Impacts des dépositions atmosphériques de fer sur les assemblages phytoplanctoniques et la production de diméthylsulfure dans le Pacifique Nord-Est contemporain et dans le contexte de l’acidification des océans

La croissance du phytoplancton est limitée par les faibles concentrations de fer (Fe) dans près de 40% de l’océan mondial. Le Pacifique subarctique Nord-Est représente une de ces zones limitées en fer et désignées High Nutrient - Low Chlorophyll (HNLC). Cet écosystème, dominé par des cellules de petite taille telles les prymnésiophytes, est caractérisé par de très faibles concentrations estivales de chlorophylle a et de fortes concentrations de macronutriments. Il a été maintes fois démontré que les ajouts de fer, sous différentes formes chimiques (habituellement FeSO4), dans les zones HNLC, stimulent la croissance et modifient la structure des communautés planctoniques en favorisant la croissance des cellules de grande taille, notamment les diatomées. Ces effets sur la communauté planctonique ont le potentiel d’influencer les grands mécanismes régulateurs du climat, tels la pompe biologique de carbone et la production de diméthylsulfure (DMS). Les poussières provenant des déserts du nord de la Chine sont reconnues depuis longtemps comme une source sporadique importante de fer pour le Pacifique Nord-Est. Malgré leur importance potentielle, l’influence directe exercée par ces poussières sur l’écosystème planctonique de cette zone HNLC n’a jamais été étudiée. Il s’agit d’une lacune importante puisque le fer associé aux poussières est peu soluble dans l’eau de mer, que la proportion biodisponible n’est pas connue et que les poussières peuvent avoir un effet inhibiteur chez le phytoplancton. Cette thèse propose donc, dans un premier temps, de mesurer pour la première fois l’effet de la fertilisation de la communauté planctonique du Pacifique Nord-Est par un gradient de concentrations de poussières désertiques naturelles. Cette première expérimentation a démontré que le fer contenu dans les poussières asiatiques est biodisponible et qu’une déposition équivalente à celles prenant place au printemps dans le Pacifique Nord-Est peut résulter en une stimulation significative de la prise de nutriments et de la croissance du phytoplancton. Mes travaux ont également montré que l’ajout de 0,5 mg L-1 de poussières peut résulter en la production d’autant de biomasse algale que l’ajout de FeSO4, l’espèce chimique utilisée lors des expériences d’enrichissement en fer à grande échelle. Cependant, les ajouts de FeSO4 favorisent davantage les cellules de petite taille que les ajouts de poussières, observation démontrant que le FeSO4 n’est pas un proxy parfait des poussières asiatiques. Dans un deuxième temps, je me suis intéressée à une source alternative de fer atmosphérique, les cendres volcaniques. Mon intérêt pour cette source de fer a été attisé par les observations d’une floraison spectaculaire dans le Pacifique Nord-Est, ma région d’étude, associée à l’éruption de 2008 du volcan Kasatochi dans les îles Aléoutiennes. Forte de mon expérience sur les poussières, j’ai quantifié l’effet direct de ces cendres volcaniques sur la communauté planctonique du Pacifique Nord-Est. Mes résultats ont montré que le fer contenu dans les cendres volcaniques est également biodisponible pour le phytoplancton. Ils ont également montré que cette source de fer peut être aussi importante que les poussières désertiques dans la régulation de la croissance du phytoplancton dans cette partie de l’océan global à l’échelle millénaire. Dans un troisième temps, j’ai estimé comment l’acidification des océans modulera les réponses des communautés planctoniques aux dépositions naturelles de fer mises en évidence lors de mes expériences précédentes. Pour ce faire, j’ai effectué des enrichissements de poussière dans de l’eau de mer au pH actuel de 8.0 et dans l’eau de mer acidifiée à un pH de 7.8. Mes résultats ont montré une diminution du taux de croissance du phytoplancton dans le milieu acidifié mais pas de changement notable dans la structure de la communauté. Les ajouts de poussières et de cendres, de même que les variations de pH, n’ont pas eu d’effet significatif sur la production de DMS et de son précurseur le diméthylsulfoniopropionate (DMSP), probablement en raison de la courte durée (4 jours) des expériences. L’ensemble des résultats de cette thèse montre que le fer contenu dans diverses sources atmosphériques naturelles est biodisponible pour le phytoplancton du Pacifique Nord-Est et que des taux de déposition réalistes peuvent stimuler la croissance de manière notable dans les premiers jours suivant une tempête désertique ou une éruption volcanique. Finalement, les résultats de mes expériences à stresseurs multiples Fer/acidification suggèrent une certaine résistance des communautés phytoplanctoniques à la diminution du pH prédite d’ici la fin du siècle pour les eaux de surface des océans.

