Abstracts Category : Other

Thermal acclimation and light-harvesting complex expression in Symbiodinium

by Sarah Louise Gierz

Endosymbioses observed between photosynthetic dinoflagellates of the genus Symbiodinium and reef-building (Scleractinian) corals are crucial to the success of diverse reef ecosystems. Dysfunction of this symbiotic relationship can occur under a number of stressors (including elevated sea surface temperatures and ocean acidification), resulting in the expulsion of Symbiodinium from host cells or loss of photosynthetic pigments, a process known as coral bleaching. While ocean temperatures fluctuate on a daily basis, the mean ocean temperature is predicted to rise approximately 1 2 C over the next century and is expected to lead to more mass coral bleaching events.Within coral bleaching experiments, elucidation of sites of thermal sensitivity within Symbiodinium has focused on potential points where damage may originate. One of these potential sites are the integral light-harvesting protein complexes (LHCs), which bind chlorophylls and accessory pigment molecules with roles in lightharvesting by receiving and transferring light energy to photosystems, and photoprotection by dissipating excess energy under stress conditions. Little is known about the response of the diversified integral LHC gene family (acpPCs) in Symbiodinium to thermal stress, as only short term (24 h), light stress and dissociation experiments have been reported. Additionally, few studies have examined the broad transcriptional response of Symbiodinium to thermal stress conditions.Therefore, the aims of this research were to examine the effect of extended thermal stress on Symbiodinium to determine variations in gene expression and morphology both in vitro and in hospite and to link this to observed physiological parameters. To achieve these aims thermal stress experiments were performed on cultured Symbiodinium sp. (clade F), and in hospite utilising Acropora aspera harbouring Symbiodinium clade C3. A targeted quantitative PCR approach was utilised to determine the expression of five integral LHC genes within Symbiodinium in hospite and a transcriptome approach was utilised to identify differentially expressed transcripts within Symbiodinium sp. (clade F) in vitro. Variations in Symbiodinium morphology were characterized following exposure of A. aspera to thermal stress using confocal laser scanning microscopy.Exposure of Symbiodinium sp. (clade F) cultures to a twenty-eight day thermal stress regime (~31 C) elicited a stress response measured as reduced cell growth from day four onwards (p < 0.01) and decreased dark-adapted yield on days fourteen (p < 0.05), nineteen (p < 0.001) and twenty-eight (p < 0.001). Whole transcriptome sequencing of Symbiodinium cells on days four, nineteen and twenty-eight identified 23,654 unique genes (FDR < 0.05), though 92.49% differentially expressed genes displayed 2-fold change in expression. The transcriptional response included differential expression of genes encoding photosynthetic machinery subunits, integral LHCs, fatty - acid desaturases, metabolic enzymes, and components of stress response