Baker, A. C. | Search bioRxiv

bioRxiv2020

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.

ecology↗

bioRxiv2020

Host genotype and stable differences in algal symbiont communities explain patterns of thermal stress response of Montipora capitata following thermal pre-exposure and across multiple bleaching events

As sea surface temperatures increase worldwide due to climate change, coral bleaching events are becoming more frequent and severe, resulting in reef degradation. Leveraging the inherent ability of reef-building corals to acclimatize to thermal stress via pre-exposure to protective temperature treatments may become an important tool in improving the resilience of coral reefs to rapid environmental change. We investigated whether historical bleaching phenotype, coral host genotype, and exposure to protective temperature treatments would affect the response of the Hawaiian coral Montipora capitata to natural thermal stress. Fragments were collected from colonies that demonstrated different bleaching responses during the 2014-2015 event in K[a]neohe Bay (Oahu, Hawaii) and exposed to four different artificial temperature pre-treatments (and a control at ambient temperature). After recovery, fragments experienced a natural thermal stress event either in laboratory conditions or their native reef environment. Response to thermal stress was quantified by measuring changes in the algal symbionts photochemical efficiency, community composition, and relative density. Historical bleaching phenotype was reflected in stable differences in symbiont community composition, with historically bleached corals containing only Cladocopium symbionts and historically non-bleached corals having mixed symbiont communities dominated by Durusdinium. Mixed-community corals lost more Cladocopium than Cladocopium-only corals during the natural thermal stress event, and preferentially recovered with Durusdinium. Laboratory pre-treatments exposed corals to more thermal stress than anticipated, causing photochemical damage that varied significantly by genotype. While none of the treatments had a protective effect, temperature variation during treatments had a significant detrimental effect on photochemical efficiency during the thermal stress event. These results show that acclimatization potential is affected by fine-scale differences in temperature regime, host genotype, and relatively stable differences in symbiont community composition that underpin historical bleaching phenotypes in M. capitata.

ecology↗