Abstract
The dinoflagellate algal symbiont Durusdinium trenchii (Family Symbiodiniaceae) is known to confer greater thermal tolerance to its coral hosts than other common Caribbean algal symbionts and can increase in abundance on reefs following bleaching events. Adult corals can be induced to change the composition of their algal symbiont communities through controlled bleaching and recovery, but the stability and persistence of these new communities is not well understood. Recent studies have developed our understanding of the competitive dynamics that govern symbiont community shifts (and reversions) within coral hosts. This project aimed to further elucidate these interactions by tracking the spread (or loss) of different symbiont genera in colonies under natural reef conditions.
The first set of experiments, repeated in spring and in fall, tracked changes in symbiont communities as experimental cores collected from field colonies bleached and recovered under laboratory conditions, focusing on changes in dominance of D. trenchii. When comparing between species, a lack of initial D. trenchii was associated with a delay in symbiont ‘shuffling’ to D. trenchii in Montastraea cavernosa. Within Orbicella faveolata and Siderastrea siderea, a lack of initial D. trenchii was associated with reduced D. trenchii dominance after recovery. Initial symbiont communities were also found to shape bleaching responses between seasonal experiment batches, with M. cavernosa corals collected in the fall hosting higher symbiont densities and showing greater bleaching sensitivity.
In the second part of the project these manipulated cores (and associated controls), were grafted back into their original field colonies. As implants healed back into their respective colonies we tracked symbiont community changes in these implants and also in the surrounding coral tissue. Background D. trenchii spread into colonies to a distance of at least a 1.25cm from 2.5cm M. cavernosa implants over the course of 7 months, supporting the findings of previous laboratory-based experiments, and indicating that symbionts can be transferred between implant and parent colony in the field. This transfer was accelerated by, but not dependent on, the healing of implants, suggesting a role for both endogenous (gastrovascular transfer) and exogenous (seawater transfer) mechanisms, in the migration of symbionts through coral colonies. Due to the higher thermal tolerance of D. trenchii, this project is not only relevant to the competitive dynamics of symbionts in wild colonies but could also inform potential interventions for increasing the bleaching resistance of massive corals using tissue implants to introduce thermotolerant symbionts.