Thermal preconditioning slows down the bleaching rate in P. acuta
We split colonies collected in Kaneohe Bay into three parts for preconditioned (PC), non-preconditioned (NPC) and control treatments. After 3 weeks of acclimatization to indoor tanks environment at winter ambient temperature (26ºC), PC part of each colony was exposed to the usual summer maximal temperature (29ºC) for 3 days and then rested at ambient 26ºC for two weeks (Fig.1A). To assess thermal tolerance, we measured bleaching rate in each coral during a sudden heat stress (32ºC) using fluorescence confocal microscopy (Fig.1B, C). Each fragment of PC, NPC and control coral was live imaged at the beginning of the experiment and after 3 and 6 days. Red (algae) to green (host) signal ratio was calculated to quantify the density of Symbiodinium cells in coral tissue. Our results show that corals previously exposed to sublethal temperatures are more tolerant to acute heat stress and bleach at slower rate than non-preconditioned corals (two-way ANOVA, signal_ratio ~ time * conditioning, p = 0.000) (Fig. 1C). To eliminate that the shift in red to green signal ratio is caused by increased level of pGFP in coral tissue as response to heat stress, we analyzed pGFP expression via qPCR and observed no differences (data not shown).
Previous experiments have already shown the ability of corals to increase their tolerance to heat stress after exposure to sublethal temperatures (Ainsworth et al., 2016; Bay & Palumbi, 2015; Bellantuono et al., 2012). It seems that the length of sublethal temperature exposure along with the length and temperature of the resting period are more important for the acquired thermal resilience than the preconditioning temperature. Our experiment is unique because it combines relatively short preconditioning period with relatively long resting time.