Oceanic 18S rDNA sequences from picoplankton reveal unsuspected eukaryoticdiversity

Picoplankton-cells with a diameter of less than 3 mum-are the dominant cont ributors to both primary production and biomass in open oceanic regions(1,2 ). However, compared with the prokaryotes(3), the eukaryotic component of p icoplankton is still poorly known. Recent discoveries of new eukaryotic alg al taxa based on picoplankton cultures(4,5) suggest the existence of many u ndiscovered taxa. Conventional approaches based on phenotypic criteria have limitations in depicting picoplankton composition due to their tiny size a nd lack of distinctive taxonomic characters(6). Here we analyse, using an a pproach that has been very successful for prokaryotes(7) but has so far sel dom been applied to eukaryotes(8), 35 full sequences of the small-subunit ( 18S) ribosomal RNA gene derived from a picoplanktonic assemblage collected at a depth of 75 min the equatorial Pacific Ocean, and show that there is a high diversity of picoeukaryotes. Most of the sequences were previously un known but could still be assigned to important marine phyla including prasi nophytes, haptophytes, dinoflagellates, stramenopiles, choanoflagellates an d acantharians. We also found a novel lineage, closely related to dinoflage llates and not previously described.