UCLA

Coral reefs are among the world’s most biodiverse and valuable ecosystems, but they are in global decline. One major concern for the future of reefs is mass coral bleaching, a phenomenon where corals turn white after expelling their endosymbiotic algae (Symbiodinium) during periods of elevated sea surface temperatures. Unfortunately, the frequency and severity of mass coral bleaching is increasing as climate change results in global sea surface temperature rise. Extensive research has examined how Symbiodinium variation influences bleaching susceptibility, yet Symbiodinium is only one part of a complex coral holobiont that also include bacteria and archaea.

This dissertation investigates variation in coral microbiomes in the world’s largest and most diverse coral reef ecosystem, the Indonesian Archipelago. Using a common garden approach, Chapter 1 investigates how the microbiome of Porites lobata varies with water temperature among two nearby reef ecosystems that experience different levels of coral bleaching—Amed and Pemuteran, Bali. Results reveal significant local variation in microbiomes, but no consistent changes in microbiomes with temperature. Chapter 2 examines how the coral microbiomes of three species of coral with different bleaching susceptibility (Acropora millepora, Pocillopora verrucosa, and Porites lobata) respond to elevated water temperatures in a single location, Manokwari, West Papua. Results show significant differences in microbiomes in all three species, indicating species-specific relationships between coral host and microbial symbionts. However, while microbiomes changed with elevated temperature, there were no consistent patterns by species or by propensity to bleach. Lastly, chapter 3 examines natural variation in microbiomes of these same three species of coral across 5000 km of the Indonesian Archipelago, including reefs with historically low, moderate, and high bleaching susceptibility. Results varied by species: microbiomes of Acropora millepora showed no significant geographic variation; microbiomes of Pocillopora verrucosa were significantly different among most locations, but not among regions; and microbiomes of Porites lobata showed significant variation based on regional variation in bleaching susceptibility, but not among individual localities. Combined, these studies show that coral microbiomes are complex, varying by species, reef, and geographic region, providing novel insights into the nature and specific relationship of microbiomes to coral host and their environment.