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.