Fatty acids associated with the frustules of diatoms and their fate during degradation -- A case study in Thalassiosira weissflogii

International audienceDiatoms are major actors in the export of organic carbon out of the euphotic zone. Yet, the processes linking biogenic silica and carbon sedimentation fluxes to deep oceanic layers remain unclear. Analysing organic fractions in biominerals is challenging because efficient cleaning often led to structural alteration of organic molecules. Hence, although lipids are widely used as biogeochemical markers in ocean flux study, few studies have dealt with the lipids that are associated with frustules. In the present study, a protocol was set up to extract and quantify the fatty acids associated to the frustule of the diatom species Thalassiosira weissflogii. The protocol involves solvent extraction of diatom external lipids, followed by clean frustule dissolution by 4%NaOH during 1h at 95°C and subsequent solvent re-extraction of frustule-associated lipids. Results confirmed that this protocol was efficient first, to isolate the frustule from the rest of the cellular organic carbon and second to extract and quantify fatty acids (FA) associated to frustules of this species. FA composition of the frustules was significantly different from that of the whole cells consisting primarily of 14:0, 16:0 and 18:0 FA, as well as a smaller portion of 16:1 and 18:1 unsaturated FA. Frustule-associated FA constituted 7% of the total FA and 1.8% of the total POC. The 30 days T.weissflogii degradation/dissolution experiment suggested that frustule FA14:0 and 16:0 were mainly associated with the bSiO2 phase dissolving slowly as no degradation of this pool was measured despite 78% frustule dissolution. At the end of the degradation experiment, this pool constituted 5.8% of the remaining total POC suggesting an effective protection by the frustule through strong interaction with the biogenic silica which is consistent with the correlation observed at depth between Si and POC sedimentation fluxes

Introduction to the French GEOTRACES North Atlantic transect (GA01): GEOVIDE cruise

© 2018 Author(s). The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this special issue (https://www.biogeosciences.net/special-issue900.html), results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among 18 articles. Here, the scientific context, project objectives, and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue

Copepods Boost the Production but Reduce the Carbon Export Efficiency by Diatoms

The fraction of net primary production that is exported from the euphotic zone as sinking particulate organic carbon (POC) varies notably through time and from region to region. Phytoplankton containing biominerals, such as silicified diatoms have long been associated with high export fluxes. However, recent reviews point out that the magnitude of export is not controlled by diatoms alone, but determined by the whole plankton community structure. The combined effect of phytoplankton community composition and zooplankton abundance on export flux dynamics, were explored using a set of 12 large outdoor mesocosms. All mesocosms received a daily addition of minor amounts of nitrate and phosphate, while only 6 mesocosms received silicic acid (dSi). This resulted in a dominance of diatoms and dinoflagellate in the +Si mesocosms and a dominance of dinoflagellate in the -Si mesocosms. Simultaneously, half of the mesocosms had decreased mesozooplankton populations whereas the other half were supplemented with additional zooplankton. In all mesocosms, POC fluxes were positively correlated to Si/C ratios measured in the surface community and additions of dSi globally increased the export fluxes in all treatments highlighting the role of diatoms in C export. The presence of additional copepods resulted in higher standing stocks of POC, most probably through trophic cascades. However it only resulted in higher export fluxes for the +Si mesocosms. In the +Si with copepod addition (+Si +Cops) export was dominated by large diatoms with higher Si/C ratios in sinking material than in standing stocks. During non-bloom situations, the grazing activity of copepods decrease the export efficiency in diatom dominated systems by changing the structure of the phytoplankton community and/or preventing their aggregation. However, in flagellate-dominated system, the copepods increased phytoplankton growth, aggregation and fecal pellet production, with overall higher net export not always visible in term of export efficiency