Villefranche sur Mer: multistressors experiment March 2012

The effect of ocean warming and acidification was investigated on a natural plankton assemblage from an oligotrophic area, the bay of Villefranche (NW Mediterranean Sea). The assemblage was sampled in March 2012 and exposed to the following four treatments for 12 days: control ( 360 µatm, 14°C), elevated pCO2 ( 610 µatm, 14°C), elevated temperature ( 410 µatm, 17°C), and elevated pCO2 and temperature ( 690 µatm, 17°C). Nutrients were already depleted at the beginning of the experiment and the concentrations of chlorophyll a (chl a), heterotrophic prokaryotes and viruses decreased, under all treatments, throughout the experiment. There were no statistically significant effects of ocean warming and acidification, whether in isolation or combined, on the concentrations of nutrients, particulate organic matter, chl a and most of the photosynthetic pigments. Furthermore, 13C labelling showed that the carbon transfer rates from 13C-sodium bicarbonate into particulate organic carbon were not affected by seawater warming nor acidification. Rates of gross primary production followed the general decreasing trend of chl a concentrations and were significantly higher under elevated temperature, an effect exacerbated when combined to elevated pCO2 level. In contrast to the other algal groups, the picophytoplankton population (cyanobacteria, mostly Synechococcus) increased throughout the experiment and was more abundant in the warmer treatment though to a lesser extent when combined to high pCO2 level. These results suggest that under nutrient-depleted conditions in the Mediterranean Sea, ocean acidification has a very limited impact on the plankton community and that small species will benefit from warming with a potential decrease of the export and energy transfer to higher trophic levels.

Use of lactulose as prebiotic and its influence on the growth, acidification profile and viable counts of different probiotics in fermented skim milk

Lactulose can be considered as a prebiotic, which is able to stimulate healthy intestinal microflora. In the present work, the use of this ingredient in fermented milk improved quality of skim milk fermented by Lactobacillus acidophilus, Lactobacillus rhamnosus, Lactobacillus bulgaricus and Bifidobacterium lactis in co-culture with Streptococcus thermophilus. Compared to control fermentations without lactulose, the addition of such a prebiotic in skim milk increased the counts of all probiotics, with particular concern to B. lactis (bifidogenic effect), the acidification rate and the lactic acid acidity, and concurrently reduced the time to complete fermentation (t(pH4.5)) and the pH at the end of cold storage for 1 to 35 days. (c) 2010 Elsevier B.V. All rights reserved.

Growth and acidification performance of probiotics in pure culture and co-culture with Streptococcus thermophilus: The effect of inulin

Inulin was used as a prebiotic to improve the quality and consistency of skim milk fermented by co-cultures and pure Cultures of Lactobacillus acidophilus, Lactobacillus rhamnosus, Lactobacillus bulgaricus and Bifidobacterium lactis with Streptococcus thermophilus. We compared, either in the presence or absence of 4 g inulin/100 g, the results of the main kinetic parameters, specifically the generation time (t(g)), the maximum acidification rate (V(max)). and the times to reach V(max) (t(max)), to attain pH 5.0 (t(pH5.0)) and to complete the fermentation (t(pH4.5)). Post-acidification, lactic acid formation and cell counts were also determined and compared, either 1 day after the fermentation was complete or after 7 day storage at 4 degrees C. In general, inulin addition to the milk increased in co-cultures V(max), decreased t(max), t(g) and t(pH4.5), favored post-acidification, exerted a bifidogenic effect, and preserved almost intact cell viability during storage. In addition, S. thermophilus was shown to stimulate the metabolism of the other lactic bacteria. Contrary to co-cultures, most of the effects in pure Cultures were not statistically significant. The most important aspect of this paper is the use of the generation time as a toot to investigate the microbial response to inulin addition. (c) 2009 Elsevier Ltd. All rights reserved.

