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Distribution and abundance of uncultured heterotrophic flagellates in the world oceans

Authors

  • Ramon Massana,

    Corresponding author
    1. Department of Marine Biology and Oceanography, Institut de Ciències del Mar, CSIC, Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Catalonia, Spain.
      *E-mail ramonm@icm.csic.es; Tel. (+34) 93 2309500; Fax (+34) 93 2309555.
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  • Ramon Terrado,

    1. Départment de Biologie, Université Laval, Québec City, Québec G1K 7P4, Canada.
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  • Irene Forn,

    1. Department of Marine Biology and Oceanography, Institut de Ciències del Mar, CSIC, Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Catalonia, Spain.
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  • Connie Lovejoy,

    1. Départment de Biologie, Université Laval, Québec City, Québec G1K 7P4, Canada.
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  • Carlos Pedrós-Alió

    1. Department of Marine Biology and Oceanography, Institut de Ciències del Mar, CSIC, Passeig Marítim de la Barceloneta 37-49, 08003 Barcelona, Catalonia, Spain.
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*E-mail ramonm@icm.csic.es; Tel. (+34) 93 2309500; Fax (+34) 93 2309555.

Summary

Heterotrophic flagellates play fundamental roles in marine ecosystems as picoplankton grazers. This recognized importance contrasts with our ignorance of the taxonomic composition of this functional group, which remains mostly unidentified by microscopical and culturing approaches. Recent molecular marine surveys based on 18S rDNA genes have retrieved many sequences unrelated to cultured organisms and marine stramenopiles were among the first reported uncultured eukaryotes. However, little is known about the organisms corresponding to these sequences. Here we determine the abundance of several marine stramenopile lineages in surface marine waters using molecular probes and fluorescent in situ hybridization. We show that these protists are free-living bacterivorous heterotrophic flagellates. They were widely distributed, occurring in the five world oceans, and accounted for a significant fraction (up to 35%) of heterotrophic flagellates in diverse geographic regions. A single group, MAST-4, represented 9% of cells within this functional assemblage, with the intriguing exception of polar waters where it was absent. MAST-4 cells likely contribute substantially to picoplankton grazing and nutrient re-mineralization in vast areas of the oceans and represent a key eukaryotic group in marine food webs.

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