Caspase activity assay
Coral fragments (size ~0.5 – 1.5cm) were dry-frozen and kept at -80°C. Prior 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 centrifugated at 500g for 5min at 4°C. 50μl of host cell containing supernatant was transferred to a new tube and lysed with Cell Lysis Buffer according to instructions in EnzChekTM Caspase-3 Assay Kit #1, Z-DEVD-AMC substrate (ThermoFisher Scientific). Protein concentration was measured with Qubit Protein Assay Kit (Life Technologies). 50μg of protein in volume of 50μl or lower was used to measure caspase 3 activity following the kit instructions. Fluorescence was measured with SpectraMax2 (Molecular devices) using SoftMax Pro v4.8 software. Caspase activity was analyzed in 6 colonies for heat stress experiment, and in 4 colonies for the venetoclax and colchicine experiments.