Total RNA was extracted from dry frozen coral samples (size ~0.5 - 1.5cm) with RiboZolTM RNA Extraction Reagent (VWR Life Science) followed by DNase treatment (DNaseI, Zymo Research) and phenol-chloroform extraction. We performed the DNase treatment in 50 μl with 1U of DNase I at 37°C for 15 min and – to increase the efficiency – with additional 1U for 25 min. 1μg of RNA was reverse transcribed with High-Capacity cDNA reverse transcription kit (ThermoFisher Scientific) with RiboLock RNase inhibitor (ThermoFisher Scientific). Reverse transcription quantitative PCR (RT-qPCR) reactions were run in 12 μl with PowerUpTM SYBRTM Green Master Mix (Applied Biosystems) for 40 cycles. Four gene expression profiles – pa-Actin, pa-EF-1a (elongation factor 1), pa-Calm (calmodulin), and pa-AHC (AdenosylHomoCysteinase) - were compared in five different treatments and pEF-1a showed the highest expression stability upon heat stress and thus chosen as the reference gene. All primer sequences are listed in Table S1.