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Les diatomées sont des micro-algues qui participent à hauteur de 35 à 75 % à la production primaire océanique et qui sont les acteurs majeurs du cycle biogéochimique du silicium (Si) dans l’océan. Comprendre les mécanismes qui affectent la dissolution de la silice biogénique (bSiO2) constituant le frustule des diatomées est nécessaire afin d’améliorer la compréhension du cycle océanique du Si. En période estivale, la majorité de l’océan ouvert est limitée par de faibles disponibilités en éléments nutritifs. Les travaux réalisés dans cette thèse ont donc pour objectif majeur d’étudier l’effet de l’environnement nutritionnel des diatomées sur la dissolution et l’export de silice biogénique (bSiO2). Cette étude s’est focalisée sur les limitations en fer (Fe), en Si et en azote (N), c'est-à-dire sur les éléments nutritifs dont les faibles concentrations en période estivale limitent la production de diatomées dans une grande majorité de l’océan mondial. Une des originalités de cette thèse consiste en l’étude de la limitation en cuivre (Cu), dont les impacts sur la composition élémentaire et la dissolution des diatomées ont été peu étudiés. Les effets des conditions limitantes en micronutriments ont été étudiés sur la diatomée pennée Pseudo-nitzschia delicatissima tandis que les limitations en macronutriments ont été étudiées sur la diatomée centrique Thalassiosira weissflogii. La première étape de ce travail a consisté en l'étude de l'effet de conditions nutritives limitantes sur deux échelles du frustule des diatomées : celle de la cellule, avec l’étude du degré de silicification des diatomées, et celle du frustule, avec l’étude de sa structure et sa composition fine, au moyen de la spectroscopie InfraRouge à Transformée de Fourier (IRTF). La seconde étape de ce travail a consisté en l’étude du devenir post-mortem de ces cellules à travers l’étude de la cinétique de dissolution de la bSiO2 constituant leur frustule. Les résultats obtenus par IRTF indiquent clairement qu’à la plasticité du contenu global en bSiO2 du frustule des diatomées, s’ajoute la plasticité du frustule à l’échelle moléculaire, en fonction des conditions de croissance. Le degré d’organisation et le degré de réactivité du réseau siliceux sont tous deux affectés par l’environnement nutritif des diatomées. La quantité relative de matière organique associée au frustule varie également avec la disponibilité des éléments nutritifs. A travers ces changements, l’environnement nutritionnel affecte la dissolution du frustule des diatomées qui, dans les six expériences de dissolution, se déroulent en deux étapes, illustrant la composition biphasique du frustule. Les résultats indiquent que les conditions de croissance vont affecter la proportion et les vitesses de dissolution de la bSiO2 de ces phases. Par la modification des propriétés intrinsèques des phases de bSiO2, le frustule des diatomées limitées en nutriments auront une propension à la dissolution moins importante que celle des diatomées non-limitées. De ce fait, l’environnement nutritif des diatomées affecte l’export de bSiO2. Pour P. delicatissima, seuls 9 % de la bSiO2 initiale des cellules non limitées sont préservés après 22 jours de dissolution, tandis qu’environ 25 % sont préservés en cas de carence stricte en Cu ou de limitation en Fe durant la croissance. Les frustules de T. weissflogii limitées en macronutriment sont également mieux préservés après un mois de dissolution, avec 41 % et 51 % de la bSiO2 initiale restante pour les cellules majoritairement limitées en Si ou en N, respectivement, comparés au 20 % de préservation pour les cellules non-limitées. Ces résultats suggèrent que la dissolution de la silice dans les modèles globaux de l’océan pourrait être mieux paramétrée en tenant compte (i) de la cinétique de dissolution des deux phases de silice biogénique et (ii) de la meilleure préservation de la bSiO2 des cellules limitées en nutriments.Diatoms are microalgae that contribute up to 75% of oceanic primary production and are major players in the oceanic biogeochemical silicon (Si) cycle. Understanding the mechanisms affecting the biogenic silica (bSiO2), constituting the diatom frustule, is necessary to improve the understanding of oceanic Si cycling. In summer, most of the open ocean is limited by low nutrient availability. Thus, the main objective of this thesis is to study the effect of diatom nutritional environment on biogenic silica (bSiO2) dissolution and export. This study focused on iron (Fe), Si and nitrogen (N) limitations, i.e. nutrients whose low concentrations during summer limit diatom production in the majority of the world ocean. One originality of this thesis is to study the copper (Cu) limitation, whose impact on the elemental composition and dissolution of diatoms has rarely been studied. The effects of micronutrient limiting conditions were studied on the pennate diatom Pseudo-nitzschia delicatissima while macronutrient limiting conditions were studied on the centric diatom Thalassiosira weissflogii. The first stage of this work was to study the effect of nutrient limiting conditions at two different scales of diatoms frustule: the cell scale, with the study of the silicification degree of diatoms, and the frustule scale, with the study of its structure and composition using Fourier Transform InfraRed spectroscopy (FTIR). The second stage of this work consisted of the study of the diatom post-mortem fate through the study of the dissolution kinetics of bSiO2 from diatom frustule. The FTIR results clearly indicate that, in addition to the plasticity of global bSiO2 content, diatom frustule also has plasticity at the molecular scale, depending on growth conditions. The organization degree and the reactivity degree of the silica lattice are both affected by diatom nutritional environment. The relative amount of organic matter associated with frustule is also affected by nutrient availability. Through these changes, the nutritional environment affects the dissolution of diatom frustules, which showed a two-stage dissolution in all six dissolution experiments, illustrating the biphasic composition of the frustule. The results indicate that the growth conditions affect both the proportion and the dissolution rates of these bSiO2 phases. By changing the intrinsic properties of the bSiO2 phases, nutrient limited-frustules will be dissolving less than non-limited diatom frustule. Therefore, the diatom nutrient environment affects bSiO2 export. In P. delicatissima, only 9 % of the initial bSiO2 remained from replete cells at the end of the twenty-two days dissolution experiment, while about 25 % remained from Cu-starved and Fe-limited cells. Macronutrient-limited T. weissflogii were also more preserved after one month of dissolution with 41 % and 51 % of the initial bSiO2 remaining for cells predominantly Si- or N-limited, respectively, whereas 20% of the initial bSiO2 was preserved in replete cells. These results suggest that the biogenic silica dissolution in global ocean models could be better parameterized taking into account (i) the dissolution kinetics of the two bSiO2 phases and (ii) an enhanced bSiO2 preservation of nutrient-limited cells

