Date of Award

12-2013

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Marine Biology

Advisor

Malcolm Shick

Second Committee Member

Mark Wells

Third Committee Member

Rebecca Van Beneden

Abstract

Coral bleaching is a global threat that has profound effects not only on coral health and longevity, but also on reef diversity and human economies. Bleaching is the loss of symbiotic dinoflagellates or their pigments from corals under stress, including a combination of high temperature and high irradiance that creates a superabundance of reactive oxygen species (ROS) as the photosynthetic electron transport systems are disrupted. Because of the absolute necessity of iron, copper, and manganese to both the electron transport proteins and to key antioxidant enzymes [including ascorbate peroxidase (APx), catalase (CAT), and the three forms of superoxide dismutase (Fe-SOD, Mn-SOD, and CuZn-SOD)] that detoxify ROS, I posited that corals and their symbionts exposed to low trace-metal availabilities would be more susceptible to bleaching than those exposed to metal-replete conditions. This hypothesis was tested by subjecting specimens of Stylophora pistillata collected from Davies Reef and the Palm Islands on Australia’s Great Barrier Reef to increased water temperature and supersaturating irradiance in either a metal-replete or metal-depleted experimental medium. Separate experiments explored depletion of manganese, and of copper and zinc. In both experiments, symbiont photosynthetic health was monitored (by measuring F0 and Fv/Fm indicators of photosynthetic performance and photodamage) daily, and on the final day of each experiment specific enzyme activities were assayed (at a standard temperature of 25°C) in both the animal and the algae.

In the manganese experiment a slight rise in F0 coupled with a decline in Fv/Fm, indicated photodamage. In the algal samples exposure to high temperature and manganese-replete seawater enhanced both APx and total SOD activities. CuZn-SOD was detected only when manganese was available during exposure to high temperature. Fe-SOD was significantly increased by manganese availability regardless of exposure temperature. In the host, there was a differential effect of manganese availability on APx and CuZn-SOD activities depending on exposure temperature.

In the copper and zinc experiment, an increase in F0 and a decline in Fv/Fm values indicated photodamage, especially at high temperature in the metal-depleted condition. In the algae, high temperature and Cu and Zn depletion was associated with significantly higher observed activity in CAT, total SOD, and Mn-SOD. In the host, enzyme activity was enhanced by exposure to high temperature for every assayed enzyme except CAT. Further, exposure to high and low temperature elicited differential effects of Cu and Zn availability on total SOD activity measured at the common temperature of 25°C.

The manganese experiment showed that high availabilities of trace metals rendered corals robust to bleaching stress, suggesting that corals or their dinoflagellate symbionts have a ‘luxury’ metal uptake system. The copper-zinc experiment reinforced the finding that trace metal availabilities do have important impacts on stress leading to bleaching, but that these impacts do not necessarily affect antioxidant enzyme activities in the way that I hypothesized. In the copper-zinc experiment, the effects of metal depletion on photosynthetic capacity may have been manifested through loss of carbonic anhydrase (which contains Zn) activity, enhanced by compromised ability to synthesize electron transfer proteins.

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