Pan-oceanic distribution of new highly diverse clades of deep-sea diplonemids

Molecular rRNA gene surveys reveal a considerable diversity of microbial eukaryotes in different environments. Even within a single clade, the number of distinct phylotypes retrieved often goes beyond previous expectations. Here, we have used specific 18S rRNA PCR primers to investigate the diversity of diplonemids, a poorly known group of flagellates with only a few described species. We analysed surface and deep-sea plankton samples from different oceanic regions, including the water-column in the Marmara Sea. We retrieved a large diversity of diplonemid phylotypes, most of which formed two novel distinct clades without cultured representatives. Although most marine diplonemid phylotypes appeared to be cosmopolitan, they showed a marked stratified distribution through the water column, being very scarce or absent in surface waters. The small and specific diplonemid diversity found in surface samples and the fact that most sequences of uncultured diplonemids found in other studies came from deep-sea environments suggest that the two major uncultured diplonemid clades group species preferentially inhabit the deep ocean.