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Symbiodinium symbioses in Hawaiʻi: Diversity, biogeography, and endemism
The Research Problem
Symbiodinium are small, unicellular, photosynthetic dinoflagellates that live symbiotically within the cells of other organisms, such as those of corals and marine invertebrates like jelly fish. Corals can only survive if they host the microscopic Symbiodinium in their tissues and because of the fundamental role they play in reef health, Symbiodinium are some of the most important and abundant organisms on coral reefs.
There are currently nine genetic groups of Symbiodinium (clades A-I). This high diversity translates to differences among Symbiodinium types that ultimately affects the performance and survival of their animal host. For example, the rates at which corals grow is affected by the type of Symbiodinium found in their tissues. Also, whether or not a coral bleaches when exposed to high seawater temperatures can be explained in part, by the type of Symbiodinium it associates with. Some Symbiodinium (clade D) make corals more tolerant of temperature stress while others (clade C) make the coral susceptible. Not surprisingly then, there is an increase in the abundance of clade D in corals on reefs that have been impacted by high temperature and other stressors. In Hawai‘i, and particularly in the Northwestern Hawaiian Islands, there is very little known about the diversity of Symbiodinium. One of our research goals is to investigate and discover the types of Symbiodinium present in Hawai‘i.
We use molecular tools to genotype Symbiodinium present in tissues of coral species and foraminiferans that are common on Hawaiian reefs. We then analyze the distribution of Symbiodinium over space (e.g. island and latitude) and in the context of past ocean temperatures (past twenty years), as well as in different coral species that are both healthy and diseased.
The diversity of Symbiodinium in corals across the Northwestern Hawaiian Islands is similar to other Pacific reefs; clade C seems to be the dominant symbiont in corals and clade F in foraminiferans. We have also identified new Symbiodinium diversity in Hawai‘i that has not been found elsewhere. The abundance of clade D in the Northwestern Hawaiian Islands is very low, which is expected because there is a low incidence of elevated temperature events in those waters compared to other reefs around the world. This contrasts with the Main Hawaiian Islands where the coral Montipora capitata (rice coral) associates with Symbiodinium clade C and D in Kāne‘ohe Bay and only clade C in the Northwestern Hawaiian Islands. This suggests that corals in Kāne‘ohe Bay are exposed to more stressful conditions that corals in the Northwestern Hawaiian Islands.
Diseased Acropora at French Frigate Shoals primarily associate with Symbiodinium belonging to clade A, while healthy Acropora associated with clade C. We explored this finding in more detail using biochemical approaches and discovered that clade A produces less food for the host to use than clade C, and the lack of nutrition explained the occurrence of disease in corals harboring clade A.
Due to the different Symbiodinium that affect host growth rate and tolerance to thermal stress, understanding how host-symbiont associations are distributed on different reefs and how past environmental conditions have shaped these distribution patterns is important to understanding and predicting how climate change will impact Hawaiian reef ecosystems.