Diversité et succession des protistes dans l'océan Arctique

L'Arctique est la région du globe où le réchauffement climatique est le plus prononcé. L'étude de la diversité des microorganismes, leur dynamique de communauté et les facteurs environnementaux qui agissent sur eux s'avèrent donc importants pour comprendre comment ces communautés vont réagir à des changements environnementaux. Cette thèse explore la diversité des protistes et leur dynamique dans l'océan Arctique sur une échelle temporelle ainsi que spatiale. La méthodologie utilisée dans cette étude est basée sur l'analyse des séquences du 18S ADN. L'étude temporelle de la communauté des protistes de la zone mésopélagique montre une communauté dynamique avec des assemblages distincts à l'hiver-printemps et à l'été-automne. Cette dynamique est associée à des changements hydrographiques et montre aussi un couplage avec la zone euphotique, qui est reflétée par une plus grande proportion des séquences associées à des dinoflagellés hétérotrophes pendant la période été-automne. Dans la zone euphotique, la transition printanière est caractérisée par une augmentation de la biomasse des organismes autotrophes. Toutefois, lorsqu'un maximum de chlorophylle a est atteint, l'activité des ciliés et dinoflagellés hétérotrophes est stimulée. L'étude de la diversité des protistes à la fin de la saison productive dans la zone euphotique montre que des masses d'eau, avec une origine commune, présentent des assemblages de protistes plus similaires. Néanmoins, un groupe divers de taxons se retrouve distribué de façon pan-arctique. Parmi ceux-ci, il y a des organismes autotrophes, hétérotrophes et parasitiques. Cette thèse a permis d'augmenter la connaissance de la dynamique des organismes autotrophes dans les milieux pélagiques arctiques et offre une nouvelle vision de l'écologie des protistes hétérotrophes et parasitiques. Les résultats soulignent que ces protistes hétérotrophes doivent être considérés dans l'étude de la réponse des cycles biogéochimiques au réchauffement climatique dans l'océan Arctique

Seasonal changes in planktonic bacterivory rates under the ice-covered coastal Arctic Ocean

Bacterivory was determined in surface waters of Franklin Bay, western Arctic, over a seasonal ice-covered period (winter-spring, 2003-2004). The objectives were to obtain information on the functioning of the microbial food web under the ice, during winter (from 21 December 2003 to 21 March 2004) and during spring (from 22 March 2004 to 29 May 2004), and to test whether bacterial losses would increase after the increase in bacterial production following the spring phytoplankton bloom. Chl a concentrations ranged from 0.04 to 0.36 mu g L(-1), increasing in March and reaching a peak in April. Bacterial biomass showed no consistent trend for the whole period, and protist biomass followed a pattern similar to that of Chl a. Bacterial production increased 1 week after Chl a concentrations started to increase, while bacterivory rates increased very slightly. Average bacterivory rates in winter (0.16 +/- 0.07 mu g C L(-1) d(-1)) were not significantly different from those in spring (0.29 +/- 0.24 mu g C L(-1) d(-1)). Average bacterial production, on the other hand, was similar to bacterivory rates in winter (0.19 +/- 0.38 mu g C L(-1) d(-1)), but higher than bacterivory in spring (0.93 +/- 0.28 mu g C L(-1) d(-1)). Therefore, bacterial production was controlled by grazers during winter and by substrate concentration in spring

Kajian Waktu Strangulasi Terhadap Pembungaan Jeruk Pamelo ‘Cikoneng\u27 (Citrus Grandis (L.) Osbeck)

