The impact of environmental stress on Symbiodinium spp.: A molecular and community-scale investigation

• Main Author: McGinley, Michael P
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2012
• Subjects: Biological oceanography; Environmental science
• ISBN: 9781267611239; 1267611235

Coral reefs are among the most delicate ecosystems in regards to the adverse effects of global climate change. The symbiosis between scleractinian corals and their endosymbiotic algae (Symbiodinium) is especially sensitive to elevated sea surface temperatures, which can induce a breakdown in the mutualistic association and ultimately the expulsion of the algal cells (coral bleaching). Here, I initially use real-time PCR to monitor the transcript abundance of two chloroplast encoded genes, psbA (encoding the D1 protein of photosystem II) and psaA (encoding the P700 protein of photosystem I), in both cultured and in hospite Symbiodinium spp. exposed to thermal stress. The down-regulation of both genes in two thermally sensitive types could be the result of several different physiological mechanisms, but may ultimately contribute to diminished repair rates of photosynthetic proteins, rendering some Symbiodinium spp. especially susceptible to periods of thermal stress. Furthermore, to investigate how the transcription of chloroplast encoded genes is influenced under varying environmental conditions, the transcript abundance of psbA and psaA was also monitored while chemically altering the redox state of the plastoquinone pool in the photosynthetic electron transport chain, during a low to high light acclimation period, and over a natural diel light cycle. In general, the expression patterns of both genes were not well conserved between the cultured Symbiodinium spp. and the overall incongruence of these patterns with other well-established plastid models from various microalgae and higher plants suggests that Symbiodinium is more reliant on post-transcriptional mechanisms for homeostatic regulation of its photosynthetic proteins. I also examined the impact of coral bleaching on the community dynamics of in hospite Symbiodinium spp. from the western Caribbean and eastern Pacific, respectively. In the former location, I monitored the prevailing Symbiodinium communities before, during and after two experimental bleaching events (repeated during subsequent years) in three Caribbean corals (Montastraea faveolata, Porites divaricata, and Porites astreoides). P. astreoides exhibited a stable and specific symbiosis throughout the experiment; whereas, M. faveolata and P. divaricata displayed a stress-induced shift that favored the formation of different opportunistic and/or stress-tolerant symbioses, along with the concomitant decline of several previously dominant, but stress-prone, Symbiodinium spp. Repetitive bleaching events could reinforce these patterns by continually "resetting" the recovery of these corals and leading to the competitive exclusion of some Symbiodinium types. Lastly, in the eastern Pacific, the Symbiodinium community of Pocilloporid corals was examined following an anomalous cold-water bleaching event. This study suggests that these corals form temporally stable and specific associations with a dominant Symbiodinium spp., despite the high potential for change and environmental conditions favoring a shift towards a stress-tolerant symbiont.