Abundance and diversity of prymnesiophytes in the picoplankton community from the equatorial Pacific Ocean inferred from 18S rDNA sequences

Picoplankton, i.e., cells smaller than 2-3 mu m, dominate in most open ocea nic regions, such as in the Pacific Ocean. In these areas, the dominant car otenoid of photosynthetic eukaryotes is 19'-hexanoyloxyfucoxanthin (19HF), considered to be a diagnostic marker for prymnesiophytes. This suggests tha t this class could be a major component of eukaryotic picoplankton. despite the fact that virtually no prymnesiophyte has been described to date from this size das;. To address this question, we assessed prymnesiophyte divers ity and abundance in natural picoplankton communities, using a molecular ap proach. Total genomic DNA was isolated from 3-mu m-filtered samples collect ed in the Pacific Ocean. Small subunit (18S) ribosomal RNA genes (rDNA) wer e amplified by the polymerase chain reaction (PCR) using universal eukaryot ic primers. The relative abundance of 18S rDNA from prymnesiophytes was qua ntified using group-specific and eukaryotic 18S rDNA probes. The percentage of the prymnesiophyte versus total 18S rDNA was much lower than the percen tage of prymnesiophytes calculated on the basis of pigment analyses for the same samples. 18S rDNA libraries from five samples were screened using a p rymnesiophyte-specific oligonucleotide probe, and 14 nearly complete 18S rD NA sequences were retrieved. Phylogenetic analysis of these sequences estab lished the presence of several prymnesiophyte lineages with no equivalent a mong cultivated species.