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A Comparative Approach To Elucidating The Physiological Response In Symbiodinium To Changes In Temperature
McGinty, Elizabeth S.
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Coral reefs are one of the most biodiverse ecosystems, and play a large role in the functioning of oceanic and coastal ecosystems. They are currently declining around the world, largely due to the effects of climate change and local anthropogenic issues. Temperature stress in particular has been associated with significant coral mortality over the last 30 years due to adverse effects on the relationship between reef-building corals and their algal endosymbionts from the genus Symbiodinium. Periods of elevated temperature have led to the phenomenon of coral bleaching, a disruption of the symbiosis that is characterized by the loss of the algal symbionts or their pigments. Variations in bleaching responses have been observed that correlate with the identity of the algal symbiont, but the mechanisms behind these differences remain unresolved. To examine how Symbiodinium physiology may be contributing to these observed differences, eight Symbiodinium cultures, or Symbiodinium types, were exposed to elevated temperature stress, and several parameters that impact the bleaching response were measured. These included the production of reactive oxygen species (ROS), the activity of antioxidants responsible for scavenging ROS, oxygen uptake rates (V́́o2) as a measure of metabolism, growth rates, and loss of the photosynthetic chlorophyll a (Chl a) pigment. Each Symbiodinium type was found to display unique responses to elevated temperature stress, demonstrating a gradient of sensitivity. Symbiodinium types that were sensitive to elevated temperatures had increased production of ROS without a corresponding increase in antioxidants to scavenge them, increases in V́́o2, and decreases in growth rate concomitant with a loss in Chl a at relatively low temperatures. Tolerant attributes were found in other Symbiodinium types including low production of reactive oxygen, sometimes coupled with higher activity of antioxidants, stable V́́o2 and continued growth at relatively higher temperatures, and resistance to loss of the Chl a. In addition, the functional differences of intact associations from four coral species collected in La Parguera, Puerto Rico, were examined with regard to host and symbiont factors linked to resistance to bleaching and disease. Components of the melanin cascade in host tissue can confer resistance to bleaching as well as being important factors in the immune response, and antioxidant activity of both the host and symbiont have a role in preventing damage due to ROS produced by symbionts and by the host during the melanin cascade. Significant differences were found among different coral-Symbiodinium associations, and correlations were found in coral-Symbiodinium responses, including symbiont antioxidant activity and host melanin cascade. This indicates that each partner in the relationship can significantly influence the response of the other to environmental stress and pathogens. The culmination of this research demonstrates that the Symbiodinium stress response is a complex interaction of the stability of their metabolism, disruption to their photosynthetic apparatus, and influences from their host when in symbiosis. As temperatures increase, some types of Symbiodinium are physiologically robust, some are able to compensate, and others are negatively affected. These results suggest that stressors such as elevated temperature may also be influencing the relationship between the host and symbiont, potentially shifting the relationship from a mutualism to a parasitism.