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Résumé : We review available data on archaea, bacteria and small eukaryotes in anattempt to provide a general picture of microbial diversity, abundances and microbedrivenprocesses in Lake Kivu surface and intermediate waters (ca. 0–100 m). Thevarious water layers present contrasting physical and chemical properties andharbour very different microbial communities supported by the vertical redoxstructure. For instance, we found a clear vertical segregation of archaeal and bacterialassemblages between the oxic and the anoxic zone of the surface waters. Thepresence of speci fi c bacterial (e.g. Green Sulfur Bacteria) and archaeal (e.g.ammonia-oxidising archaea) communities and the prevailing physico-chemicalconditions point towards the redoxcline as the most active and metabolically diversewater layer. The archaeal assemblage in the surface and intermediate water columnlayers was mainly composed by the phylum Crenarchaeota , by the recently de fi nedphylum Thaumarchaeota and by the phylum Euryarchaeota . In turn, the bacterialassemblage comprised mainly ubiquitous members of planktonic assemblages offreshwater environments ( Actinobacteria , Bacteroidetes and Betaproteobacteria among others) and other less commonly retrieved phyla (e.g. Chlorobi , Clostridiumand Deltaproteobacteria ). The community of small eukaryotes (<5 m m) mainlycomprised Stramenopiles , Alveolata , Cryptophyta , Chytridiomycota , Kinetoplasteaand Choano fl agellida , by decreasing order of richness. The total prokaryoticabundance ranged between 0.5 × 10 6 and 2.0 × 10 6 cells mL −1 , with maxima locatedin the 0–20 m layer, while phycoerythrin-rich Synechococcus -like picocyanobacteriapopulations were comprised between 0.5 × 10 5 and 2.0 × 10 5 cells mL −1 in the samesurface layer. Brown-coloured species of Green Sulfur Bacteria permanentlydeveloped at 11m depth in Kabuno Bay and sporadically in the anoxic waters of thelower mixolimnion of the main basin. The mean bacterial production was estimatedto 336 mg C m −2 day −1 . First estimates of the re-assimilation by bacterioplankton ofdissolved organic matter excreted by phytoplankton showed high values of dissolvedprimary production (ca. 50% of total production). The bacterial carbon demand cantotally be fuelled by phytoplankton production. Overall, recent studies have revealeda high microbial diversity in Lake Kivu, and point towards a central role of microbesin the biogeochemical and ecological functioning of the surface layers, comprisingthe mixolimnion and the upper chemocline.

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