Impact des conditions nutritionnelles sur la dissolution de la silice biogénique des diatomées à travers l'étude de la variabilité de la structure biphasique du frustule

Diatoms are microalgae that contribute up to 75% of oceanic primary production and are major players in the oceanic biogeochemical silicon (Si) cycle. Understanding the mechanisms affecting the biogenic silica (bSiO2), constituting the diatom frustule, is necessary to improve the understanding of oceanic Si cycling. In summer, most of the open ocean is limited by low nutrient availability. Thus, the main objective of this thesis is to study the effect of diatom nutritional environment on biogenic silica (bSiO2) dissolution and export. This study focused on iron (Fe), Si and nitrogen (N) limitations, i.e. nutrients whose low concentrations during summer limit diatom production in the majority of the world ocean. One originality of this thesis is to study the copper (Cu) limitation, whose impact on the elemental composition and dissolution of diatoms has rarely been studied. The effects of micronutrient limiting conditions were studied on the pennate diatom Pseudo-nitzschia delicatissima while macronutrient limiting conditions were studied on the centric diatom Thalassiosira weissflogii. The first stage of this work was to study the effect of nutrient limiting conditions at two different scales of diatoms frustule: the cell scale, with the study of the silicification degree of diatoms, and the frustule scale, with the study of its structure and composition using Fourier Transform InfraRed spectroscopy (FTIR). The second stage of this work consisted of the study of the diatom post-mortem fate through the study of the dissolution kinetics of bSiO2 from diatom frustule. The FTIR results clearly indicate that, in addition to the plasticity of global bSiO2 content, diatom frustule also has plasticity at the molecular scale, depending on growth conditions. The organization degree and the reactivity degree of the silica lattice are both affected by diatom nutritional environment. The relative amount of organic matter associated with frustule is also affected by nutrient availability. Through these changes, the nutritional environment affects the dissolution of diatom frustules, which showed a two-stage dissolution in all six dissolution experiments, illustrating the biphasic composition of the frustule. The results indicate that the growth conditions affect both the proportion and the dissolution rates of these bSiO2 phases. By changing the intrinsic properties of the bSiO2 phases, nutrient limited-frustules will be dissolving less than non-limited diatom frustule. Therefore, the diatom nutrient environment affects bSiO2 export. In P. delicatissima, only 9 % of the initial bSiO2 remained from replete cells at the end of the twenty-two days dissolution experiment, while about 25 % remained from Cu-starved and Fe-limited cells. Macronutrient-limited T. weissflogii were also more preserved after one month of dissolution with 41 % and 51 % of the initial bSiO2 remaining for cells predominantly Si- or N-limited, respectively, whereas 20% of the initial bSiO2 was preserved in replete cells. These results suggest that the biogenic silica dissolution in global ocean models could be better parameterized taking into account (i) the dissolution kinetics of the two bSiO2 phases and (ii) an enhanced bSiO2 preservation of nutrient-limited cells.Les diatomées sont des micro-algues qui participent à hauteur de 35 à 75 % à la production primaire océanique et qui sont les acteurs majeurs du cycle biogéochimique du silicium (Si) dans l’océan. Comprendre les mécanismes qui affectent la dissolution de la silice biogénique (bSiO2) constituant le frustule des diatomées est nécessaire afin d’améliorer la compréhension du cycle océanique du Si. En période estivale, la majorité de l’océan ouvert est limitée par de faibles disponibilités en éléments nutritifs. Les travaux réalisés dans cette thèse ont donc pour objectif majeur d’étudier l’effet de l’environnement nutritionnel des diatomées sur la dissolution et l’export de silice biogénique (bSiO2). Cette étude s’est focalisée sur les limitations en fer (Fe), en Si et en azote (N), c'est-à-dire sur les éléments nutritifs dont les faibles concentrations en période estivale limitent la production de diatomées dans une grande majorité de l’océan mondial. Une des originalités de cette thèse consiste en l’étude de la limitation en cuivre (Cu), dont les impacts sur la composition élémentaire et la dissolution des diatomées ont été peu étudiés. Les effets des conditions limitantes en micronutriments ont été étudiés sur la diatomée pennée Pseudo-nitzschia delicatissima tandis que les limitations en macronutriments ont été étudiées sur la diatomée centrique Thalassiosira weissflogii. La première étape de ce travail a consisté en l'étude de l'effet de conditions nutritives limitantes sur deux échelles du frustule des diatomées : celle de la cellule, avec l’étude du degré de silicification des diatomées, et celle du frustule, avec l’étude de sa structure et sa composition fine, au moyen de la spectroscopie InfraRouge à Transformée de Fourier (IRTF). La seconde étape de ce travail a consisté en l’étude du devenir post-mortem de ces cellules à travers l’étude de la cinétique de dissolution de la bSiO2 constituant leur frustule. Les résultats obtenus par IRTF indiquent clairement qu’à la plasticité du contenu global en bSiO2 du frustule des diatomées, s’ajoute la plasticité du frustule à l’échelle moléculaire, en fonction des conditions de croissance. Le degré d’organisation et le degré de réactivité du réseau siliceux sont tous deux affectés par l’environnement nutritif des diatomées. La quantité relative de matière organique associée au frustule varie également avec la disponibilité des éléments nutritifs. A travers ces changements, l’environnement nutritionnel affecte la dissolution du frustule des diatomées qui, dans les six expériences de dissolution, se déroulent en deux étapes, illustrant la composition biphasique du frustule. Les résultats indiquent que les conditions de croissance vont affecter la proportion et les vitesses de dissolution de la bSiO2 de ces phases. Par la modification des propriétés intrinsèques des phases de bSiO2, le frustule des diatomées limitées en nutriments auront une propension à la dissolution moins importante que celle des diatomées non-limitées. De ce fait, l’environnement nutritif des diatomées affecte l’export de bSiO2. Pour P. delicatissima, seuls 9 % de la bSiO2 initiale des cellules non limitées sont préservés après 22 jours de dissolution, tandis qu’environ 25 % sont préservés en cas de carence stricte en Cu ou de limitation en Fe durant la croissance. Les frustules de T. weissflogii limitées en macronutriment sont également mieux préservés après un mois de dissolution, avec 41 % et 51 % de la bSiO2 initiale restante pour les cellules majoritairement limitées en Si ou en N, respectivement, comparés au 20 % de préservation pour les cellules non-limitées. Ces résultats suggèrent que la dissolution de la silice dans les modèles globaux de l’océan pourrait être mieux paramétrée en tenant compte (i) de la cinétique de dissolution des deux phases de silice biogénique et (ii) de la meilleure préservation de la bSiO2 des cellules limitées en nutriments

