Intra- and interspecific variation and phenotypic plasticity in thylakoid membrane properties across two Symbiodinium clades

Coral photosynthetic endosymbionts (Symbiodinium) are phylogenetically very diverse, yet the extent of inter- and intraspecific functional variation within clades remains largely underexplored. Understanding this variability will be critical for future research on climate change mediated responses. A properly functioning thylakoid membrane is essential for optimal photosynthetic performance both in free living and in hospite conditions. Here, we analyse the thylakoid membrane melting points of 13 Symbiodinium strains from species in Clades B and A, grown at both control (26 °C) and high temperature (31 °C). We observed a broad range of responses to thermal stress regardless of taxonomic rank. Our results support and augment a growing body of the literature demonstrating that functional differences among Symbiodinium spp. are as distinct at lower taxonomic levels (i.e. interspecific) as they are among major clades. These findings highlight the importance of assessing the variability of plastid traits across the Symbiodinium tree.