An Assessment Time of Strangulation on Flowering Induction of Pummelo ‘Cikoneng\u27 (Citrus grandis (L.) Osbeck). Field assessment was conducted at Farmer\u27s Field Bantarmara Village, Cisarua, Sumedang region, West Java from August 2007 to May 2008, to determine the effect of different strangulation times on flowering induction of Pummelo during flowering-off season. Three strangulation times (1,2 and 3 months after harvest, MAH) were arranged in randomized complete block design with 6 (six) replications. An assessment results showed that strangulation at 3 MAH effectively induced flowering appearance (83%) and increased numbers of generative shoots, flowering cluster, bud, blooming, fruit formation and fruit setting. On the other hand, all treated plants had higher C/N ration than control. Kajian lapang dilaksanakan di Tanah Petani (TP) Bantarmara, kecamatan Cisarua, kabupaten Sumedang, Jawa Barat dari bulan Agustus 2007 sampai Mei 2008, untuk melihat waktu strangulasi yang tepat untuk menginduksi bunga diluar musim. Tiga waktu strangulasi (1, 2 dan 3 bulan setelah panen, BSP) disusun menurut rancangan acak kelompok dan 6 (enam) ulangan. Hasil kajian menunjukkan bahwa waktu strangulasi 3 BSP cukup efektif menginduksi jumlah muncul bunga (83%), tetapi mempengaruhi jumlah tunas generative, kluster bunga, kuncup bunga, bunga mekar, buah terbentuk dan fruit set masing-masing 32,67; 28,17; 220,00; 143.33; 61,17 dan 39,02%. Di samping itu, rasio C/N daun pada semua perlakuan lebih tinggi dari control

Autotrophic and heterotrophic acquisition of carbon and nitrogen by a mixotrophic chrysophyte established through stable isotope analysis

Collectively, phagotrophic algae (mixotrophs) form a functional continuum of nutritional modes between autotrophy and heterotrophy, but the specific physiological benefits of mixotrophic nutrition differ among taxa. Ochromonas spp. are ubiquitous chrysophytes that exhibit high nutritional flexibility, although most species generally fall towards the heterotrophic end of the mixotrophy spectrum. We assessed the sources of carbon and nitrogen in Ochromonas sp. strain BG-1 growing mixotrophically via short-term stable isotope probing. An axenic culture was grown in the presence of either heat-killed bacteria enriched with ^(15)N and ^(13)C, or unlabeled heat-killed bacteria and labeled inorganic substrates (^(13)C-bicarbonate and ^(15)N-ammonium). The alga exhibited high growth rates (up to 2 divisions per day) only until heat-killed bacteria were depleted. NanoSIMS and bulk IRMS isotope analyses revealed that Ochromonas obtained 84–99% of its carbon and 88–95% of its nitrogen from consumed bacteria. The chrysophyte assimilated inorganic ^(13)C-carbon and ^(15)N-nitrogen when bacterial abundances were very low, but autotrophic (photosynthetic) activity was insufficient to support net population growth of the alga. Our use of nanoSIMS represents its first application towards the study of a mixotrophic alga, enabling a better understanding and quantitative assessment of carbon and nutrient acquisition by this species

Microbial food web responses to light and nutrients beneath the coastal Arctic Ocean sea ice during the winter-spring transition

14 pages, 4 tables, 5 figures.-- Printed version published Dec 2008.-- Issue title: "Sea ice and life in a river-influenced arctic shelf ecosystem".We measured the abundance and biomass of phototrophic and heterotrophic microbes in the upper mixed layer of the water column in ice-covered Franklin Bay, Beaufort Sea, Canada, from December 2003 to May 2004, and evaluated the influence of light and nutrients on these communities by way of a shipboard enrichment experiment. Bacterial cell concentrations showed no consistent trends throughout the sampling period, averaging (± SD) 2.4 (0.9) × 108 cells L−1; integrated bacterial biomass for the upper mixed layer ranged from 1.33 mg C m−3 to 3.60 mg C m−3. Small cells numerically dominated the heterotrophic protist community in both winter and spring, but in terms of biomass, protists with a diameter > 10 µm generally dominated the standing stocks. Heterotrophic protist biomass integrated over the upper mixed layer ranged from 1.23 mg C m−3 to 6.56 mg C m−3. Phytoplankton biomass was low and variable, but persisted during the winter period. The standing stock of pigment-containing protists ranged from a minimum value of 0.38 mg C m−3 in winter to a maximal value of 6.09 mg C m−3 in spring and the most abundant taxa were Micromonas-like cells. These picoprasinophytes began to increase under the ice in February and their population size was positively correlated with surface irradiance. Despite the continuing presence of sea ice, phytoplankton biomass rose by more than an order of magnitude in the upper mixed layer by May. The shipboard experiment in April showed that this phototrophic increase in the community was not responsive to pulsed nutrient enrichment, with all treatments showing a strong growth response to improved irradiance conditions. Molecular (DGGE) and microscopic analyses indicated that most components of the eukaryotic community responded positively to the light treatment. These results show the persistence of a phototrophic inoculum throughout winter darkness, and the strong seasonal response by arctic microbial food webs to sub-ice irradiance in early springFinancial support for this study was provided by the Natural Sciences and Engineering Research Council of Canada, the Canada Research Chair program, Fonds québécois de recherche sur la nature et les technologies and Indian and Northern Affairs Canada. We thank the officers and crew of the CCGS AmundsenPeer reviewe

