Phylogeny, Heat-Stress and Enzymatic Heat-Sensitivity in the Antarctic Psychrophile, Chlamydomonas sp. UWO 241

Psychrophilic species, micro-organisms which are unable to grow at temperatures of or above 20°C, are abundant in perennially cold ecosystems across the globe. Intensifying scientific investigation of these organisms from ecological to molecular scales has underscored the ability of life on Earth to adapt to environments which seem inhospitable due to high or low temperatures, high salinity, pressure and light, and ultra-low nutrient availability. Psychrophilic organisms that are also photosynthetic represent a much more limited group than psychrophiles generally, as their habitats must be well buffered against the warming influence of the infra-red energy accompanying sunlight. Photosynthetic psychrophiles face the added challenge of balancing the biophysical capture and biochemical transformation of light energy. The psychrophilic green alga Chlamydomonas sp. UWO 241 (hereafter UWO 241) has been the subject of numerous publications, however these are mainly focused on adaptations which facilitate life at low temperature as opposed to restricting it from moderate ones. In this thesis I critically examine previously published phylogenies placing UWO 241 as a strain of Chlamydomonas raudensis, investigate the changes in mRNA abundance in UWO 241 in response to high steady-state growth temperatures and heat shock using targeted and transcriptome-wide approaches, and investigate the regulatory enzymes of the carbon-fixing Calvin-Benson-Bassham (CBB) cycle for signs of heat sensitivity using the techniques of Homology Modelling and Molecular Dynamics. These investigations revealed that, contrary to previously published reports, UWO 241 is not closely related to C. raudensis SAG 49.72. Here, I also show that mRNA abundance in UWO 241 does not respond to different steady-state growth temperatures or a heat-shock temperature shift regime in ways similar to the related mesophile, Chlamydomonas reinhardtii. Additionally, analysis of the regulatory CBB cycle enzymes of UWO 241 for indications of heat-lability indicated that glyceraldehyde-3-phosphate dehydrogenase may be the heat-limited enzyme in this pathway. These findings deepen our understanding of the reasons that this Antarctic alga is unable to grow at moderate temperatures.