Abstract
Coral reef ecosystems are highly biodiverse and hold great ecological and economic value. However, these ecosystems are under threat of local and regional extinction as the scleractinian corals that build coral reefs are declining due to increased sea surface temperatures. This has led coral managers and scientists to explore intervention methods that can complement the ongoing restoration activities to boost climate resilience. One potential intervention is the application of sublethal doses of bleaching stress to induce compensatory responses in reef corals during recovery that would increase their resistance to subsequent thermal stress (“stress hardening”). Here we trialed different methods for stress-hardening corals as part of a restoration program in Miami, FL, and used quantitative PCR (qPCR) and chlorophyll fluorometry (I-PAM) to assess changes in the structure and function of algal symbiont communities (Symbiodiniaceae spp.). We tested the effect of high irradiance and the herbicide Diuron (DCMU: 3-(3,4-dichlorophenyl)-1,1-dimethylurea) on the bleaching and recovery of 10 genotypes of the threatened Caribbean staghorn coral (Acropora cervicornis) and multiple non-acroporids. For high light stress, we exposed corals to ~2100 umol m-2 sec-1 which decreased photochemical efficiency (Fv/Fm) and resulted in visible bleaching within 3 weeks. These corals were then allowed to recover in the field and further changes monitored using qPCR. This experiment was repeated in the field by raising coral colonies to 1ft below the water’s surface on a man-made raft structure. We also tested the effect of short-term exposure (1h, 2h, 5h, 24hr, 48hr, 72hr and 5 days) to three concentrations of DCMU (0.45mg/L, 4.5mg/L and 41.85mg/L) on coral bleaching and recovery. All methods used were effective in bleaching coral colonies of staghorn, evidenced through qPCR and I-PAM measurements, but there were no detectable changes in the symbiont type to more thermally tolerant types (Durusdinium). Two genotypes, Cheetos A and Cheetos D, exhibited increased resistance to high light stress and may serve to increase the success of local restoration activities during acute stress events. Non-acroporid species used in this study were found to contain Durusdinium prior to treatment and no significant bleaching was observed in any treatment type but this suggests that wild colonies may already be shifting to Durusdinium-dominance. This study shows the significance of “fast-fail” approaches to evaluate potential field-based techniques for stress hardening coral colonies. This will become increasingly important as climate change continues to drive coral declines globally and scientists search for ways to improve coral resilience.