Abstract Summary
There is increasing interest in developing interventions that enhance thermotolerance of restored corals in anticipation of continued climate change. One such method involves controlled bleaching and recovery to manipulate corals algal symbionts in favor of more thermotolerant types. Data from natural bleaching events suggest that disturbed symbiont communities return to their original symbionts over months-years, but the factors influencing the long-term persistence of these symbionts are not well understood. Using real-time PCR, we assessed the longevity of symbiont communities dominated by thermotolerant Symbiodinium trenchii (initially type D1a) in 24 replicate cores from each of 6 colonies of the massive starlet coral (Siderastrea siderea) collected from Emerald Reef, FL following a natural bleaching event. We monitored the symbiont community for three years, including an initial period at control temperatures (7 months at 25-28°C), followed by a thermal challenge applied to a subset of cores (4 weeks at 32-33°C), recovery at control temperatures (1 year), a second thermal challenge applied to all cores (4 weeks at 31-32°C) and a final recovery period (1 year). We found that, under control temperatures, corals containing >1% Symbiodinium clade C rapidly became dominated by these clade (< 4 months). However, corals containing < 1% clade C remained dominated by S. trenchii for >1 year and bleached less severely (measured as declines in symbiont abundance and function [Fv/Fm]) when exposed to heat stress. During both bleaching events, cores with mixed communities experienced shifts in favor of S. trenchii dominance, and cores already dominated by S. trenchii became virtually exclusive on S. trenchii, thereby increasing the long-term persistence of these thermotolerant symbionts. These data suggest that the longevity of S. trenchii communities increases if competing symbionts are reduced below 1%, which may be unlikely to occur across entire colonies because sensitive symbionts likely persist in coral microhabitats (e.g., shaded vertical colony edges) during bleaching events. Instead, these dynamics indicate that the symbiont communities of S. siderea likely persist long-term as complex mosaics whose specific composition in any particular polyp is governed by its history of disturbance and that of its neighbors.
