Publication Date

2019-05-03

Availability

Embargoed

Embargo Period

2021-05-02

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Marine Biology and Ecology (Marine)

Date of Defense

2018-12-07

First Committee Member

Andrew C. Baker

Second Committee Member

Peter W. Glynn

Third Committee Member

Chris Langdon

Fourth Committee Member

Larry Brand

Fifth Committee Member

Ross Cunning

Abstract

Ocean warming, acidification, and nutrient pollution are threatening the persistence of coral reef ecosystems worldwide. This dissertation assessed the effects of these stressors on the disruption of the coral-algal symbiosis (bleaching) and aimed to identify factors that promote more resistant holobiont partnerships. I used highly sensitive qPCR assays to monitor dynamic changes in Symbiodiniaceae algal communities during heat stress and recovery, and during the exposure to elevated nutrients and CO2. During natural bleaching events in Florida and Panama, I evaluated the potential for multiple coral species to increase their temperature tolerance through the acquisition of thermotolerant symbionts (Chapters 2 and 4). Heat stress favored the acquisition of thermotolerant algal taxa (Durusdinium) in two coral species, including one believed not to be able to change its symbiont partner. However, I also documented no increase in Durusdinium in three coral species exposed to back-to-back bleaching events in consecutive years. Since annual bleaching conditions are expected to become common in the coming decades, I monitored the persistence of thermotolerant symbionts under non-stressful temperatures, and evaluated how the frequency of heat disturbances affects the trajectory of symbiont community recovery (Chapters 2 and 3). Symbiont communities dominated by Durusdinium were generally unstable under non-bleaching conditions and were replaced by thermosensitive algae over a period of months. However, Durusdinium-dominated communities persisted longer when this symbiont accounted for >98% of the initial community, resulting in more stable communities that were maintained for 2-3 years. Finally, because global warming will likely interact with other local and global stressors, I assessed the effect of elevated nutrients and CO2 on pre-bleaching symbiont communities and further bleaching susceptibility (Chapters 5 and 6). The effects of elevated nutrients and CO2 were small in comparison with the effects of heat stress. However, these stressors strongly impaired coral host performance, and sometimes exacerbated the effects of subsequent heat stress. Between predicted back-to-back bleaching events and the high likelihood that global carbon emissions will exceed RCP4.5, understanding the dynamic relationship between corals and their algal symbionts is key to helping researchers protect the future of coral reefs. This dissertation emphasizes differential effects of warming, acidification, and nutrification that vary depending on coral species and algal symbiont communities, suggesting that multispecies approaches should be taken to understand how coral reefs will respond to the expected future conditions.

Keywords

Coral bleaching; symbiosis; Durusdinium; Symbiodinium; global warming; ocean acidification; nutrification

Available for download on Sunday, May 02, 2021

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