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WOS:000502962100001; International audience; In marine ecosystems, carbon export is driven by particle flux which is modulated by aggregation, remineralization, and grazing processes. Zooplankton contribute to the sinking flux through the egestion of fast sinking fecal pellets but may also attenuate the flux by tearing apart phytoplankton aggregates into small pieces through swimming activity or direct ingestion. Freely suspended cells, artificial monospecific aggregates from two different diatom species (Chaetoceros neogracile and Skeletonema merinol) and natural aggregates of Melosira sp. were independently incubated with five different copepod species (Acadia clausi, Temora longicomis, Calanus helgolandicus, Euterpina acutifrons, and Calanus hyperboreus). During the grazing experiments initiated with free diatoms, E. acutifrons feeding activity evidenced by ingestion rates of 157 +/- 155 ng Chl a ind(-1) d(-1), induced a significant increase of S. marinoi aggregation. Transparent exopolymeric particles (TEP) production was only slightly boosted by the presence of grazers and turbulences created by swimming may be the main trigger of the aggregation processes. All copepods studied were able to graze on aggregates and quantitative estimates led to chlorophyll a ingestion rates (expressed in Chla a equivalent, i.e., the sum of chlorophyll a and pheopigments in their guts) ranging from 4 to 23 ng Chl a(eq) ind(-1) d(-1). The relation between equivalent spherical diameters (ESDs) and sinking velocities of the aggregates did not significantly change after grazing, suggesting that copepod grazing did not affect aggregate density as also shown by Si:C and C:N ratios. Three main trends in particle dynamics could be identified and further linked to the copepod feeding behavior and the size ratio between prey and predators: (1) Fragmentation of S. marinoi aggregates by the cruise feeder T. longicomis and of Melosira sp. aggregates by C. hyperboreus at prey to predator size ratios larger than 15; (2) no change of particle dynamics in the presence of the detritic cruise feeder E acutifrons; and finally (3) re-aggregation of C. neogracile and S. marinoi aggregates when the two filter feeders A. clausi and C. helgolandicus were grazing on aggregate at prey to predator size ratios lower than 10. Aggregation of freely suspended cells or small aggregates was facilitated by turbulence resulting from active swimming of small copepods. However, stronger turbulence created by larger cruise feeders copepods prevent aggregate formation and even made them vulnerable to breakage.

International audience; Diatom aggregates contribute significantly to the vertical sinking flux of particulate matter in the ocean. These fragile structures form a specific microhabitat for the aggregated cells but their internal chemical and physical characteristics remain largely unknown. Studies on the impact of aggregation on Si cycle led to what appears to be inconsistency. Despite a lower biogenic silica (bSiO2) dissolution rate and a diffusion of the silicic acid (dSi) similar in aggregate and in seawater, dSi surprisingly accumulate in aggregates. A reaction-diffusion model helps to clarify this incoherence by reconstructing dSi accumulation measured during batch experiments with aggregated and non-aggregated Skeletonema marinoi and Chaetoceros decipiens. The model calculates the effective bSiO2 dissolution rate as opposed to the experimental apparent bSiO2 dissolution rate, which results on the effective bSiO2 dissolution and transport of dSi out of the aggregate. In the model, dSi transport out of the aggregate is modulated considering alternatively retention -decrease of dSi diffusion constant- and adsorption -reversible chemical bounds between dSi and the aggregate matrix- processes. Modelled bSiO2 dissolution is modulated by the impact of dSi concentration inside aggregate and diatom viability, as enhanced persistence of metabolically active diatom has been observed in aggregates. Adsorption better explains dSi accumulation within and outside aggregates, raising the possible importance of dSi travelling within aggregates to the deep sea (potentially 20% of the total silica flux). Moreover, the model states that bSiO2 dissolution is