FURLA, P.*; SHICK, J.M.; ALLEMAND, D.; ZOCCOLA, D.; Univ. of Nice, France; Univ. of Maine, Orono; Univ. of Nice, France; Centre Scientifique de Monaco: The symbiotic cnidarian: a physiological chimera of alga and animal

Cnidarians (corals and sea anemones) harboring photosynthetic algae (dinoflagellates and chlorophytes) derive several benefits from the association. Nevertheless, phototrophic symbiosis imposes evolutionary pressures more relevant to photosynthetic organisms than to metazoans. E.g., such symbiosis modifies the metabolism of the host animal to provide its endosymbionts with nutrients such as CO₂ to ensure optimal photosynthesis, so that host oral tissues possess a carbon-concentrating mechanism to supply the symbionts� type II Rubisco with a high concentration of CO₂. Furthermore, living in symbiosis requires that the host expose itself to potentially detrimental environmental factors normally avoided by lower metazoans, such as bright sunlight (including UVR) and hyperoxia. As a consequence of the endosymbiont�s photosynthesis, a large amount of O₂ is produced in the animal�s cells, leading to a diurnal transition between light hyperoxia and dark hypoxia. To cope with the potential photo-oxidative stress, hosts possess an array of superoxide dismutase (SOD) enzymes, some having unusual characteristics. Expression of these enzymes appears to be regulated by symbiotic interactions. To ameliorate the effects of solar UVR while still allowing photosynthetically available radiation to reach the algae, symbiotic cnidarians concentrate ectodermally an array of colorless UV sunscreens, the mycosporine-like amino acids (MAAs). These are initially algal products that may be converted by the host to a suite of secondary MAAs differing chemically from the acidic algal MAAs and their precursors, some of which are also antioxidants. Aspects of these symbiosis-dependent host adaptations will be discussed in the light of a parallel evolution of plants and symbiotic cnidarians.