Enhanced photoprotection pathways in symbiotic dinoflagellates of shallow-water corals and other cnidarians
Reynolds, Jennifer Carrye McCabe
MetadataShow full item record
Endosymbiotic dinoflagellates of the genus Symbiodinium provide their coral hosts up to 100% of their energy requirements for reef building. Symbiodinium is a symbiont common to many cnidarians including corals, jellyfish, and anemones, as well as mollusk species including giant clams. Here, we provide evidence for the first time using the novel serial irradiation pulse (SIP) chlorophyll fluorescence technique that most members of clade A Symbiodinium, but not clades B–D or F, exhibit enhanced capabilities for alternative photosynthetic electron transport pathways including cyclic electron transport. Unlike other clades, clade A Symbiodinium engage cyclic electron transport and regularly undergo pronounced light-induced dissociation of antenna complexes from photosystem II (PSII) reaction centers. These photoprotections promote the survival of cnidarians with clade A Symbiodinium at high irradiance intensities and warm ocean temperatures, conditions that commonly cause coral bleaching. Laboratory experiments using cultured Symbiodinium investigated the effects of long-term warm water temperatures on the ability to utilize the photoprotections unique to clade A Symbiodinium. Clade A Symbiodinium yielded the lowest amounts of the integral D1 protein, a common site of photosynthetic damage, but showed recovery when temperatures were reduced. In addition, field studies were conducted to determine the distinct physiological responses of Symbiodinium to long-term, seasonal ocean temperature changes. During all field seasons and in each colony harboring clade A Symbiodinium, cyclic electron transport was actively engaged. As a result, these corals maintained the highest maximum quantum yield and chlorophyll-a densities even during high irradiance and warm ocean temperatures. Cyclic electron transport and light harvest complex dissociation are unique photoprotections that confer resistance to bleaching conditions that conspicuously impacted corals in symbiosis with non-clade A Symbiodinium. Such photoprotections are now easily identified through the use of the non-invasive SIP chlorophyll fluorescence technique. The results presented here show that clade A Symbiodinium may be better suited to withstand high irradiance and warm ocean temperatures, environmental conditions that may enhance coral bleaching.