effectively decreased in aggregates mainly due to higher diatom viability but also to other parameters discussed here; Les agrégats de diatomées contribuent de façon significative au flux vertical de matière dans l'océan. Ces fragiles structures forment un microhabitat particulier dont les caractéristiques physico-chimiques internes sont pourtant encore peu connues. Les études menées jusqu'à présent sur l'impact de l'agrégation sur le cycle du silicium conduisent à des conclusions à première vue incohérentes: malgré une vitesse de dissolution de la silice biogénique (bSiO2) plus faible dans les agrégats de diatomées, et une vitesse de diffusion de la silice dissoute (dSi) similaire à celle de l'eau de mer, la dSi s'accumule dans l'eau intersticielle des agrégats pour atteindre des valeurs jusqu'à 200 fois supérieures à celle de l'eau de mer environnante. En utilisant un modèle de reaction-diffusion qui reproduit l'accumulation de la dSi mesurée expérimentalement à l'intérieur et à l'extérieur d'agrégats monospécifiques de diatomée Skeletonema marinoii et Chaetoceros decipiens, le modèle calcule la vitesse effective de dissolution de la bSiO2 dans un agrégat. Par comparaison, la vitesse de dissolution accessible experimentalement et appelée dans cette étude "vitesse de dissolution apparente" résulte de la vitesse de dissolution effective et du transport de dSi hors de l'agrégat. Dans le modèle, le transport de dSi est modulé en considérant alternativement deux processus: la rétention qui implique un ralentissement de la diffusion de la dSi et de la constante diffusion, et l'adsorption qui est un processus reversible impliquant des liaisons faibles entre la dSi et la matrice de l'agrégat. la dissolution de la bSiO2 est modélisée en considérant l'impact de la concentration en dSi à l'intérieur de l'agrégat et la viabilité des diatomées. Les résultats montrent que des processus d'adsorption permettent de mieux reconstruire la dissolution de la bSiO2 dans un agrégats. Cette conclusion souligne l'importance du transport vertical de silice dissoute par les agrégats jusqu'aux couches profondes océaniques (potentiellement 20% du flux total de sédimentation du silicium). Deplus, le modèle tend à confirmer la diminution de la vitesse effective de dissolution de la bSiO2 dans les agrégats et non plus seulement de la vitesse de dissolution apparente. Ce ralentissement serait principalement le resultat d'une viabilité maintenue plus longtemps dans les agrégats que dans lorsque les cellules sont libres, mais aussi d'autres processus non-identifiés mais discutés dans cette étude.

International audience; Because aggregated diatoms sink rapidly through the water column, leaving little time for dissolution, aggregation influences the balance between recycling of biogenic silica (bSiO2) and its sedimentation and preservation at the seafloor. Additionally, aggregation may directly impact dissolution rates of opal. Laboratory experiments were conducted to investigate the influence of aggregation on bSiO2 dissolution rates using 3 different batch cultures of the diatoms Chaetoceros decipiens, Skeletonema costatum, and Thalassiosira weissflogii. Specific dissolution rates of bSiO2 of aggregated and freely suspended diatoms were compared. Additionally, the influences of the dissolved silicon (dSi) concentration in the pore water of aggregates, the viability of diatoms, and the concentrations of transparent exopolymer particles (TEP) and of bacteria on bSiO2 dissolution rates were determined. Initial specific dissolution rates of diatom frustules were significantly lower for aggregated diatoms (2.9% d–1) than for freely suspended diatoms (6.6% d–1). Lower specific dissolution rates in aggregates were attributed to elevated dSi concentrations in aggregate pore water (maximum 230 vs. 20 µmol l–1) and to the fact that aggregated diatoms remained viable for longer than freely suspended diatoms. Specific bSiO2 dissolution was significantly correlated to viability of cells independent of treatment. Bacterial concentrations in both treatments appeared high enough, so that after cell death the coating protecting the silica frustule was degraded without measurable delay. The TEP content of aggregates appeared to affect dissolution rates, possibly by retaining solutes within aggregates.