Making a Model - Investigating the Molecular Machinery of the Coral Symbiosis Model System Aiptasia

Abstract

Coral reefs are the most diverse marine ecosystems of significant ecological and economic importance, globally. Increasing environmental stress imposed by global warming, ocean acidification and pollution has led to the continuous decline of coral reefs. For reefs to thrive and survive, they rely on the stable endosymbiosis between coral animal and photosynthetic algae. The fragile symbiotic relationship is dependent on a balanced metabolic exchange, which is easily disturbed by stress, consequently leading to the loss of the endosymbiotic algae - a process known as bleaching. Since corals energetically rely on the algae, the breakdown of symbiosis can have fatal consequences. However, the underlying molecular and cellular mechanisms of this symbiosis are thus far poorly understood. The small sea anemone Aiptasia has provided an experimentally tractable model organism, furthering our understanding on the function of symbiosis and hence, coral susceptibility and resilience to stress. Nonetheless, this model organism is comparatively young and therefore, requires innovative approaches as well as establishment and optimization of protocols. In this thesis, we applied transcriptomic, proteomic and epigenomic tools in Aiptasia, with the aim to assess the dynamics of symbiosis and thermotolerance. Heat stress studies, on Aiptasia's originating from geographically distinct regions, provided insight into the cnidarian-algae symbiosis mechanisms and the role of metabolic compatibility in symbiosis. Furthermore, findings elucidated that associating with thermotolerant algae can improve the cnidarian host's tolerance, potentially acting as a form of local adaptation. Finally, the role of epigenetic mechanisms in cnidarian symbiosis was investigated, by optimizing Chromatin Immunoprecipitation (ChIP) and establishing the genomic landscape of histone 3 lysine 9 acetylation (H3K9ac) in Aiptasia. These new results will enable progressing Aiptasia further as a model organism and thus, advance our understanding on the complex mechanisms of coral symbiosis.