Diversity in populations of free-living Symbiodinium from a Caribbean and Pacific reef

The presence and diversity of free-living dinoflagellates belonging to the endosymbiotic genus Symbiodinium were explored in seawater samples collected above coral reefs in Kāne‘ohe Bay, O‘ahu, Hawai‘i, and Puerto Morelos, Quintana Roo, Mexico. Two genetic markers were used to assess Symbiodinium diversity in the water column: the internal transcribed spacer 2 region of the nuclear ribosomal array (ITS2), and a hypervariable region in domain V of the large subunit (23S) of the chloroplast ribosomal array (cp23S-HVR). Sequencing of cloned gene fragments reveals that clades B, C, D, and H Symbiodinium are detectable in the seawater samples. In addition to the previously described types B1, C3, C15, C21, and D1, novel Symbiodinium sequences belonging to clades B and C were also retrieved. The majority of Symbiodinium sequences recovered from Kāne‘ohe Bay belonged to clade C and those from Puerto Morelos to clade B, a pattern that reflects the dominant types of Symbiodinium found in endosymbiosis with scleractinian corals in these two areas. This study represents the first direct assessment of Symbiodinium diversity in waters adjacent to coral reefs located in the Caribbean and the Pacific and confirms the presence of Symbiodinium in this compartment of the ecosystem. These data provide context for future studies examining spatial and temporal patterns in the availability of Symbiodinium in the water column, work that will ultimately promote a greater understanding of the interactions between symbiotic dinoflagellates and their environmentally sensitive benthic hosts. Marine invertebrates representing at least five phyla form symbiotic relationships with endosymbiotic dinoflagellates belonging to the genus Symbiodinium (Stat et al. 2006). As a result of this close coupling of primary producers and consumers, coral reef ecosystems maintain high levels of productivity despite being surrounded by relatively oligotrophic waters (Hoegh-Guldberg 1999). Scleractinian corals form the structural framework of coral reef ecosystems and are the most ecologically important and best-studied examples of these symbioses. The respiratory requirements of the coral host are almost exclusively met by the translocation of photosynthetic materials provided by the endosymbionts and are thus considered to be in obligate symbiosis with Symbiodinium (Muscatine and Porter 1977). Therefore, reductions in chlorophyll concentrations per symbiont cell or a loss of resident endosymbionts (or both) results in decreased fitness and possible mortality for the host. This condition, which occurs in response to a variety of environmental stressors, causes the external coloration of the host to appear pale and is commonly described as coral bleaching (HoeghGuldberg and Smith 1989). Recovery from bleaching is possible if symbiont populations re-establish in the host in