Meeting Abstract
Corals that take-up algal symbionts from the environment anew each generation (horizontal transmitters) may have a winnowing period during which specific symbiont associations become established from initially diverse infections, and this process could be modulated by the environment. Acropora tenuis, a common horizontal transmitting coral on the Great Barrier Reef, associates with a stress-sensitive species of Cladocopium and a stress-resistant species of Durusdinium as adults. We examined infection of A. tenuis larvae with four genera of Symbiodiniaceae with different thermal maxima (Cladocopium, Durusdinium, Fugacium, and Gerakladium) over two weeks at three different temperatures (27C, 30C, & 31C). Larvae were exposed to a single genus or an assortment of all four and individual larvae were flash frozen at days 3, 7 and 14. The type and number of symbionts/larva was measured in individual larvae by digital droplet PCR. All four genera were successful at initially infecting larvae at all temperatures in single infections and mixed infections. At 27C, Durusdinium was the dominant symbiont in 68% of larvae at day 3, and 79% at day 14. However, at 30C and 31C, larvae were dominated by either Gerakladium or Durusdinium at approximately equal frequency (approx. 40-43% each) by the end of the trials. Fugacium, which has the highest thermal maximum in culture, did not proliferate within the larvae at any temperature. If sea surface temperatures continue to warm during the onset of symbiosis, symbionts that were relatively rare or absent in A. tenuis adults, like Gerakladium, may begin to compete, which could have implications for holobiont performance under thermal stress.
