Autotrophy versus heterotrophy: The origin of carbon determines its fate in a symbiotic sea anemone

Cnidarians – corals and their relatives – dominate the shallow, illuminated sea-floor in tropical nutrient-poor seas, mainly because their food sources are both heterotrophic, by predation, and autotrophic, from their intracellular symbiotic microalgae, the zooxanthellae. This trophic flexibility is rare in the animal kingdom, and the cellular usage and biological lifetime of the organic food molecules derived from both sources have never been investigated. Here we show that autotrophically-derived carbon is stored mostly in fatty tissue and is quickly consumed by the organism, while predation-derived carbon is incorporated throughout the animal’s body and remains there for a much longer time. We hypothesize that the simple photosynthate molecules are used primarily for immediate energetic processes like respiration, whereas the more complex heterotrophic organic molecules are utilized to build cellular structures such as proteins and membranes. Our study reveals, for the first time, the different fates and roles of autotrophy and heterothrophy in zooxanthellate coelenterates.