over euryarchaeotal ones (7 OTUs). Sequences affiliated with the kingdom Euryarchaeota were mainly recovered from the anoxic water compartment and mostly grouped into methanogenic lineages (Methanosarcinales and Methanocellales). In turn, crenarchaeal phylotypes were recovered throughout the sampled epipelagic waters (0-to 100-m depth), with clear phylogenetic segregation along the transition from oxic to anoxic water masses. Thus, whereas in the anoxic hypolimnion crenarchaeotal OTUs were mainly assigned to the miscellaneous crenarchaeotic group, the OTUs from the oxic-anoxic transition and above belonged to Crenarchaeota groups 1.1a and 1.1b, two lineages containing most of the ammonia-oxidizing representatives known so far. The concomitant vertical distribution of both nitrite and nitrate maxima and the copy numbers of both MCG1 16S rRNA and amoA genes suggest the potential implication of Crenarchaeota in nitrification processes occurring in the epilimnetic waters of the lake. Lake Kivu is a meromictic lake located in the volcanic region between Rwanda and the Democratic Republic of the Congo and is the smallest of the African Great Rift Lakes. The monimolimnion of the lake contains a large amount of dissolved CO 2 and methane (300 km 3 and 60 km 3 , respectively) as a result of geological and biological activity (24,73,85). This massive accumulation converts Lake Kivu into one of the largest methane reservoirs in the world and into a unique ecosystem for geomicrobiologists interested in the methane cycle and in risk assessment and management (34,71,72,85). Comprehensive studies on the diversity and activity of planktonic populations of both large and small eukaryotes and their trophic interplay operating in the epilimnetic waters of the lake are available (33,39,49). Recent surveys have also provided a deeper insight into the seasonal variations of photosynthetic and heterotrophic picoplankton (67, 68), although very few data exist on the composition, diversity, and spatial distribution of bacterial and archaeal communities. In this regard, the studies conducted so far of the bacterial/archaeal ecology in Lake Kivu have been mostly focused on the implications on the methane cycle (34, 73), but none have addressed the presence and distribution of additional archaeal populations in the lake.During the last few years, microbial ecology studies carried out in a wide variety of habitats have provided compelling evidence of the ubiquity and abundance of mesophilic archaea (4,10,13,19). Moreover, the discovery of genes encoding enzymes related to nitrification and denitrification in archaeal metagenomes from soil and marine waters (29,86,88) and the isolation of the first autotrophic archaeal nitrifier (40) demonstrated that some archaeal groups actively participate in the carbon and nitrogen cycles (56,64,69). In relation to aquatic environments, genetic markers of ammonia-oxidizing archaea (AOA) of the marine Crenarchaeota group 1.1a (MCG1) have consistently been found in water masses of several oceanic regions (...