Effect of inulin on growth and acidification performance of different probiotic bacteria in co-cultures and mixed culture with Streptococcus thermophilus

Inulin was used as a prebiotic to improve the quality and consistency of skim milk fermented by Lactobacillus acidophilus (La), Lactobacillus rhamnosus (Lr), Lactobacillus bulgaricus (Lb) and Bifidobacterium lactis (BI) with Streptococcus thermophilus (St), either in binary co-cultures or in cocktail containing all microorganisms. We compared, either in the presence of 40 mg inulin g(-1) or not, the results of the maximum acidification rate (V(max)) and the times to reach it (t(max)), to reach pH 5.0 (t(PH5.0)) and to complete the fermentation (t(f)). Post-acidification, lactic acid formation and cell counts were also compared after either 1 day (D1) or 7 days of storage at 4 degrees C (N). In co-culture, inulin addition to the milk increased V(max), decreased t(max) and t(f), favored post-acidification and exerted a bifidogenic effect. S. thermophilus proved to stimulate the metabolism of the other lactic bacteria and enhanced the product features. After D7, a significant prebiotic effect of inulin was observed in all co-cultures. Either after D1 or D7, the enumerations of Lr and BI in mixed culture markedly decreased compared to their respective co-cultures because of greater competition for the same substrates. (C) 2008 Elsevier Ltd. All rights reserved.

Acidification rates of probiotic bacteria in Minas frescal cheese whey

The acidification rates of Lactobacillus delbrueckii subsp. bulgarieus (Lb), Lactobacillus acidophilus (La), Lactobacillus rhamnosus (Lr), and Bifidobacterium animalis subsp. lactis (Bl) in co-culture with Streptococcus thermophilus (St) were studied in Minas frescal cheese whey. Effects of the co-culture composition and the final pH values on the kinetic parameters of acidification, post-acidification and counts of health promoting micro-organisms were also studied. Fermentation time to reach pH 4.5 was longer when St-Lr co-culture was used, while St-Lb had the shortest fermentation time when compared with the other co-culture combinations. All products showed development of acidity during the storage period and lowest values had been observed employing St-Bl co-culture. The technological interest of using M. frescal cheese whey for the production of a probiotic lactic beverage is discussed in this article. (C) 2007 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.

Inhibition of phenotype modulation, growth, and migration of vascular smooth muscle cells by a guanosine-rich 30-mer phosphorothioate oligodeoxynucleotide

The testing of a 30-mer dG-rich phosphorothioate oligodeoxynucleotide (LG4PS) for effects on the behaviour of vascular smooth muscle cells (VSMC) in vitro and in vivo is described. LG4PS at 0.3 mu M inhibited significantly the phenotype modulation of freshly isolated rabbit VSMC, and cell outgrowth from pig aortic explants was inhibited similar to 80% by 5 mu M LG4PS. The growth of proliferating rabbit and pig VSMC was inhibited similar to 70% by 0.3 mu M and 5 mu M LG4PS, respectively. Though less marked, the antiproliferative effects of LG4PS on human VSMC were comparable to those obtained with heparin. The cytotoxic effects of LG4PS on VSMC in vitro were low. Despite these promising results, adventitial application of 2-200 nmol LG4PS in pluronic gel failed to reduce vascular hyperplasia in balloon-injured rabbit carotid arteries, and the highest dose caused extensive mortality. (C) 1997 Academic Press Limited.

Chemical synthesis, antibacterial activity and conformation of diptericin, an 82-mer peptide originally isolated from insects

