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

Intervention strategies to enhance coral resilience include manipulating associations with algal endosymbionts. While hosting thermotolerant Durusdinium trenchii can increase bleaching thresholds in Caribbean coral adults, its effects remain largely unknown during their early life stages. 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 Montastraea cavernosa (containing Cladocopium ITS2 type C3) 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 symbiont identity. 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. 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. Furthermore, 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.

Automated summary

The study found that coral recruits can acquire symbionts from nearby corals of different species, and early exposure to high temperature can enhance the resilience of coral recruits to future thermal stress. However, hosting more D. trenchii symbionts may affect coral growth, indicating a trade-off between growth and heat tolerance. Practitioners will need to carefully weigh the benefits and costs of using donor colonies to seed coral recruits with thermotolerant symbionts in restoration strategies.