Thermally tolerant symbionts may explain Caribbean octocoral resilience to heat stress
Coral reef ecosystems are under threat from the frequent and severe impacts of anthropogenic climate change, particularly rising sea surface temperatures. The effects of thermal stress may be ameliorated by adaptation and/or acclimation of the host, symbiont, or holobiont (host + symbiont) to increased temperatures. We examined the role of the symbiont in promoting thermal tolerance of the holobiont, using Antillogorgia bipinnata (octocoral host) and Breviolum antillogorgium (symbiont) as a model system. We identified five distinct genotypes of B. antillogorgium from symbiont populations isolated from Antillogorgia colonies in the Florida Keys. Three symbiont genotypes were cultured and maintained at 26{degrees}C (ambient historical temperature) and two were cultured and maintained at 30{degrees}C (elevated historical temperature) for two years. Following culturing, we analyzed the growth rate and carrying capacity of each symbiont genotype at both ambient and elevated temperatures in culture (in vitro). All genotypes grew well at both temperatures, indicating thermal tolerance among these B. antillogorgium cultures. Prior culturing at the elevated temperature, however, did not result in increased thermal tolerance. We then inoculated juvenile A. bipinnata polyps with each of the five symbiont genotypes, and reared these polyps at both ambient and elevated temperatures (in hospite experiment). All genotypes were able to establish symbioses with polyps in both temperature treatments. Survivorship of polyps at 30{degrees}C was significantly lower than survivorship at 26{degrees}C, but all treatments had surviving polyps at 56 days post-infection, suggestive of broad thermal tolerance in B. antillogorgium, which may play a part in the increased resilience of Caribbean octocorals during heat stress events.