Les protistes : acteurs-clés du réseau trophique pélagique au Lac Tanganyika

: Le picoplancton et ses brouteurs-clés dans deux grands lacs tropicaux

Abstract

While pathogenic microbes are actually far from being the rule, in contrast microbes are generally associated to illness. Actually, microbes play crucial parts in sustaining life on Earth and preserving biodiversity. In aquatic environments, protists (microbial eukaryotes) are fundamental components of the pelagic food web, both as primary producers and as the main link between picoplanktonic carbon and the higher trophic levels, through grazing activities. The substantial role of protozoan has been particularly stressed for the oligotrophic systems, where picoplankton, inedible for metazooplankton, typically dominates planktonic biomass and productivity. Located in the East African Rift Valley, Lake Tanganyika is one of the world's largest and deepest lakes, which is also known for the unusually high yield of its pelagic fisheries. Its high water transparency and low nutrient levels favour both picophytoplankton proliferation, at the expense of larger phytoplankton, and development of an important microbial trophic network. As carbon transfer through microbial food web increases the number of trophic links and leads to large energy losses, it is expected that Lake Tanganyika microbial communities are particularly efficient in carbon use and cycling to reduce these losses and therefore sustain high fish productivity. Our first objective was thus (i) to estimate carbon biomass transiting through protozoa by focussing on protozooplancton impact on both autotrophic and heterotrophic picoplancton in Lake Tanganyika, and (ii) to determine the major carbon pathway within the microbial trophic network. Protist communities in aquatic systems can also be phylogenetically and functionally diverse. During the last decade, molecular surveys have regularly unveiled undescribed lineages, suggesting that microbial eukaryote diversity remains undersampled. Actually, small-eukaryote communities in tropical lacustrine systems had not been investigated so far. Yet, from an ecological perspective, molecular approaches for describing protistan diversity often represent the first step in characterizing the different functional roles performed within assemblages. Our second objective consisted in unveiling small-eukaryote diversity retrieved from the tropical Lake Tanganyika by using DGGE and clone libraries of 18S rDNA. Further clone libraries were also constructed to uncover small-eukaryote diversity within Lake Kivu, another freshwater system located in the Rift Valley. Differences and similarities were detected among microbial eukaryote communities recovered from these two oligotrophic tropical lakes that feature contrasting physical-chemical conditions and trophic structures. Detection of common undescribed lineages in both lakes suggested that tropical conditions could favour some taxonomic groups. Using the FISH technique, we especially focussed on novel kinetoplastids which were retrieved in both lakes and appeared to be abundant in Lake Tanganyika. The present study provides the first description of small-eukaryote diversity within a tropical freshwater ecosystem and brings substantial evidence of the crucial role of protozooplankton in carbon transfer in one of the most fascinating lacustrine systems, Lake Tanganyika.

Date of Award	19 Feb 2010
Original language	French
Awarding Institution	University of Namur
Supervisor	Jean-Pierre DESCY (Supervisor), Samuël Pirlot (Jury), Karine Van Doninck (Jury), Wim G. Vyverman (Jury) & Ramon Massana i Molera (Co-Supervisor)