Coral fragments (size ~0.5 – 1.5cm) were dry-frozen and kept at -80°C. Prior to the assay, host and algal cells were separated as follows. Frozen fragments were placed on ice in 1.5ml tubes with 250μl of ice-cold EB buffer (100mM Tris, 20mM EDTA, pH 8.0). Glass beads (Sigma Aldrich, G1152) were added to the corals, each fragment was vortexed in short pulses for 20s and then placed immediately back on ice. Samples were centrifuged at 500g for 5min at 4°C.