Unexpected diversity of small eukaryotes in deep-sea Antarctic plankton

Phylogenetic information from ribosomal RNA genes directly amplified from t he environment changed our view of the biosphere, revealing an extraordinar y diversity of previously undetected prokaryotic lineages. Using ribosomal RNA genes from marine picoplankton, several new groups of bacteria and arch aea have been identified, some of which are abundant2-4. Little is known, h owever, about the diversity of the smallest planktonic eukaryotes, and avai lable information in general concerns the phytoplankton of the euphotic reg ion. Here we recover eukaryotes in the size fraction 0.2-5 mum from the aph otic zone (250-3,000 m deep) in the Antarctic polar front. The most diverse and relatively abundant were two new groups of alveolate sequences, relate d to dinoflagellates that are found at all studied depths. These may be imp ortant components of the microbial community in the deep ocean. Their phylo genetic position suggests a radiation early in the evolution of alveolates.