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See detailMicrobial Diversity and Processes in Lake Kivu (East Africa)
Llirós, M.; Darchambeau, François ULg; Garcia-Armisen, T. et al

Conference (2011)

Lake Kivu is a deep meromictic and oligotrophic tropical African lake with a permanent thermal- and haline stratification with huge accumulations of dissolved CO2 and CH4 (ca. 300 km3 and 60 km3 ... [more ▼]

Lake Kivu is a deep meromictic and oligotrophic tropical African lake with a permanent thermal- and haline stratification with huge accumulations of dissolved CO2 and CH4 (ca. 300 km3 and 60 km3, respectively) in the deep anoxic monimolimnion (from 60 o 480 m depth). Although there are a wealth of information on the ecology of small eukaryotes and their trophic role on Kivu, available information on prokaryotic planktonic assemblages is scarce. Molecular analysis of archaeal and bacterial communities showed a vertical segregation imposed by the permanent redoxcline. In relation to Bacteria, Actinobacteria, Betaproteobacteria, Green Sulfur Bacteria and Bacteroidetes were the most commonly retrieved groups. For Archaea, a marked dominance of Thaumarchaeota and Crenarchaeota (75% of all archaeal OTUs) over Euryarchaeota was observed. In the anoxic hypolimnion, Euryarchaoeta (Methanosarcinales and Methanocellales) lineages together with Miscellaneous Crenarchaeotic Group phylotypes were mainly recovered. In turn, Thaumarchaeota phylotypes were recovered in oxic and suboxic waters. CARDFISH analyses over the first 100 m revealed the dominance of Bacteria (51.4% – 95.7% of DAPI-stained cells), especially Actinobacteria (epilimnion), Betaproteobacteria (oxic-anoxic interface) and Bacteroidetes (upper hypolimnion), over Archaea (1.0% – 4.5%; maximum abundances at the oxic-anoxic interface). In turn, flow cytometry evidenced the dominance of HNA cells in the euphotic layer, whereas the proportion of LNA cells increased with depth. HNA and LNA populations were still observed in the anoxic hypolimnion suggesting facultative or strict anaerobic metabolisms. The detection of distinct depth maxima of nitrate, nitrite, archaeal amoA and Marine Thaumarchaeota 16S gene copy numbers together with regularly detection of deep maxima of 3H-Thymidine uptake, and the presence of low-light adapted GSB species point towards a strong link of N, C, and S cycles in the redoxcline of Lake Kivu. [less ▲]

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See detailVertical distribution of ammonia-oxidizing crenarchaeota and methanogens in the epipelagic waters of Lake Kivu (Rwanda-Democratic Republic of the Congo)
Llirós, M.; Gich, F.; Plasencia, A. et al

in Applied and Environmental Microbiology (2010), 76(20), 6853-6863

Four stratified basins in Lake Kivu (Rwanda-Democratic Republic of theCongo) were sampled in March 2007 to investigate the abundance,distribution, and potential biogeochemical role of planktonic archaea ... [more ▼]

Four stratified basins in Lake Kivu (Rwanda-Democratic Republic of theCongo) were sampled in March 2007 to investigate the abundance,distribution, and potential biogeochemical role of planktonic archaea. We used fluorescence in situ hybridization with catalyzed-reported deposition microscopic counts (CARD-FISH), denaturing gradient gel electrophoresis (DGGE) fingerprinting, and quantitative PCR (qPCR) of signature genes for ammonia-oxidizingarchaea (16S rRNA for marine Crenarchaeota group 1.1a [MCG1] and ammonia monooxygenase subunit A [amoA]). Abundance of archaea ranged from 1 to 4.5% of total DAPI (4'6- diamidino-2-phenylindole) counts with maximal concentrations at theoxic-anoxic transition zone (∼50-m depth). Phylogenetic analysis of the archaeal planktonic community revealed a higher level of richnessof crenarchaeal 16S rRNA gene sequences (21 of the 28 operational taxonomic units [OTUs] identified [75%]) 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. © 2010, American Society for Microbiology. [less ▲]

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