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.