Vertical distribution and phylogenetic composition of bacteria in the Eastern Tropical North Pacific Ocean

The vertical community structure of bacteria along a depth profile in the Eastern Tropical North Pacific Ocean (13 degrees N, 104 degrees W) was studied by flow cytometry measurement and 16S rRNA gene clone libraries analysis. Picoeukaryotes and Synechococcus peaked at 30 m and decreased sharply below 50 m, while Prochlorococcus peaked at both 30 and 100 m layers and disappeared below 200 m. Heterotrophic bacteria peaked above shallow thermocline and decreased along the depth profile. Sequences of total 322 clones from four clone libraries (10, 100, 1000, and 3000 m) clustered into nine major lineages. gamma-Proteobacteria dominated all the depths and occupied almost the whole bacterial community at the 3000 m. alpha-Proteobacteria was abundant throughout the water column except near the sea bottom, and delta-Proteobacteria peaked at the 1000 m depth. Cyanobacteria were primarily limited to the photic zone, and the genetic diversity of Prochlorococcus showed a good correlation with niche adaptation. The appearance of the Cytophaga-Flexibacter-Bacteroides (CFB) group did not show a clear relationship with depth. Actinobacteria were found both in the photic zone and in deep water. Planctomyetes, Acidobacteria, and Verrucomicrobia were present as minor groups and more dominant in the deeper layers of water.