Impact of starvation on Si-N-C content, dissolution and degradation of a diatom

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Feeding on dispersed vs. aggregated particles: The effect of zooplankton feeding behavior on vertical flux

WOS:000401487700100International audienceZooplankton feeding activity is hypothesized to attenuate the downward flux of elements in the ocean. We investigated whether the zooplankton community composition could influence the flux attenuation, due to the differences of feeding modes (feeding on dispersed vs. aggregated particles) and of metabolic rates. We fed 5 copepod species-three calanoid, one harpacticoid and one poecilamastoid-microplankton food, in either dispersed or aggregated form and measured rates of respiration, fecal pellet production and egg production. Calanoid copepods were able to feed only on dispersed food; when their food was introduced as aggregates, their pellet production and respiration rates decreased to rates observed for starved individuals. In contrast, harpacticoids and the poecilamastoid copepod Oncaea spp. were able to feed only when the food was in the form of aggregates. The sum of copepod respiration, pellet production and egg production rates was equivalent to a daily minimum carbon demand of ca. 10% body weight-(1) for all non-feeding copepods; the carbon demand of calanoids feeding on dispersed food was 2-3 times greater, and the carbon demand of harpacticoids and Oncaea spp. feeding on aggregates was \textgreater7 times greater, than the resting rates. The zooplankton species composition combined with the type of available food strongly influences the calculated carbon demand of a copepod community, and thus also the attenuation of vertical carbon flux

Dissolution of the biogenic silica: another look on silica structure.

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Impact of copper starvation and of iron limitation on the frustule composition and dissolution of the pseudo-nitzschia diatom

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Impact Of Growth Stress On Diatom BSiO2 Structure And Recycling

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