Combined effects of thermal stress and symbiont identity on carbon flux in a model cnidarian-dinoflagellate symbiosis

Reef corals rely on their symbiotic relationship with dinoflagellates of the family Symbiodiniaceae for their metabolic energy demands, but this relationship is sensitive to thermal stress which can induce bleaching. We investigated the flux of 14C-labeled photosynthate in the sea anemone Exaiptasia diaphana ('Aiptasia') - a model for the cnidarian-dinoflagellate symbiosis - under low, control and high temperatures (15, 25, 33 °C) when colonized by either its native symbiont Breviolum minutum or the non-native but thermally tolerant Durusdinium trenchii or Breviolum psygmophilum. B. psygmophilum formed the most thermally stable symbiosis, maintaining a higher population density, and fixing and translocating similar or more carbon than the other two species at both high and low temperatures. In contrast, anemones containing D. trenchii markedly bleached at both high and low temperatures, confirming the sensitivity of this host-symbiont pairing to thermal extremes. Elevated temperature led to a significant increase in the percentage of fixed carbon (%) translocated from the symbiont to the host with both Breviolum species, however the total amount of carbon released to the host by the symbiont population (translocated C per mg of host protein) remained consistent between elevated and control temperatures due to the reduced rate of total photosynthetic carbon fixation. The highest rates of photosynthesis and carbon translocation by the symbiont population were seen at the low temperature, reflecting the maintenance or elevation of photosynthesis and/or symbiont density relative to the control. Overall, our findings underscore the significance of symbiont identity in conjunction with host-symbiont specificity in dictating the thermal resilience of the cnidarian-dinoflagellate symbiosis and emphasizing the potential of B. psygmophilum as an interesting candidate for future studies.