The small amounts of antibacterial peptides that can be isolated from insects do not allow detailed studies of their range of activity, side-chain sugar requirements, or their conformation, factors that frequently play roles in the mode of action. In this paper, we report the solid-phase step-by-step synthesis of diptericin, an 82-mer peptide, originally isolated from Phormia terranovae. The unglycosylated peptide was purified to homogeneity by conventional reversed-phase high performance liquid chromatography, and its activity spectrum was compared to that Of synthetic unglycosylated drosocin, which shares strong sequence homology with diptericin's N-terminal domain. Diptericin appeared to have antibacterial activity:for only a limited number of Gram-negative bacteria. Diptericin's submicromolar potency against Escherichia coli strains indicated that, in a manner similar to drosocin, the presence of the carbohydrate side chain is not,necessary to kill bacteria. Neither the N-terminal, drosocin-analog fragment, nor the C-terminal, glycine-rich attacin-analog region was active against any of the bacterial strains studied, regardless of whether the Gal-GalNAc disaccharide units were attached. This suggested that the active site of diptericin fell outside the drosocin or attacin homology domains. In addition, the conformation of diptericin did not seem to play a role in the antibacterial activity, as was demonstrated by the complete lack of ordered structure by two-dimensional nuclear magnetic resonance spectroscopy and circular dichroism. Diptericin completely killed bacteria within I h, considerably faster than drosocin and the attacins; unlike some other, fast-acting antibacterial peptides, diptericin did not lyse normal mammalian cells. Taken together, these data suggest diptericin does not belong to any known class of antibacterial peptides.

Oropouche virus entry into HeLa cells involves clathrin and requires endosomal acidification

Oropouche virus (ORO), family Bunyaviridae, is the second most frequent cause of arboviral febrile illness in Brazil. Studies were conducted to understand ORO entry in HeLa cells. Chlorpromazine inhibited early steps of ORO replication cycle, consistent with entry/uncoating. The data indicate that ORO enters HeLa cells by clathrin-coated vesicles, by a mechanism susceptible to endosomal acidification inhibitors. Transmission electron microscopy and immunofluorescence indicated that ORO associates with clathrin-coated pits and can be found in association with late endosomes in a time shorter than 1 h. (C) 2008 Elsevier B.V. All rights reserved.

Effects of ocean acidification on the swimming ability, development and biochemical responses of sand smelt larvae

Ocean acidification, recognized as a major threat to marine ecosystems, has developed into one of the fastest growing fields of research in marine sciences. Several studies on fish larval stages point to abnormal behaviours, malformations and increased mortality rates as a result of exposure to increased levels of CO2. However, other studies fail to recognize any consequence, suggesting species-specific sensitivity to increased levels of CO2, highlighting the need of further research. In this study we investigated the effects of exposure to elevated pCO2 on behaviour, development, oxidative stress and energy metabolism of sand smelt larvae, Atherina presbyter. Larvae were caught at Arrábida Marine Park (Portugal) and exposed to different pCO2 levels (control: ~600μatm, pH=8.03; medium: ~1000μatm, pH=7.85; high: ~1800μatm, pH=7.64) up to 15days, after which critical swimming speed (Ucrit), morphometric traits and biochemical biomarkers were determined. Measured biomarkers were related with: 1) oxidative stress - superoxide dismutase and catalase enzyme activities, levels of lipid peroxidation and DNA damage, and levels of superoxide anion production; 2) energy metabolism - total carbohydrate levels, electron transport system activity, lactate dehydrogenase and isocitrate dehydrogenase enzyme activities. Swimming speed was not affected by treatment, but exposure to increasing levels of pCO2 leads to higher energetic costs and morphometric changes, with larger larvae in high pCO2 treatment and smaller larvae in medium pCO2 treatment. The efficient antioxidant response capacity and increase in energetic metabolism only registered at the medium pCO2 treatment may indicate that at higher pCO2 levels the capacity of larvae to restore their internal balance can be impaired. Our findings illustrate the need of using multiple approaches to explore the consequences of future pCO2 levels on organisms.

[10 phot. de France: Montbéliard, Montoire (phot. Yvon-Abel, phot. à Loches), la Mer de glace près Chamonix (phot. ancienne maison Martinet, 172, rue de Rivoli, Paris), le lac Chambon (Puy-de-Dôme), La Rochelle (phot. D. Tallon Larente, phot. dans cette ville), un dolmen près de Lodève (Hérault) (phot. E. Cartailhac), environs de Chambéry (phot. J.L. Perrot, phot. à Chambéry), le Pont du Gard, la Maladetta et le Port de Vénasque (Haute-Garonne Aragon), Cirque de Gavarnie (Hautes-Pyrénées). Don E. Reclus en 1886]

Donateur : Reclus, Élisée (1830-1905)