Photoacclimation responses of a symbiotic sea anemone reveal an important host cellular plasticity

The high productivity of coral reef ecosystems is largely attributed to the mutualistic symbiosis between reef-building corals and their intracellular dinoflagellate in the genus Symbiodinium commonly referred to as zooxanthellae. These photosynthetic algae translocate a majority of their photosynthetically fixed carbon to the host and contribute to their metabolic needs and the calcification process. <i>Symbiodinium</i> must maintain a balance between the energy derived from the light reactions in the chloroplast and the amount of energy used during dark reactions and other metabolic processes. Nevertheless, in the natural environment the holobiont have to cope with daily and seasonal changes in light intensity, upsetting that balance and creating a stress that induces a physiological response (photoacclimation) to optimize growth rates.
After a ten day exposition to high and very low light intensity, morphological and photophysiological analysis conducted on the symbiotic sea anemone, Anemonia manjano, reveal significant modifications of the host tissues ultrastructure and the Symbiodinium metabolic processes (photosynthesis, respiration). Those results highlight particularly important gastrodermal and ectodermal plasticity in which symbiotic cnidarians acclimate to the Symbiodinium physiological status (mainly photosynthesis) by varying the density of particular cellular types (e.g.: cnidocytes, gastrodermal cells) contained in their tissues.