Archaeal dominance in the mesopelagic zone of the Pacific Ocean

The ocean's interior is Earth's largest biome. Recently, cultivation-independent ribosomal RNA gene surveys have indicated a potential importance for archaea 1 in the subsurface ocean 2 , 3 , 4 . But quantitative data on the abundance of specific microbial groups in the deep sea are lacking 5 , 6 . Here we report a year-long study of the abundance of two specific archaeal groups (pelagic euryarchaeota and pelagic crenarchaeota) 2 in one of the ocean's largest habitats. Monthly sampling was conducted throughout the water column (surface to 4,750 m) at the Hawai'i Ocean Time-series station 7 . Below the euphotic zone (> 150 m), pelagic crenarchaeota comprised a large fraction of total marine picoplankton, equivalent in cell numbers to bacteria at depths greater than 1,000 m. The fraction of crenarchaeota increased with depth, reaching 39% of total DNA-containing picoplankton detected. The average sum of archaea plus bacteria detected by rRNA-targeted fluorescent probes ranged from 63 to 90% of total cell numbers at all depths throughout our survey. The high proportion of cells containing significant amounts of rRNA suggests that most pelagic deep-sea microorganisms are metabolically active. Furthermore, our results suggest that the global oceans harbour approximately 1.3 × 10 28 archaeal cells, and 3.1 × 10 28 bacterial cells. Our data suggest that pelagic crenarchaeota represent one of the ocean's single most abundant cell types.