Calcium and hydrogen peroxide differentially contribute to the two heat-stress pathways in the marine red alga Pyropia yezoensis

The marine red alga Pyropia yezoensis employs two heat-stress signaling pathways, one dependent and one independent of membrane fluidity, each targeting distinct sets of heat stress–inducible genes, but the underlying factors are unclear. Here, to assess the importance of Ca2+ in the heat-stress response of P. yezoensis, we treated all three stages of its life cycle with Ca2+ chelators, which blocked the upregulation of both membrane fluidity–dependent and –independent genes. This repression was rescued by adding CaCl2. In addition, treatment with the calcium ionophore A23187 induced the expression of these genes at 15°C and 25°C. Thus, extracellular Ca2+ influx is a shared factor of both signaling pathways. Moreover, H2O2 contents increased upon exposure to 25°C and when membrane fluidity increased due to treatment with benzyl alcohol, which induced the expression of only membrane fluidity–dependent genes. Thus, H2O2 production is specifically involved in the membrane fluidity–dependent pathway. Furthermore, the heat stress–dependent production of H2O2 was inhibited by treatment with Ca2+ chelators, placing extracellular Ca2+ influx upstream of H2O2 production in the membrane fluidity–dependent pathway. Therefore, Ca2+ contributes to both pathways, whereas H2O2 is only involved in the membrane fluidity–dependent pathway in P. yezoensis.