Vertical distribution of bacteria and archaea in a CO₂-rich meromictic lake: A case study of Lake Monoun

In order to assess the diversity and spatial distribution of bacterial and archaeal communities in a CO₂-rich and meromictic lake, samples were collected along the water column of Lake Monoun and analyzed using PCR-DGGE and quantitative analyses of the 16S rRNA gene. The retrieved sequences were affiliated to 6 bacterial phyla and two archaeal phyla which plausible environmental functions map with the physico-chemical parameters of the lake. Unclassified sequences were also detected. This suggests heterogeneity in community composition and existence of potential candidate divisions. For instance, amongst the bacterial sequences, 18.2% matched with methanotrophic bacteria of the order methylococcales and amongst the archaeal sequences, 16.6% matched with methanogenic species of the order methanomicrobiales. Hence, evidencing the existence of methane-related prokaryotes in the lake, a finding that would play a key role in our understanding of the methane puzzle of Lake Monoun. Other groups capable of a wide range of metal and nutrients transformations were also detected, as well as those of unknown functions. The layering of microbial communities also appeared to directly or indirectly depend on oxygen availability. DGGE and qPCR analyses both suggested a scarcity of archaea in the surface samples. Furthermore, qPCR revealed that bacteria were numerically more important than archaea in all the samples. The general distribution along the water column indicated that archaeal abundance increases with depth.