Abstract Summary
Stress hardening, a technique using controlled stress to induce net positive physiological change, has been proposed as a scalable tool to improve the heat tolerance of nursery-grown corals through bleaching and recovery. Public-facing experiments in the Inventors in Residence Lab at the Phillip and Patricia Frost Museum of Science in Miami, FL, are testing various stress hardening methods using nursery-grown Acropora cervicornis while showcasing coral restoration science to thousands of visitors. These iterative studies include assessing the feasibility of stress hardening using high irradiance while testing which stress and recovery conditions minimize tradeoffs. 165 coral fragments from eight genotypes of A. cervicornis were haphazardly assigned to either a control or a high light treatment (250 or 2000 umol photon m-2s-1) at one of two temperatures (25 or 29.5°C). Following the light stress, bleached corals were allowed to recover at either 25 or 29.5°C in a fully crossed design. Tissue samples, photochemical efficiency, and buoyant weight measurements were taken to assess changes in symbiont community structure (qPCR) and function (I-PAM) and to quantify coral growth. Bleached corals lost approximately 95% of their algal symbionts, with symbiont community function declining more severely at 25 than at 29.5°C. Control corals at 25°C had slower growth rates and lower symbiont abundance and function compared to controls at 29.5°C. While light-stressed corals held at 25°C throughout the study also showed lower total growth and reduced symbiont community function, corals that were light-stressed and recovered at 29.5°C did not show the same significant declines in growth or photochemical efficiency. These results indicate that recovery environment can mitigate potential tradeoffs caused by hormetic approaches such as stress hardening. Exposure of naïve controls and recovered fragments to subsequent stress will elucidate whether stress hardening improves resistance to, or recovery from, bleaching. This pilot study highlights the importance of seasonally timing potential interventions to maximize their net benefit and to maintain restoration efficiency by preserving key physiological functions such as coral growth rate.
