Evolution des activités enzymatiques dans le tube digestif au cours de la vie larvaire du bar (Dicentrarchus labrax). Variations des protéinogrammes et des zymogrammes

The development of the digestive system of larvae of Dicentrarahus labrax has been studied. The changes in activities of digestive enzymes from hatching to 30 days have been investigated. 8 activities have been recorded : esterases, a-glucosidase, trypsin, chymotryp­sin, leucinaminopeptidase, alkalin and acid phosphatase, using polyacrylamid gel electropho­resis. All the activities are very weak at hatching j the activities of trypsin, a-glucosidase, alkalin phosphatase increase between 0 and 5 days, decrease slowly and are quite stable from 15 to 30 days. The other activities increase steadily from 0 to 20 days. Relationships between the developmental process of the digestive system and changes in activities are investigated.Quelques activités enzymatiques digestives de larves de loups ou bars (Dicentrarahus labrax) ont été étudiées, en utilisant les techniques d'électrophorèse. Les larves ont été prélevées de cinq jours en cinq jours à partir de l'éclosion jusqu'à trente jours. Des Varia­tions dans le nombre des isozymes actifs vis-à-vis des substrats testés sont enregistrées. Toutes les activités semblent être présentes, mais très faibles, dès l'éclosion ; les princi­pales variations sont décelées dans les quinze premiers jours. Les relations avec le dévelop­pement fonctionnel du tractus digestif des larves sont discutées

Uptake and accumulation of ammonium by Alexandrium catenella during nutrient pulses

Field observations in Thau Lagoon, southern France, indicate that the growth of natural populations of Alexandrium catenella during blooms is limited by nitrogen and exhibits a storage rather than a growth response to an ammonium pulse. Therefore, ammonium uptake and accumulation under transient conditions were investigated in detail in laboratory cultures. Following nitrogen exhaustion from the medium, ammonium pulses of varying magnitudes were induced, and measurements of extra- and intra-cellular ammonium were carried out for 24–72h along with measurements of ammonium incorporation (15N tracer) and inorganic carbon fixation (13C tracer). During vegetative growth, values of intra-cellular ammonium reached 30% (Strain TL01) and 2% (Strain ACT03) of cell nitrogen. When ammonium was available in the culture medium, ammonium uptake estimated by the 15N tracer technique accounted for only 65% of the decrease in external ammonium. This discrepancy is probably due to organic nitrogen excretion. Once external ammonium was exhausted, the observed isotopic dilution of both 13C and 15N cell content indicated uptake of a compound containing both unlabelled carbon and nitrogen atoms

Impact of the oyster Crassostrea gigas on a microbial community in Atlantic coastal ponds near La Rochelle

To assess the in situ impact of oysters Crassostrea gigas on planktonic protist and bacteria communities and the potential contribution of protozoa to their food resource intake, the abundance and the diversity of protists and bacteria were followed in 2 Atlantic coastal ponds, with and without oysters. The protist biomass in such ponds was high, with a maximum in spring of 982 mug C l(-1) and a minimum in winter of 179 pg C l(-1). Whatever the season, the presence of oysters (20 m(-2) corresponding to an average of 23 mg dry weight m(-2)) induced a significant decrease in >5 pm protist abundance. On the contrary, planktonic organisms <5 pm, such as Chlorophyta flagellates and bacteria, developed similarly in both ponds. It can be assumed that such depletion in micro-sized protists was especially related to the grazing activity of C. gigas, which efficiently retains >5 pm particles. In spring, oyster grazing triggered dramatic changes in the protist community by lowering the taxonomic diversity. In autumn and winter, the presence of oysters deeply influenced the taxonomic structure of the protist communities: > Fun protists could only develop in the control pond, whereas they were removed by filtration in the oyster pond; on the contrary, >5 Fun protists that were not retained were favoured in the oyster pond. The results showed that hetero/mixotrophic protists represent an important potential resource in coastal ponds: flagellates >5 pm were the main protist resource for C. gigas; ciliates represented the second resource, with a substantial contribution in autumn; diatoms and dinoflagellates, though efficiently removed, represented a weak carbon resource. Our study supports the hypothesis that oysters may access the strong bacterioplanktonic production through hetero/mixotrophic protists, which would thus allow the transfer of carbon from the microbial loop towards C. gigas

