Abstract
With increasing rates of greenhouse gas emissions, tropical coral reefs are suffering from increasingly frequent and severe episodes of temperature-induced coral bleaching. Algal symbionts in the family Symbiodiniaceae play a large part in shaping reef coral response to heat stress, and some symbionts, such as Durusdinium trenchii, can provide the coral host with an increased resistance to coral bleaching. This experiment analyzed the effect of the initial proportions of D. trenchii on the bleaching response of three coral species (Montastraea cavernosa, Orbicella faveolata, and Siderastrea siderea) to heat stress. A total of 60 replicate cores from 10 colonies of each of the three species were collected from Emerald Reef, Miami-Dade County, Florida, in the spring and fall of 2019, and their symbiont communities were analyzed to assess the initial proportions of D. trenchii. These cores were then exposed to 32°C for 25 days and bleaching response was measured as declines in symbiont to host cell ratio (using quantitative PCR) and photochemical efficiency (using chlorophyll fluorometry). Results indicated there were no significant differences in the initial proportions of D. trenchii between fall and spring in all species, but there was a significant difference in the initial proportions of D. trechii between M. cavernosa and both O. faveolata and S. siderea but not between the latter two species. Photochemical efficiency declined in response to heat stress in both fall and spring corals, and in corals that were dominated by Durusdinium as well as corals dominated by other symbiont genera. The dominant symbiont genus significantly influenced declines in symbiont abundance under heat stress, with Durusdinium having lower declines than Breviolum or Cladocopium. Furthermore, the initial proportion of D. trenchii significantly affected the maximum amount of heat stress needed to bleach corals. These results suggest that the initial proportions of D. trenchii may be a predictor of bleaching severity in mixed assemblages. This may indicate that D. trenchii provides greater thermotolerance, compared to other genera, but previous studies suggest that it may be a weak competitor. The benefits and trade-offs of Durusdinium necessitate research on the long-term implications of hosting these heat-tolerant symbionts.