School of BioSciences - Theses

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    Lack of transferability of experimentally enhanced thermal tolerance of photosymbionts between coral larvae and sea anemone hosts
    Sakamoto, Rumi ( 2021)
    Coral reefs support a wide range of marine species and are crucial for our economy. However, current climate change and associated summer heatwaves cause reduced coral health and high mortality rates. An important aspect of coral reefs is a mutualism between the cnidarian host and endosymbiotic algae in the family Symbiodiniaceae, which provide more than 90 % of the coral energy through photosynthesis under the healthy mutualistic relationship. However, stressful environmental conditions such as elevated ocean temperature cause coral bleaching, which is the loss of Symbiodiniaceae from coral tissues. To increase the thermal tolerance of the corals, heat-evolved strains of the Symbiodiniaceae species Cladocopium C1acro were previously obtained through experimental evolution in vitro. Of the ten heat-evolved Symbiodiniaceae strains, three strains confer enhanced bleaching tolerance to Acropora tenuis coral larvae. To test whether these strains improve the thermal tolerance of another host species, I exposed sea anemones, Exaiptasia diaphana, inoculated with each of eight strains of Symbiodiniaceae (the homologous symbiont, Breviolum minutum; the wild-type and six heat-evolved strains of Cladocopium C1acro) to elevated temperature. My findings showed that the thermal tolerance of E. diaphana varied depending on the inoculated Symbiodiniaceae strains, however, the thermal tolerance ranking differed from that observed in the previous coral larval experiment. Metabolomics analysis of the host fraction indicated that the translocation of sugar was significantly lower under the elevated temperature treatment for all host-symbiont pairs, which supports one of the bleaching hypotheses that coral bleaching is caused Symbiodiniaceae becoming parasitic under elevated temperature. This study revealed important insight into the cnidarian-Symbiodiniaceae interaction under elevated temperature and provided an insight into the coral bleaching hypothesis.