Marine diatoms sustain growth of bivalves in a Mediterranean lagoon

International audienceCarbon stable isotopes and fatty acids were measured in the suspended particulate organic matter (POM) of the Thau lagoon to study its qualitative temporal changes in relation to environmental factors and to identify the food sources of bivalves over a one-yr-cycle in relation to their growth. Reciprocally, the impact of shellfish farming on POM was also studied. Oysters and mussels were sampled and measured for biometry, stable isotopes and fatty acid composition. Water samples were collected at two sites, both inside and outside of the shellfish farming area, to determine concentrations in POM, chlorophyll a (Chl a) and stable isotopes. Carbon isotopes and fatty acids in bivalves reflected seasonal changes in food sources, which varied consistently with the environment. Seasonal changes in delta C-13 and fatty acids in the bivalves suggested that dietary phytoplankton contribution varied according to season. Terrestrial organic matter and bacteria can contribute to the diet of bivalves during non-bloom periods. Mussels seemed to rely more on diatoms and less on terrestrial organic matter and bacteria than oysters did, particularly when phytoplankton biomass was low during the summer. Although one- and two-yr-old oysters showed similar delta C-13, their fatty acid dynamics differed slightly. Periods of high growth rate in bivalves were mainly fuelled by diatoms, thus highlighting the importance of seasonal blooms of microphytoplankton during the critical period of bivalve growth and gamete production. Although there was no significant effect of shellfish farms on Chl a and POM delta C-13, consistent differences indicate that stable isotopes could be used successfully to investigate the effects of bivalve aquaculture. (C) 2011 Elsevier B.V. All rights reserved

Red coloration of oysters along the French Atlantic coast during the 1998 winter season: implication of nanoplanktonic cryptophytes

International audienceDuring a 4-week experiment on oyster grazing in coastal ponds (claires) near La Rochelle (France), a red coloration was observed in the digestive gland of oysters. This pigmentation was attributed to an intensive grazing on cryptophyte nanoflagellates. Three different techniques led to this conclusion, mentioning for the first time free-living cryptophytes as responsible for such a phenomenon. Spectrofluorometry was used to demonstrate that the red coloration was due to the presence of a phycoerythrin, characteristic of cryptophytes. Microscopic observations and accessory pigments analyses in water ponds confirmed that the source of this pigment was of cryptomonadal origin. Oyster grazing on these algae is evidenced by large differences in alloxanthin concentrations and flagellate abundances between the two experimental ponds. The winter occurrence of this phenomenon is of importance for oyster commercialization and a short-term depuration is suggested to remove the undesirable pigmentation.

Heterotrophic protists as a trophic link between picocyanobacteria and the pearl oyster Pinctada margaritifera in the Takapoto lagoon (Tuamotu Archipelago, French Polynesia)

International audiencePearl oysters are farmed in oligotrophic tropical atoll lagoons where planktonic communitiesare dominated by production from cyanobacteria smaller than 2 μm. Paradoxically, the pearloyster Pinctada margaritifera only retains particles larger than 2 μm. In this study, we assess the relativecontribution of hetero/mixotrophic microbiota to the available planktonic resource. In TakapotoAtoll, picocyanobacteria are the dominant biomass (20 μg C l–1). The carbon biomass of ciliates anddinoflagellates ranges from 1 to 24 and 0.5 to 5 μg C l–1 respectively, with a mean of 6 μg C l–1 for ciliatesand 2 μg C l–1 for dinoflagellates. The possible retention by P. margaritifera on a natural protistsuspension was investigated. Due to its high clearance rates (ca 20 l h–1 g–1) the pearl oyster retained85 μg C h–1 g–1 from ciliates and 65 μg C h–1 g–1 from dinoflagellates. Conversely, cyanobacteria werenot efficiently retained by the bivalve and did not efficiently contribute to its diet. From our experiments,we concluded that hetero/mixotrophic protists rapidly and efficiently process the picoplanktonicresource towards filter-feeders, particularly pearl oysters

ASSESSMENT OF CRYPTIC SPECIES DIVERSITY WITHIN BLOOMS AND CYST BANK OF THE ALEXANDRIUM TAMARENSE COMPLEX (DINOPHYCEAE) IN A MEDITERRANEAN LAGOON FACILITATED BY SEMI-MULTIPLEX PCR

International audienceThe occurrence of Alexandrium catenella related to paralytic shellfish poisoning in the French Mediterranean Thau lagoon has been known since 1998. Blooms are recurrent and usually occur each year in spring and/or autumn. Taxonomic diversity of resting cysts and vegetative cells has been studied through morphological examination and molecular typing of 558 clonal strains sampled in 2004 and 2007. Sequencing the nuclear rRNA fragment including ITS1, the 5.8S rRNA gene, ITS2, and the D1/D2 28S rRNA gene permitted the detection of two species A. catenella and A. tamarense which are difficult to distinguish morphologically (cryptic species). In order to carry out massive and accurate molecular determinations, an original semi-multiplex PCR method, using new ribotype-specific primers targeting the 18S-28S rRNA ITS region has been developed. The relative abundance of each species was then established in seawater in 2007 and in the sediment collected in 2004. The co-occurrence of A. catenella (Group IV), which is known as the main species responsible for toxic PSP events since 1998 and of A. tamarense (Group III) (non-toxic), which was not formally recognized by microscopic observation since 1995, was examined for several months