Microbial Ecology of Lake Kivu

We review available data on non-phytoplankton microorganisms in an attempt to provide a general picture of microbial diversity, abundances and microbe-driven processes in Lake Kivu surface and intermediate waters (ca. 0–100 m). The various water layers present contrasting physical and chemical properties and harbour very different microbial communities supported by the vertical redox structure. For instance, we found a clear vertical segregation of archaeal and bacterial assemblages between the oxic and the anoxic zone of the surface waters. The presence of specific bacterial (e.g. Green Sulfur Bacteria) and archaeal (e.g. ammonia-oxidising ar-chaea) communities and the prevailing physico-chemical conditions point towards the redoxcline as the most active and metabolically diverse water layer. The archaeal assemblage in the surface and in-termediate water column layers was mainly composed by the King-dom Crenarchaeota, by the recently defined phylum Thaumarchae-ota and by the Kingdom Euryarchaeota. In turn, the bacterial as-semblage comprised mainly ubiquitous members of planktonic as-semblages of freshwater environments (Actinobacteria, Bacteroidetes and Betaproteobacteria among others) and other less commonly retrieved phyla (e.g. Chlorobi, Clostridium and Deltaproteobacteria). The community of small eukaryotes (< 5 µm) mainly comprised Stramenopiles, Alveolata, Cryptophyta, Chytridiomycota, Kinetoplastea and Choanoflagellida, by decreasing order of richness. The total prokaryotic abundance ranged between 0.5×106 and 2.0×106 cells mL-1, with maxima located in the 0–20 m layer, while phycoerythrin-rich picocyanobacteria populations were comprised between 0.5×105 and 2.0×105 cells mL-1 in the same surface layers. Brown-coloured species of Green Sulfur Bacteria permanently de-veloped at 11-m depth in Kabuno Bay and sporadically in the anoxic waters of the lower mixolimnion of the main basin. The mean bacte-rial production was 336 mg C m-2 d-1, i.e. close to the mean particu-late phytoplankton production. First estimates of the re-assimilation by bacterioplankton of dissolved organic matter excreted by phyto-plankton showed high values of dissolved primary production (ca. 50 % of total production). The bacterial carbon demand can totally be fuelled by phytoplankton production. Overall, recent and current studies have revealed a high microbial diversity in Lake Kivu, and point to a central role of microbes in the biogeochemical and ecolog-ical functioning of the surface layers, comprising the mixolimnion and the upper chemocline.