Symbiont type influences trophic plasticity of a model cnidarian-dinoflagellate symbiosis.

The association between cnidarians and photosynthetic dinoflagellates within the genus Symbiodinium is a prevalent relationship in tropical and subtropical marine environments. Although the diversity of Symbiodinium provides a possible axis for niche diversification, increased functional range and resilience to physical stressors such as elevated temperature, how such diversity relates to the physiological balance between autotrophy and heterotrophy of the host animal remains unknown. Here, we experimentally show interspecific and intraspecific variability of photosynthetic carbon fixation and subsequent translocation by Symbiodinium to the model cnidarian host Aiptasia pallida. By using a clonal anemone line harboring different species of Symbiodinium, we determined that symbiont identity influences trophic plasticity through its density, capacity to fix carbon, quantity of translocated carbon and ultimately the host's capacity to ingest and digest prey. Symbiont carbon translocation and host prey ingestion were positively correlated across symbiont combinations that consisted of different isoclonal lines of Symbiodinium minutum, while a combination with type D4-5 Symbiodinium displayed lower carbon translocation, and prey capture and digestion more similar to Aiptasia lacking symbionts. The absence of a shift toward greater heterotrophy when carbon translocation is low suggests that the metabolic demand of feeding and digestion may overwhelm nutritional stores when photosynthesis is reduced, and amends the possible role of animal feeding in resistance to or recovery from the effects of climate change in more obligate symbioses such as reef-building corals.