Éditeur(s) : Actes Symposium, Bordeaux, France, 8-14 septembre 1981, pp 389-395
Résumé : Estimations of algal growth potential (AGP) of oyster-pond waters have been made by the use of bioassays and nutrient analysis. Data obtained demonstrated that nitrogen is the nutrient Iimiting AGP and that an important part of algal biomass is supported by organic nutrients. Such an uptake ranges from 0.6 to 30.7 µg-at.I-1 nitrogen ; which represents up to six times the total amount of inorganic nitrogen taken up. U.V oxydation and subsequent chemical analysis shown DON concentrations in oyster-pond waters varied with oyster presence or absence: values recorded were 10-40 µg-at N.I-1 when oysters were not present, and up to 50-60 µg-at N.I-1 when oysters were covering the bottom of the pond. The oyster Crassostrea gigas was demonstrated to excrete 77-93 % DON, versus 10-33 % N-NH4; urea excretion often represented three folds the NH4 excretion. The capability to take up organic nutrients varies with algal species ; the diatom Navicula ostrearia appeared to be the most efficient one.
L'utilisation de tests biologiques dans l'étude de la fertilité des eaux de claires ainsi que l'analyse chimique des réserves en sels nutritifs des mêmes eaux montrent qu'une part importante de la biomasse micro-algale peut être produite dans ces bassins, par assimilation par les cellules de substances organiques dissoutes. Ainsi, pour l'azote, des estimations indirectes aboutissent à des quantités de cet élément assimilé sous forme organique variant entre 0,6 et 30,7 µg-at.I-1 selon les modes d'évaluation, les espèces et l'origine des eaux, c'est-à-dire jusqu'à six fois les quantités d'ions minéraux prélevés par les algues. Ces valeurs sont tout à fait en concordance avec les teneurs en azote organique dissous effectivement présentes dans les eaux de claires: entre 10 et 40 µg-at.I-1 et jusqu'à 50 à 60 µg-at.I-1 dans les bassins où des huîtres sont immergées. On ne peut donc exclure une relation entre ces fortes valeurs en azote organique dissous et les huîtres. D'ailleurs, il est démontré que l'huître Crassostrea gigas peut excréter 77 à 93 % de rejets azotés sous forme organique, l'ammoniaque n'en représentant que 10 à 33 % et la quantité d'urée excrétée atteignant jusqu'à trois fois celle d'ammoniaque. L'utilisation de ces formes organiques de l'azote par les algues des claires varie avec les espèces; Navicula ostrearia semble être la mieux adaptée à ce type d'assimilation.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/1981/acte-2946.pdf
http://archimer.ifremer.fr/doc/00000/2946/

Éditeur(s) : Wiley-blackwell
Résumé : Alexandrium tamarense (M. Lebour) Balech strains isolated in spring 2007 from a single bloom in Thau lagoon have been grown in nonaxenic artificial media. For three strains showing large oscillations in biomass (crashes followed by recoveries) on a scale of several days, a significant relationship was observed between changes in cell densities (as in vivo fluorescence) and changes in nitrate concentrations. Increases in cell densities were accompanied by decreases in nitrate, while decreases in cell densities corresponded to increases in nitrate, presumably due to nitrification. Net increases in nitrate could reach up to 15 mu mol N . L(-1) . d(-1) indicating a very active nitrifying archaeal/bacterial population. However, following population crashes, algal cells can recover and attain biomass levels similar to those reached during the first growth phase. This finding indicates that those archaea/bacteria do not compete for nutrients or do not hamper algal growth under those conditions. In contrast to diatoms, dinoflagellates such as A. tamarense do not excrete/exude dissolved organic matter, thus preventing excessive bacterial growth. This mechanism could help explain the recovery of this species in the presence of bacteria.
Journal Of Phycology (0022-3646) (Wiley-blackwell), 2011-10 , Vol. 47 , N. 5 , P. 1057-1062

Droits : 2011 Phycological Society of America
http://archimer.ifremer.fr/doc/00049/16046/13585.pdf
DOI:10.1111/j.1529-8817.2011.01031.x
http://archimer.ifremer.fr/doc/00049/16046/

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
DNA cytosine methylation is a widely conserved epigenetic mark in eukaryotes that appears to have critical roles in the regulation of genome structure and transcription. Genome-wide methylation maps have so far only been established from the supergroups Archaeplastida and Unikont. Here we report the first whole-genome methylome from a stramenopile, the marine model diatom Phaeodactylum tricornutum. Around 6% of the genome is intermittently methylated in a mosaic pattern. We find extensive methylation in transposable elements. We also detect methylation in over 320 genes. Extensive gene methylation correlates strongly with transcriptional silencing and differential expression under specific conditions. By contrast, we find that genes with partial methylation tend to be constitutively expressed. These patterns contrast with those found previously in other eukaryotes. By going beyond plants, animals and fungi, this stramenopile methylome adds significantly to our understanding of the evolution of DNA methylation in eukaryotes.
ISSN: 2041-1723

hal-01545322
https://hal.archives-ouvertes.fr/hal-01545322
DOI : 10.1038/ncomms3091

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Deposit feeders are able to process a considerable volume of sediment, containing large 9 quantities of associated bacteria. However, conclusions concerning the trophic role played by 10 benthic bacteria in marine sediments are still not fully elucidated. This study deals with 11 bacterivory by the gastropod Hydrobia ulvae, one of the most abundant deposit-feeding 12 species in intertidal mudflats in Western Europe. Ingestion rates of bacteria were determined 13 during grazing experiments using 15 N pre-enriched bacteria. Grazing experiments were 14 performed in order to measure effects of abiotic (temperature, salinity and luminosity) and 15 biotic (bacterial and algal abundances) factors on ingestion rates of bacteria by H. ulvae of an 16 intertidal mudflat (Brouage, Marennes-Oléron, France). The mean ingestion rate of bacteria 17 by H. ulvae was 1149 ngC ind-1 h-1. The general trend showed a temperature effect with an 18 optimum around 30°C, and the assimilation rate was significantly lower at 5°C. Bacterial 19 assimilation did not significantly differ between salinity 18 and salinity 31. Ingestion was the 20 same in light and in dark conditions. Results were compared with those of other grazing 21 experiments conducted simultaneously in similar conditions with two other grazers with 22 different size and feeding modes: the foraminifera Ammonia tepida and a nematode 23 community from the superficial sediment of the Brouage mudflat. Hydrobia ulvae and 24 nematodes presented a feeding behavior less influenced by environmental changes than A. tepida. Hydrobia ulvae ingested bacteria at a higher rate than smaller meiofaunal grazers and seemed to have a lower ability to selectively ingest diatoms than meiofaunal grazers.
ISSN: 1385-1101

hal-01248039
https://hal.archives-ouvertes.fr/hal-01248039
https://hal.archives-ouvertes.fr/hal-01248039/document
https://hal.archives-ouvertes.fr/hal-01248039/file/Pascal%20et%20al%202008.pdf
DOI : 10.1016/j.seares.2008.05.003