high DDA biomass only occurring when water parcels spent >23 days in the optimal habitat niche.

Les coraux constructeurs de récifs, sont à la fois autotrophes (ils vivent en symbiose avec des microalgues de la famille des Symbiodiniaceae) et hétérotrophes, c’est à dire qu’ils sont capables de se nourrir sur une gamme de proies allant de la matière organique dissoute, au plancton. Les récifs coralliens sont menacés par le réchauffement climatique, qui perturbe la symbiose entre les coraux et leurs symbiontes, entraînant un blanchissement massif des coraux. Les diazotrophes planctoniques, capables de fixer le diazote (N2) atmosphérique en azote (N) biodisponible (l’ammonium, NH4+), et de transférer cet N Dérivé de la Diazotrophie (appelé DDN) le long du réseau trophique, pourraient constituer une source alternative de nutriments pour les coraux. Seule une étude préliminaire a montré qu’une espèce corallienne pouvait se nourrir sur ces diazotrophes planctoniques (Benavides et al., 2016). Des diazotrophes symbiotiques vivent également en association avec les coraux et leurs transfèrent aussi du DDN. Dans le contexte du changement climatique, où les coraux sont menacés à la fois par l’acidification (AO) et le réchauffement des océans, ce travail se propose d’étudier le rôle de la diazotrophie planctonique et symbiotique dans l’acquisition d’N par les coraux et dans leur résistance/résilience à ces changements. (i) Dans un premier temps, nous avons déterminé les quantités d’N apportées par l’ingestion de diazotrophes planctoniques et symbiotiques au sein de trois espèces coralliennes. Nos résultats ont révélé que l’apport d’N par l’ingestion de diazotrophes planctoniques était répandu chez les coraux et que les taux d’assimilation par cette voie étaient mille fois plus importants que ceux obtenus par les diazotrophes symbiotiques. (ii) Nous avons également montré que des coraux blanchis étaient capables d’augmenter leur consommation de plancton diazotrophe mais également d’augmenter plus spécifiquement leur ingestion de Synechococcus. (iii) Une expérience menée in situ, sur des coraux se développant dans des résurgences naturelles de CO2 en Papouasie-Nouvelle Guinée, où la pCO2 est proche de celle prévue d’ici la fin du siècle, nous a permis de démontrer que les taux d'assimilation de DDN étaient plus élevés que dans le site contrôle, avec une communauté de diazotrophes spécifique à ce site, les Alphaprotéobactéries. (iv) Enfin, nous avons quantifié en laboratoire, les effets d’un stress de température sur les principaux paramètres physiologiques de coraux non nourris, ou nourris exclusivement avec des diazotrophes planctoniques. Ces coraux bénéficiant d’un apport en DDN par le plancton diazotrophe seraient plus résistants au blanchissement. Ils conservent davantage leurs Symbiodiniaceae et maintiennent leurs taux de croissance et de transfert d’électrons. Étant donné la forte abondance de picoplancton dans les eaux des lagons oligotrophes, nos résultats suggèrent que les coraux, capables d'ajuster la communauté de leur diazotrophes symbiotiques, et d'exploiter les sources de plancton riches en N, seraient plus résistants face au réchauffement climatique et à l’AO
Reef building corals are both autotroph (they live in symbiosis with intracellular dinoflagellate of the Symbiodiniaceae family, which provide them with photosynthates) and heterotroph, they are able to feed on a wide range of prey from organic matter to plankton. Coral reefs are threatened by global warming, which disrupts the symbiosis between corals and their symbionts, leading to mass coral bleaching. Planktonic diazotrophs, which have the particularity of fixing the atmospheric dinitrogen (N2) and transferring this diazotroph-derived nitrogen (N) (called DDN) along the food web, could be an alternative nutrient source for corals. Only a preliminary study has shown that one coral species could feed on planktonic diazotrophs (Benavides et al., 2016). Symbiotic diazotrophs also live in association with corals and transfer them some DDN. In the context of climate change, where corals are threatened by both acidification (OA) and ocean warming, this work proposes to study the role of planktonic and symbiotic diazotrophs in N acquisition by corals and in their resistance/resilience to these changes. (i) First, we quantified DDN assimilation rates through heterotrophic nutrition of diazotrophs and symbiotic diazotrophs. Our results reveal the importance of N intake through heterotrophy on planktonic diazotrophs as N assimilation rates through this way were a thousand times higher than those obtained via endosymbiotic diazotrophs. (ii) We also showed for the first time that thermally stressed corals are able to increase not only their consumption of planktonic diazotrophs and plankton that likely benefited from N2 fixation, but also more specifically their ingestion of a very specific taxonomic group of picoplankton : the ubiquitous marine cyanobacterium Synechococcus. (iii) An experiment conducted in situ, on corals growing in natural CO2 vents in Ambitle (Papua New Guinea), where the pCO2 is close to that expected by the end of the century, allowed us to demonstrate that DDN assimilation rates in the Symbiodiniaceae were significantly higher in comparison to an ambient CO2 site, concomitant with a restructured diazotroph community and the particular prevalence of Alphaproteobacteria. (iv) Finally, in laboratory conditions, we have quantified the effects of temperature stress on corals fed exclusively with planktonic diazotrophs. Corals benefiting from DDN supplied by diazotrophic plankton, would be more resistant to bleaching, they retain more of their Symbiodiniaceae and maintain their growth and electron transfer rates (in Photosystem II, ETR) compared to unfed corals. Given the high abundance of picoplankton in oligotrophic waters at large, our results suggest that corals capable of adjusting their diazotrophic communities and exploiting N-rich picoplankton sources to offset their increased N requirements, may be able to cope better with OA and global warming

