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Soutenance de thèse par Katia-Louiza ASMANI

Cette thèse aura lieu le lundi 20 janvier 2014 à 14h à l'IUEM, amphi A

Sujet : "Etude et contrôle des populations bactériennes associées aux élevages larvaires de Cassostrea gigas en système d'eau recyclée


This study allowed defining the parameters of larval density, rate of water exchange to optimize larval rearing oyster Crassostrea gigas in continuous system with recycled water (RAS: Recycling Aquaculture System). Thus larval density of 50 to 150 larvae mL-1 with 10 % seawater renewal per hour, ensure good survival and growth of larvae, however, the rearing performances are less than those of the open circuit (100% seawater renewal per hour.) or FTS (Flow through System). Bacterial populations associated with these rearings were examined by pyrosequencing targeted on the 16S rRNA gene.

First, it appears that bacterial populations of different niches analyzed: water, algae, larvae, bioreactor, and biofilm had little OTUS common between them and the microbiomes of each compartment had its own dynamics independently conditions tested during the experiment. The diversity of these populations is generally high with Shannon indices ranging from 3.5 to 5.8 with a few notable exceptions to the algal cultures or water. This diversity drops dramatically when Bacteriovorax sp (Deltaproteobacteria) settled in dominant flora. Gammaproteobacteria rate is generally low even though this group tends to become more important at the end of rearing. It hardly includes Vibrionaceae which are very poorly represented whatever the compartment. The bacteria called "environmental" which are without specific affiliation can be invasive.

A control test of microflora by adding bacteria resulted in a decrease in performances and changes in the microbiome of larvae and water indicating the possible impact of bacteria without being pathogens. The hypothesis was that the inferior results of RAS could indeed be due to these bacterial biofilms that develop on the walls of tanks and recycling system. The study of biofilm has shown a succession of populations starting with pioneering Alphaproteobacteria bacteria to the formation of more or less dense network of filamentous bacteria Anaerolineae (Chloroflexi) and Saprospirae (Bacteroidetes). Although several studies have been made of the biological filter coupled to detections amoA genes of bacteria and archaebacteria, it could not be detected in significant numbers of autotrophic nitrifying bacteria and any capable of oxidizing ammonia. The presence of nitrates in a system at least attests however that nitrification was occurred.

These first analyzes did not show a stable microbiota is established during larvae to postlarvae stages but rather a succession of microbiomes that keep very few close them, in these conditions, the concept of the holobiont can be applied ? If we consider only specific aspect we can be positive but to be sure it is necessary to achieve a functional metagenomic and experimentation."


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(C) Pascale Lherminier / Ifremer