Photosymbiosis in marine planktonic protists

Abstract

Some of the most enigmatic components in the plankton are the diverse eukaryotic protists that live in close association with one or more partners. Mutualistic and commensal planktonic interactions are most commonly encountered in the oligotrophic open ocean at tropical and subtropical latitudes. They are functionally and ecologically distinct, and involve a great taxonomic diversity of single-celled partners. Protists like Foraminifera, Radiolaria, dinoflagellates and diatoms can all harbor microalgal symbionts of eukaryotic and prokaryotic origin inside (endosymbiosis) and/or outside (ectosymbiosis) their cytoplasm. Such symbioses (photosymbioses) combine phototrophy, heterotrophy and sometimes dinitrogen (N2 ) fixation (reduction of N2 to ammonium). Symbiotic microorganisms therefore represent an important component of marine ecosystems and play a role in the food web and biogeochemical cycling (e.g., carbon and nitrogen). Despite their important ecological function and early recognition in the late nineteenth century, our knowledge about the diversity, distribution, and metabolic exchanges for many of the photosymbioses remains rudimentary compared with the other marine and terrestrial symbioses. Recent technical advances in Single-Cell genomics and imaging have greatly improved our understanding about planktonic symbioses. This review aims to present and compare many Eukaryote-Eukaryote and Eukaryote-Prokaryote photosymbioses described so far in the open ocean with an emphasis on their ecology and potential function in the ecosystem.