Neighboring colonies influence uptake of thermotolerant endosymbionts in threatened Caribbean coral recruits

Intervention strategies to enhance coral resilience include manipulating the association between corals and their algal endosymbionts. While hosting thermotolerant Durusdinium trenchii can increase bleaching thresholds in adults, its effects remain largely unknown during the early life stages of Caribbean corals. Here, we tested if Orbicella faveolata recruits could establish symbiosis with D. trenchii supplied by nearby "donor" colonies and examined the resulting ecological trade-offs to evaluate early Symbiodiniaceae manipulation as a scalable tool for reef restoration. We exposed aposymbiotic recruits to 29{degrees}C or 31{degrees}C and to fragments of either Montastraea cavernosa (containing Cladocopium) or Siderastrea siderea (containing D. trenchii). After 60 days, recruits reared with D. trenchii donors hosted nearly three times more D. trenchii than those with Cladocopium donors, suggesting that recruits can acquire Symbiodiniaceae from nearby corals of different species. Temperature did not affect D. trenchii uptake. Next, donor colonies were removed and surviving recruits were maintained for three months at ambient temperatures, after which a subset was exposed to a 60-day heat stress trial. Recruits previously reared at 31{degrees}C survived twice as long at 34{degrees}C as those reared at 29{degrees}C, suggesting that pre-exposure to heat can prime recruits to withstand future thermal stress. In addition, recruits hosting primarily D. trenchii survived twice as long at 34{degrees}C as those hosting little or no D. trenchii. However, the proportion of D. trenchii hosted was negatively correlated with polyp size and symbiont density, indicating a trade-off between growth (of both host and symbiont) and heat tolerance. These findings suggest that, while donor colonies may be effective sources for seeding coral recruits with thermotolerant symbionts, practitioners will need to balance the likely benefits and costs of these approaches when designing restoration strategies.