Nitrogen fixation is no longer considered to be a minor factor of the nitrogen cycle in oceanic ecosystems. Recent geochemical and biological efforts have led to a significant increase in the estimated input of nitrogen to marine ecosystems by biological fixation, while molecular studies have increased our knowledge of the number and diversity of nitrogen fixers known to be active in the ocean. Although Trichodesmium spp. have long been viewed as the primary marine nitrogen fixers, recent efforts have shown that various members of the picoplankton community are also actively involved in nitrogen fixation. The relative abundance of different nitrogen fixers is an important ecosystem parameter since nitrogen fixers may differ significantly in their physiology, life history and ecology. Here we combine rate measurements and stable isotope natural abundance measurements to constrain the impact of N2 fixation in the waters north of Australia. Samples were collected in the Coral, Arafura, and East Timor Seas, thus spanning three distinct hydrographic regions. Our data show that Trichodesmium has a significant influence on the stable nitrogen isotope ratios of particulate and zooplankton biomass and suggest that Trichodesmium is a significant source of nitrogen for the pelagic ecosystem. Based on stable carbon isotope ratios, it is also likely that the pathways are indirect and nitrogen fixed by Trichodesmium enters the higher trophic levels via decomposition as dissolved organic and inorganic nitrogen. Picocyanobacteria showed high diazotrophic activity at some stations, but unlike Trichodesmium, their N2 fixation rate was not reflected in the stable N isotope ratios of particulate and zooplankton biomass. Our results suggest an important N contribution to biomass by diazotrophs in the Coral Sea, Arafura Sea and East Timor Sea.

Ce travail s'inscrit dans le cadre du projet OUTPACE visant à caractériser les eaux du Pacifique tropical Sud-Ouest (WTSP) en termes de stocks et flux biogéochimiques, et de diversité biologique des diazotrophes le long d'un transect longitudinal Ouest-Est. Il est le fruit de la combinaison étroite de deux approches, l'une expérimentale et l'autre basée sur la modélisation, dans le but d'étudier le rôle de la diazotrophie dans la dynamique planctonique et les cycles biogéochimiques des eaux de surface du WTSP. Les analyses de plusieurs grandeurs mesurées lors de la campagne, et notamment la production primaire (PP) et du temps de turnover du phosphate inorganique dissous (DIP), ont permis d'observer un gradient Ouest-Est de productivité et de disponibilité nutritive, étroitement lié à la variabilité spatiale des taux de fixation de N$_2$. L'utilisation d'un modèle biogéochimique mécaniste (implémenté dans la plateforme Eco3M) incluant explicitement deux compartiments de diazotrophes, couplé à un modèle physique 1D vertical, a permis de mettre en évidence le fait que l'absence/présence de diazotrophie permettait d'expliquer le contraste observé entre les régions de l'Ouest de l'archipel Mélanésien (WMA) et de l'Ouest de la gyre du Pacifique Sud (WGY). Les résultats du modèle ont montré que les organismes non diazotrophes bénéficiaient de l'apport d'azote nouveau apporté par la fixation de N$_2$, et que la production planctonique de surface dépendait significativement de l'activité des diazotrophes, cette dernière contrôlée à l'Ouest par la disponibilité en phosphate et à l'Est par la disponibilité en fer
This work is part of the OUTPACE project which aimed to characterize the western tropical south Pacific (WTSP) in terms of biogeochemical stocks and fluxes and biological diversity of diazotrophs along a West-East longitudinal transect. This work combines an experimental with a modeling approach in order to study the role of diazotrophy in the planktonic dynamics and biogeochemical cycles of the WTSP surface waters. The values measured during the campaign, iespecially those of primary production (PP) and dissolved inorganic phosphate (DIP) turnover time, revealed a West-East gradient of productivity and nutrient availability, closely related to the spatial variability of N$_2$ fixation rates. The use of a mechanistic biogeochemical model (implemented in the Eco3M platform) explicitly including two compartments of diazotrophs and coupled with a vertical 1D physical model, allowed to highlight the fact that the absence / presence of diazotrophy could explain the contrast between the western regions of the Melanesian Archipelago (WMA) and the west of the south Pacific gyre (WGY). ). The model results showed that non-diazotrophic organisms benefited from the new nitrogen supply provided by nitrogen fixers, and that the surface planktonic production depended significantly on diazotroph activity, which is controlled by the phosphate availability in the west and by the iron availability in the east of the WTSP

