Physiological Responses of Pelagic <i>Sargassum</i> (Phaeophyceae) to Thermal Stress Vary by Species and Morphotype

Sargassum (common name referring to pelagic species of the genus <i>Sargassum</i>) has increased in abundance across the Atlantic Ocean since 2011, with rising sea surface temperatures and nutrients hypothesized as potential drivers. However, a detailed understanding of sargassum thermal responses remains limited. We cultured the three species/morphotypes that compose sargassum (<i>S. fluitans</i> var. <i>fluitans</i>, <i>S. natans</i> var. <i>natans</i>, and <i>S. natans</i> var. <i>wingei</i>) from south Florida in temperatures between 20°C and 35°C. Physiological parameters (growth rate and <i>in vivo</i> chlorophyll fluorescence) were evaluated every two days and pigment concentrations were analysed at the experiment conclusion. Each morphotype showed distinct thermal responses, with all morphotypes displaying mass loss at and above 32°C. Across tested temperatures, <i>S. fluitans</i> var. <i>fluitans</i> and <i>S. natans</i> var. <i>natans</i> had a thermal optimum for growth at 20°C while <i>S. natans</i> var. <i>wingei</i> had a thermal optimum of 29°C. Further, <i>S. natans</i> var. <i>wingei</i> showed significant differences from the other two morphotypes in all photosynthetic parameters measured. All morphotypes showed decreased photosynthetic performance at 35°C for all parameters analysed. Chl <i>a</i> and carotenoid content differed across morphotypes with <i>S. natans</i> var. <i>natans</i> exhibiting the higher Chl <i>a</i> than <i>S. fluitans</i> var. <i>fluitans</i>, but not significantly different from <i>S. natans</i> var. <i>wingei</i>. <i>S. natans</i> var. <i>natans</i> also exhibited higher carotenoid concentrations than both other morphotypes. Across all six temperatures tested, chl <i>a</i> and <i>c</i> differed with significantly lower concentrations at 20°C, 23°C, and 32°C compared to concentrations at 26°C and 29°C. Varying thermal tolerance across morphotypes has implications for the distribution and survival of pelagic sargassum with the potential for increased dominance of <i>S. natans</i> var. <i>wingei</i>, particularly in warmer waters, as temperatures and heatwaves increase in the tropical oceans.