Johan Decelle, Quentin Carradec, Xavier Pochon, Christian R. Voolstra, Patrick Wincker, Colomban de Vargas.
Decelle et al., “Worldwide Occurrence and Activity of the Reef-Building Coral Symbiont Symbiodinium in the Open Ocean”, Current Biology (2018).
The dinoflagellate microalga Symbiodinium sustains coral reefs, one of the most diverse ecosystems of the biosphere, through mutualistic endosymbioses with a wide diversity of benthic hosts . Despite its ecological and economic importance, the pres- ence of Symbiodinium in open oceanic waters re- mains unknown, which represents a significant knowledge gap to fully understand the eco-evolu- tionary trajectory and resilience of endangered Sym- biodinium-based symbioses. Here, we document the existence of Symbiodinium (i.e., the family Symbiodi- niaceae ) in tropical- and temperate-surface oceans using DNA and RNA metabarcoding of size- fractionated plankton samples collected at 109 stations across the globe. Symbiodinium from clades A and C were, by far, the most prevalent and widely distributed lineages (representing 0.1% of phytoplankton reads), while other lineages (clades B, D, E, F, and G) were present but rare. Concurrent metatranscriptomics analyses using the Tara Oceans gene catalog  revealed that Symbiodinium clades A and C were transcriptionally active in the open ocean and expressed core metabolic pathways (e.g., photosynthesis, carbon fixation, glycolysis, and ammonium uptake). Metabarcodes and ex- pressed genes of clades A and C were detected in small and large plankton size fractions, suggesting the existence of a free-living population and a symbi- otic lifestyle within planktonic hosts, respectively.
However, high-resolution genetic markers and mi- croscopy are required to confirm the life history of oceanic Symbiodinium. Overall, the previously un- known, metabolically active presence of Symbiodi- nium in oceanic waters opens up new avenues for investigating the potential of this oceanic reservoir to repopulate coral reefs following stress-induced bleaching.
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