Growth of uncultured heterotrophic flagellates in unamended seawater incubations

10 pages, 5 figures, 2 tablesUnamended dark incubations of 3 µm-filtered seawater were prepared with surface samples from the Norwegian Sea, the NW Mediterranean and the Indian Ocean. Except for the very oligotrophic Indian Ocean samples, this simple treatment promoted the growth of heterotrophic flagellates, with rates (0.3 to 1.2 d–1) typical of natural assemblages. Marine stramenopile (MAST)-1 and MAST-4 cell counts, obtained by fluorescence in situ hybridization (FISH), increased in most incubations. These yet uncultured protists were first detected in molecular surveys of marine picoplankton and have been recently shown to be globally distributed heterotrophic flagellates. Three incubations from the Norwegian Sea were studied in more detail by denaturing gradient gel electrophoresis (DGGE), 18S rRNA gene clone libraries and FISH counts for 5 MAST groups. Protist diversity changed gradually during the incubation, but the DGGE bands selected were already present at the beginning of the incubation. Clone libraries from the peaks in abundance of heterotrophic flagellates were dominated by MAST sequences. FISH counts revealed MAST-1B to be a very successful organism in the 3 incubations, accounting for 15 to 30% of heterotrophic flagellates after 6 to 8 d; MAST-1A and -1C cells were also abundant. MAST-4 cells peaked before the other groups and MAST-2 was the least represented. We concluded that unamended seawater incubations can select for heterotrophic flagellates abundant in situ but not yet isolated in pure culture. Therefore, they allow investigation of the growth requirements and dynamics of these uncultured protists, and provide promising preliminary stages for their isolationThis study was supported by projects ESTRAMAR (CTM2004-12631/MAR, MEC) and TRANSINDICO (REN2002-10951-E/MAR, MCyT) granted to R.M. and ARCTIC-2000 (REN2001-4909-E/ANT, MCyT) and Marine Genomics (FP6- 505403, EU) to C.P.A. Chief-scientists R. Ingvaldsen and D. Blackman and the crews of RV ‘Johan Hjort’ (Norwegian Marine Research Institute) and RV ‘Melville’ (Scripps Institution of Oceanography) provided an excellent sampling environmentPeer reviewe

Distribution and abundance of uncultured heterotrophic flagellates in the world oceans

