Photosynthesis is the natural process of harvesting, converting, and storing the energy of sunlight. It is essential for life on the Earth as we know it, providing chemical energy for plants, algae and bacteria, which further serve as the primary source of organic matter for the growth and metabolic demands of all other organisms in the ecosystem. Fluctuations in the incident light intensity and spectrum can pose major challenges to the photosynthetic apparatus. Aquatic photosynthetic organisms provide an excellent example of the capacity of the photosynthetic apparatus to adapt to fluctuating light conditions. This thesis covers various “survival” strategies of two major groups of aquatic photosynthetic organisms: the diatom Cyclotella meneghiniana and the cyanobacterium Synechocystis (sp. PCC 6803). Diatoms, a group of algae, are amongst the most abundant of aquatic photosynthetic organisms, whereas cyanobacteria or “blue green algae” are known for their capacity to survive extreme environmental conditions, such as hypersaline bays to Antarctic ice basins.