Mutualistic Relationship between Corals and Dinoflagellate Endosymbionts

The corals are critical for stabilization of land masses and for sustaining oceanic ecosystems that harbor a diversity of marine organisms. They are comprised of two partner organisms; an animal (cnidarian) and a unicellular alga (dinoflagellate in the genus Symbiodinium). The partner organisms establish an endosymbiotic association in which both experience specific benefits: the unicellular alga provides the animal host with fixed carbon, satisfying its need for energy and reductant, while the alga receives various nutrients from the host (e.g. nitrogen, phosphorus and sulfur) and is also maintained in a protected environment that cannot be easily accessed by predators. Unfortunately, the activities of man have elicited rapid changes in all habitats of the Earth. In particular, the burning of fossil fuels has caused the levels of atmospheric CO2 to rise, which in turn has led to rising atmospheric, terrestrial and aquatic temperatures, acidification of the oceans, increased melting of glaciers and the flow of freshwater into the oceans (which is reducing ocean salinity). These changes in the environment have led to degradation of the coral reefs in nearly all parts of the world.

Despite the prevalence and ecological importance of coral reef ecosystems, many features of the symbiotic associations that are critical for reef survival are not well understood. For example, we don’t know the molecules involved in initiation and maintenance of the coral association or the ways in which the metabolisms of the two organisms are modulated by the association (e.g. as a free-living organism the algae retains most of its fixed carbon, while in the symbiotic association most of the fixed carbon is passed to the host). We also know little about the ways in which corals respond to changing environmental conditions and the precise reasons for the death of the reefs as the temperature of the oceans rise.

Our studies of the coral mutualism, performed in collaboration with the laboratories of John Pringle and Steve Palumbi, have exploited both axenic Symbiodinium cultures, field samples and the sea anemone Aiptasia (a cnidarian that lives in a mutualistic association with Symbiodinium, like corals), which serves as a proxy for the coral system. Various cultured clades of Symbiodinium have been introduced into a clonal population of the sea anemone Aiptasia and the holobionts are characterized with respect to viability and photosynthetic activities under various environmental conditions. We are currently exploring how these associations respond to changing light, nutrient and temperature conditions with much of our current expression data using the cultured Symbiodinium available to download. Additional analysis of Symbiodinium transcripts can be found in the database Sym6