8 pages, 4 figures, 2 tablesHeterotrophic flagellates play fundamental roles in marine ecosystems as picoplankton grazers. This recognized importance contrasts with our ignorance of the taxonomic composition of this functional group, which remains mostly unidentified by microscopical and culturing approaches. Recent molecular marine surveys based on 18S rDNA genes have retrieved many sequences unrelated to cultured organisms and marine stramenopiles were among the first reported uncultured eukaryotes. However, little is known about the organisms corresponding to these sequences. Here we determine the abundance of several marine stramenopile lineages in surface marine waters using molecular probes and fluorescent in situ hybridization. We show that these protists are free-living bacterivorous heterotrophic flagellates. They were widely distributed, occurring in the five world oceans, and accounted for a significant fraction (up to 35%) of heterotrophic flagellates in diverse geographic regions. A single group, MAST-4, represented 9% of cells within this functional assemblage, with the intriguing exception of polar waters where it was absent. MAST-4 cells likely contribute substantially to picoplankton grazing and nutrient re-mineralization in vast areas of the oceans and represent a key eukaryotic group in marine food websThis study was supported by grants from the Spanish Ministry of Science and Technology (REN2000-1471-CO2-01/MAR, REN2001-4909-E/ANT, REN2001-0588/ANT, REN2001-5097-E/MAR, REN2002-10951-E/MAR and CTM2004-12631/MAR), the Canadian Natural Sciences and Engineering Research Council (JWACS and CASES) the US National Science Foundation (DMS Biocomplexity) and the French GIS (PICOCEAN)Peer reviewe

A tale of two mixotrophic chrysophytes: Insights into the metabolisms of two Ochromonas species (Chrysophyceae) through a comparison of gene expression.

Ochromonas spp. strains CCMP1393 and BG-1 are phagotrophic phytoflagellates with different nutritional strategies. Strain CCMP1393 is an obligate phototroph while strain BG-1 readily grows in continuous darkness in the presence of bacterial prey. Growth and gene expression of strain CCMP1393 were investigated under conditions allowing phagotrophic, mixotrophic, or phototrophic nutrition. The availability of light and bacterial prey led to the differential expression of 42% or 45-59% of all genes, respectively. Data from strain CCMP1393 were compared to those from a study conducted previously on strain BG-1, and revealed notable differences in carbon and nitrogen metabolism between the 2 congeners under similar environmental conditions. Strain BG-1 utilized bacterial carbon and amino acids through glycolysis and the tricarboxylic acid cycle, while downregulating light harvesting and carbon fixation in the Calvin cycle when both light and bacteria were available. In contrast, the upregulation of genes related to photosynthesis, light harvesting, chlorophyll synthesis, and carbon fixation in the presence of light and prey for strain CCMP1393 implied that this species is more phototrophic than strain BG-1, and that phagotrophy may have enhanced phototrophy. Cellular chlorophyll a content was also significantly higher in strain CCMP1393 supplied with bacteria compared to those without prey. Our results thus point to very different physiological strategies for mixotrophic nutrition in these closely related chrysophyte species

Seasonal changes in planktonic bacterivory rates under the ice in Franklin Bay (Arctic-Canada)

Symposium GLOBEC-IMBER España celebrado del 28-30 marzo de 2007 en Valencia.-- 1 pageThe dynamics of bacterivory rates were determined in surface samples in Franklin Bay, western Arctic, over a seasonal period (October 2003 - July 2004) comprising ice-covered and ice-free conditions. The main objectives were, first, to obtain information of the functioning of microbial food web under the ice, from the dark winter (December- mid March) to spring (mid March –May) and second, to test whether grazing losses of bacteria would be driven by the phytoplankton bloom, that would stimulate bacterial production. Chlorophyll a during the ice-covered period (December 2003- May 2004) ranged from 0.04 to 0.36 µ g L-1, starting to increase in early March and achieving this maximal value in April (spring bloom), whereas after ice melting it reached values between 0.22 and 1.46 µ g L-1. Average monthly bacterivory rates during the ice-covered period ranged between 0.17 ± 0.015 and 0.22±0.00 µ g C L-1 d-1 between December and February, and between 0.16±0.15 and 0.42±0.21 µ g C L-1 d-1 between March and May. Bacterial production was lower than or similar to bacterivory rates during the winter, from 0.011±0.01 to 0.33 ± 0.47 µ g C L-1 d-1 (from December to February), and higher than bacterivory from March to May (0.90±0.15 to 1.17±0.16 µ g C L-1 d-1 ). In consequence the dynamics of bacterivory rates during the ice-covered period did not show remarkable changes. However bacterial production increased immediately after chlorophyll a concentration augmented. Hence, we conclude that bacteria were substrate limited, and protists were likely feeding more on growing phytoplankton than on bacteri