A mesocosm experiment was carried out in a Norwegian fjord near Bergen in May 2006, with the main objective being the study of the effects of increasing concentrations of atmospheric CO2 (and associated effects such as increased acidification) on blooms of natural marine coastal plankton. Three mesocosms were bubbled with CO2(g) to achieve a high (~700ppm) CO2 concentration (pH ~7.8) to simulate predicted future conditions as a result of rising atmospheric CO2 concentrations. Another three mesocosms were treated as controls and bubbled with ambient air to represent a near pre-industrial scenario (atmospheric CO2 concentration ~300ppm, surface seawater pH ~8.15). Blooms in the mesocosms were stimulated by the addition of nutrients at a near-Redfield ratio ([N:P] ≈ [16:1]), and scientific measurements and analyses were carried out over the course of the blooms for approximately one month. Of particular interest in this study were the autotrophic plankton. The diversity and activities of these microorganisms under the two treatments was therefore investigated. By designing and using new degenerate primers specifically targeting ‘Green-type’ (Form IA and IB), ‘Red-type’ (Form IC and ID) and Form II RuBisCO, analysis of primary producers was carried out using PCR and either gDNA or cDNA (mRNA) templates from key time points spanning the complete duration of the blooms throughout the mesocosm experiment. Over 1250 novel RuBisCO large subunit sequences have been fully annotated and deposited in the NCBI GenBank® database. These sequences revealed distinct changes in the diversity of primary producers both over the courses of the blooms and between treatments. Particularly striking was the effect of acidification on the community structure of the eukaryotic picoplankton, Prasinophytes. A clade of prasinophytes closely related to Micromonas pusilla showed a distinct preference for the high CO2 conditions; a laboratory-based experiment confirmed the high tolerance of Micromonas pusilla to lower pH. Conversely, a clade related to Bathycoccus prasinos was almost entirely excluded from the high CO2 treatments. Clades of form II RuBisCO-containing dinoflagellates were also abundant throughout the experiment in both treatments. The high similarity of some of these clades to the toxin-producing species Heterocapsa triquetra and Gonyaulax polyedra, and apparent high tolerance of some clades to high CO2 conditions, is perhaps cause for concern in a high CO2 world and demands further research. In parallel with the RubisCO work, new primers were designed that target the gene encoding the Fe protein of nitrogenase (NifH). 82 Bergen genomic nifH sequences have been annotated and submitted to GenBank®. These sequences include those from organisms related to Alpha, Beta, and Gammaproteobacteria, and Cluster II and Cluster III sequences that align most closely with anaerobic Bacteria, Gram positive, and/or sulphur-reducing Bacteria. The biggest surprise, however, was the apparent abundance and significance of a Rhodobacter sphaeroides-like microorganism throughout the duration of the experiment in both treatments. Whilst this clade was unsurprisingly absent in the RuBisCO cDNA libraries, all but two of 128 nifH cDNA clones analysed were identical to the gene from Rhodobacter sphaeroides. This shows that this clade was potentially fixing N2 throughout the entire experiment, even in the presence of combined N added to both sets of mesocosms at the start of the experiment. A group of Rhodobacter sphaeroides-like microorganisms present at Bergen may therefore have been an unexpected source of new N during the experiment and contributed to the maintenance of the mesocosm communities as nutrients became depleted. One organism dominated the autotrophic communities after the blooms in both treatments. Synechococcus spp. Form IA rbcL clones most closely related to the coastal strain Synechococcus sp. strain CC9902 were recovered throughout the experiment but were particularly numerous toward the end of the experiment and dominated the “Green-type” libraries at this time. Initially, rbcL clones from these cyanobacteria were mostly derived from the ambient CO2 mesocosms but were equally distributed between treatments by the end of the experiment. This suggests that cyanobacteria related to strain CC9902 may be less tolerant of elevated CO2 (which was greatest at the beginning rather than the end of the experiment). However, despite the mesocosms being Pi-limited at the end of the experiment, several Synechococcus species (including those related to strain CC9902 and another coastal strain, CC9311) thrived. Following on from this observation, Pi uptake and assimilation mechanisms in a Synechococcus species were investigated in the laboratory. This led to the sequencing and characterisation of a pstS gene from the marine cyanobacterium Synechococcus sp. WH 8103. Unlike conventional pstS, it was discovered that the pstS II gene in this organism is constitutively expressed and unresponsive to or only weakly regulated by Pi supply. The use of PstS/pstS as a marker for P-limitation in natural samples, therefore, should be interpreted with caution.