The main objective of the project is to estimate bacterioplankton and cyanobacteria biomass and biodiversity of of lake Tanganyika. A second objective is to study temporal and vertical variations of those planktonic microbial components. Present studies, carried out in the framework of the SSTC CLIMLAKE project, have shown contrasted composition and biomass variations of total numbers of both phytoplankton and bacterioplankton, between the rainy season situation, when nutrient limitation is more likely, and dry season, during which deep mixing of the water column due to winds enhances nutrient availability. On the other hand, variation in nutrient concentration and organic matters in the water column are more likely to favour distinct genotypes. The present study hypothesisis that those contrasted environmental conditions are selecting microbial communities of very diverse nature. Such variations in microbial strains that occupies distinct niches are known in marine environment (e.g. Partensky et al., 1999, or Toledo et Palenik, 2003), but scarcely studied in freshwaters. Lake Tanganyika, due to its age (10-20 millions of years) and to its complexity, could house a large microbial diversity.