Effects of temperature on isomorphic haploid-diploid phases of Gracilariopsis tenuifrons (Gracilariales, Rhodophyta)

Sea surface temperature is increasing, and we still do not have a good understanding of the effects of these temperature rises on haploid-diploid phases of many isomorphic red algae. In this study, we aimed to investigate the physiological performance of unfertilized and cystocarpic female gametophytes, fertile male gametophytes, and fertile tetrasporophytes of Gracilariopsis tenuifrons at different temperatures (20°C, 25°C and 30°C). We evaluated growth rates, maximum quantum yield by in vivo chlorophyll fluorescence, pigment content (phycoerythrin, PE; phycocyanin, PC; allophycocyanin, APC; chlorophyll a; and carotenoids), thallus length, and number of branches. At 30°C, tetrasporophytes showed greater growth, PE and number of branches, when compared to the others. However, intermediate temperature (25°C) resulted in higher growth, thallus length, PC and APC in unfertilized female gametophytes when compared to tetrasporophytes, and both phases showed better physiological performance than cystocarpic female gametophytes and male gametophytes. The latter two showed reduced physiological performance possibly owing to their sensitivity to temperature plus the imposition of fertility when compared to the unfertilized female gametophytes and fertile tetrasporophytes. In general, regardless of temperature, the presence of cystocarps negatively affected the physiological performance of female gametophytes. The occurrence of a complex life cycle, in which there is alternation of independent generations with distinct physiological responses, even if isomorphic as in G. tenuifrons, can bring advantages to the species in occupying thermally variable environments, enabling the increase and survival of one or another phase throughout the year.