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Many members of the Phylum Cnidaria are mutualistic with unicellular
dinoflagellates belonging to the genus Symbiodinium. Corals are the most widely
recognized example of these associations due to their key ecological importance in
coral reef ecosystems where they serve as the structural and trophic foundation of
these rich ecosystems. Coral reefs are severely threatened by human activities worldwide
and are at great risk from global climate change, in particular the increase in seasurface
temperatures. Detailed knowledge of how corals respond to stress is scarce.
The most serious and immediate response of corals to environmental stress is a
process referred to as coral bleaching (a.k.a. cnidarian bleaching). Nevertheless, the
cellular and molecular processes by which elevated temperatures elicit the bleaching
response are poorly understood. This dissertation deals with this important question
by describing two mediators of cnidarian bleaching in the model symbiotic tropical
sea anemone Aiptasia pallida (Verril), namely nitric oxide and cyclophilin.
After an introduction to the topic of cnidarian-algal symbioses and cnidarian
bleaching (Chapter 1), I present results from a study describing the involvement of
nitric oxide (NO) in the anemone A. pallida (Chapter 2). Elevated temperature as well
as oxidative stress induces production of NO and exposure of A. pallida to NO induces
bleaching at non-stressful temperatures. Co-incubation with an NO scavenger
suppresses bleaching. I propose that the host up-regulates NO production in response
to elevated oxidative stress and that this situation leads to cytotoxicity and bleaching.
Chapter 3 examines the role of cyclophilin from A. pallida in the regulation of
the symbiosis. Cyclophilins belong to a highly conserved family peptydyl-prolyl cistrans
isomerases (PPIases). Incubation of A. pallida with cyclosporin A (CsA), a
potent inhibitor of cyclophilin resulted in bleaching and a decrease in tolerance to
elevated temperatures. Protein extracts from A. pallida exhibited CsA-sensitive
PPIase activity. Laser scanning confocal microscopy using superoxide and nitric
oxide-sensitive fluorescent dyes on live A. pallida revealed that CsA strongly induced
the production reactive oxygen species as well as NO. We tested weather the CsAsensitive
isomerase activity is important for maintaining the activity of the antioxidant
enzyme superoxide dismutase (SOD). SOD activity of protein extracts was
not affected by pre-incubation with CsA in vitro. In Chapter 4 I review what is known
about the molecular and cellular mechanisms of bleaching and describe a model of
bleaching based on the results presented herein as well as studies of non-cnidarian
models. |
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