Identifying protist consumers of photosynthetic picoeukaryotes in the surface ocean using stable isotope probing

Special issue on Marine Microbial Ecology.-- 13 pages, 3 figures, 1 table, supporting information https://doi.org/10.1111/1462-2920.14018Photosynthetic picoeukaryotes contribute a significant fraction of primary production in the upper ocean. Micromonas pusilla is an ecologically relevant photosynthetic picoeukaryote, abundantly and widely distributed in marine waters. Grazing by protists may control the abundance of picoeukaryotes such as M. pusilla, but the diversity of the responsible grazers is poorly understood. To identify protists consuming photosynthetic picoeukaryotes in a productive North Pacific Ocean region, we amended seawater with living N, C-labelled M. pusilla cells in a 24-h replicated bottle experiment. DNA stable isotope probing, combined with high-throughput sequencing of V4 hypervariable regions from 18S rRNA gene amplicons (Tag-SIP), identified 19 operational taxonomic units (OTUs) of microbial eukaryotes that consumed M. pusilla. These OTUs were distantly related to cultured taxa within the dinoflagellates, ciliates, stramenopiles (MAST-1C and MAST-3 clades) and Telonema flagellates, thus, far known only from their environmental 18S rRNA gene sequences. Our discovery of eukaryotic prey consumption by MAST cells confirms that their trophic role in marine microbial food webs includes grazing upon picoeukaryotes. Our study provides new experimental evidence directly linking the genetic identity of diverse uncultivated microbial eukaryotes to the consumption of picoeukaryotic phytoplankton in the upper oceanThis research is funded in part by the Gordon and Betty Moore Foundation through grant GBMF3307 to P.J.K., T.A.R., A.Z.W and A.E.S. United States National Science Foundation award DBI-1318455 to A.E.S, and the David and Lucile Packard Foundation (to A.Z.W.)Peer Reviewe