Onset and Breakdown of Cnidarian-Dinoflagellate Symbiosis : A Survey of the Host Transcriptomic Response and Characterization of the Sphingosine Rheostat during Symbiont Colonization and Hyperthermal Stress
The symbiosis between cnidarians, such as corals and sea anemones, and photosynthetic dinoflagellates belonging to the genus Symbiodinium spp. is one of the most productive in the marine environment. This mutualistic endosymbiosis allow reef-building corals to lay down the foundation of coral reef ecosystems, which supports a highly biodiverse community of marine organisms. The relationship between the cnidarian host and algal symbiont changes over time, from the initial contact through symbiont removal. The breakdown of the partnership can be brought on by numerous environmental stressors; most notably by elevated temperatures associated with climate change. The cellular and molecular mechanisms underlying these key events in cnidarian-dinoflagellate symbiosis are still poorly understood. Lipids play a central role in symbiosis by providing cellular structure, energy storage, signaling platforms. Specifically, the signaling lipids from the sphingosine rheostat, sphingosine and sphingosine-1-phosphate (S1P), play a pivotal role in determining cell fate. Increased sphingosine drives apoptotic activity within the cell while S1P promotes cell survival. The research presented in this dissertation addresses (1) the role of signaling sphingolipids at different stages of cnidarian-dinoflagellate symbiosis and (2) the transcriptional patterns of coral larvae undergoing onset of symbiosis while exposed to elevated seawater temperatures. In Chapter 2, the proposed cnidarian sphingosine rheostat model was functionally characterized in the sea anemone Aiptasia pallida. This study identified differential response of the sphingosine rheostat with symbiont recolonization and long-term symbiont maintenance, suggesting a role in host-symbiont interactions. In Chapter 3, a heat stress experiment revealed a biphasic sphingosine rheostat response in A. pallida where acute stress inhibits rheostat expression and activity that is recovered and shifted toward cell death with longer-term heat stress. This response was not linked to symbiont loss, but has implications for a more generalized heat stress response for long-term acclimation in cnidarians. In Chapter 4, coral Acropora digitifera larvae displayed different phenotypes and transcriptional profiles with the combined stress of elevated temperature and symbiont uptake. Larval survival, symbiont colonization and algal density were highly reduced from this treatment. These transcriptional patterns indicate immune suppression, membrane reorganization and oxidative stress. Furthermore, sphingolipid signaling differed at the onset of heat stress. Overall, the work presented here indicates that the sphingosine rheostat mediates host-symbiont interactions until symbiosis dysfunction and that the determinants of symbiosis can be altered with climate-induced stress.