Corals: Symbiosis, Photosynthesis and Bleaching

The mutualistic symbiosis between the dinoflagellate Symbiodinium and its cnidarian hosts provides the metabolic foundation for biodiverse coral-reef ecosystems. Symbiodinium provides the cnidarians with the majority of its fixed carbon, while the animal may allocate nutrients (e.g. nitrogen and phosphorus) to the alga and offer the alga a safe haven. However, many details of nutrient exchange between the host and alga and the ways in which the cnidarian recognizes, associates with and maintains its intracellular endosymbiont remain poorly understood.

We are working on a proxy system for studying the coral association using  the dinoflagellate Symbiodinium and the sea anemone Aiptasia pallida. To fully develop this model, we isolated clonal, axenic strains of various Symbiodinium clades (historically called clades but they may represent separate species and maybe even different genera) and studied their growth and host specificity. Work on these strains was published in The Journal of Phycology in 2013, Current Biology in 2013 and The Plant Journal in 2015.

                                                        Aiptasia harboring its algal endosymbiont
Courtesy of Cory Krediet

An axenic culture of Symbiodinium SSB01


Courtesy of Tingting Xiang 

One of the Symbiodinium strains used for these studies has been designated Symbiodinium SSB01. Surprisingly glucose induced cultured Symbiodinium SSB01 to bleach, with a loss of both pigmentation and photosynthetic activity. The cells can regain their pigmentation, photosynthetic activity and the capacity to grow photoautotrophically when glucose is eliminated from the medium. We are currently using RNA-seq to characterize global gene expression in Symbiodinium SSB01 during both the bleaching and recovery processes. Our studies are providing a reference analysis for elucidating plastid biogenesis and pathways for heterotrophic and photoautotrophic metabolism in dinoflagellates; the work is also generating insights into mechanisms associated with coral bleaching.

We also use biophysical methods to examine the photosynthetic activities of various Symbiodinium isolates, both those that grow photoautotrophically and don't infect Aiptasia, and those that can grow both as free living organisms and as endosymbionts. Our initial studies suggest that the animal represents a strong sink for reductant (in the form of fixed carbon) generated by the alga, as expected, and that the host has control over allocation of nutrients, such as nitrogen, to the endosymbiont.