[show abstract][hide abstract] ABSTRACT: The Clipperton lagoon in the North Pacific Ocean has been isolated from the surrounding sea for c. 160 years. It has a stratified water column that comprises an oxic and brackish upper water layer (mixolimnion) and a deep sulfuric anoxic saline layer (monimolimnion), separated by a steep pycnocline. Here, we test whether the Clipperton lagoon with its distinctive physico-chemical features, geographic isolation, recent water column stratification, and large nutrient input harbors original microbial communities. The combination of capillary electrophoresis single-strand polymorphism (CE-SSCP) fingerprinting and sequencing of cloned bacterial and archaeal 16S rRNA genes, and functional genes for methanogenesis (mcrA), methanotrophy (pmoA), and sulfate reduction (dsrAB), revealed that microbial communities and pathways were highly stratified down the water column. The mixolimnion contained ubiquitous freshwater clades of Alpha- and Betaproteobacteria, while the pycnocline contained mostly green sulfur bacteria (phylum Chlorobi). Sequences of the upper layers were closely related to sequences found in other aquatic ecosystems, suggesting that they have a strong potential for dispersal and colonization. In contrast, the monimolimnion contained new deeply branching bacterial divisions within the OP11 cluster and the Bacteroidetes, and was the most diverse of the layers. The unique environmental conditions characterizing the deep layers of the lagoon may explain the novelty of the microbial communities found at the Clipperton atoll.