| Summary: | Light and iron (Fe) Co-limitation occur in temperate, Fe-limited regions in seasons where days are short and the mixed layer is deeper than the euophotic zone. In the first part of this study, the effect of light and Fe co-limitation on Pseudo-nitzschia granii isolated from the NE Subarctic Pacific was investigated. This was one of the first studies to investigate the effect of Fe and light on the physiology of oceanic, pennate diatoms in artificial seawater. Fe-limited P. granii grew at slower rates than Fe-replete cells, and the former showed signs of chlorosis and silica deficiency. P. granii was efficient at utilizing light, especially low light. Light and Fe were found to affect different aspects of photosynthesis. Whereas Fe affected the quantum yield of photosynthesis, light affected photochemical efficiency. Despite low growth rates, P. granii was able to survive and grow under Fe-limitation. This was possibly because Fe limitation increased the activity of non-photochemical quenching mechanisms, allowing the cells protection against photodamage. Overall, growth rates were directly related to the rate of linear electron transport through the cells. In the second part of this thesis, interaction of light and temperature on the physiology of P. granii was investigated. Studies on the combined effects of light and temperature, especially in temperate phytoplankton, are rare. Both light and temperature were found to influence growth rates. Temperature dependency decreased with decreasing light. Cellular chlorophyll displayed a modal response to temperature and light. Photosynthetic yield and efficiency were generally depressed at low and high temperatures. Unlike in green algae and some cyanobacteria, low-temperature acclimation in P. granii was not similar to high light acclimation. Science, Faculty of Botany, Department of Graduate
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