Metabolic responses of diatoms to a changing environment: Pathways for energy dissipation in high light

Marine diatoms are an abundant and highly productive group of phytoplankton, contributing significantly to oceanic primary productivity and playing an important role in the global carbon cycle. Diatoms are commonly associated with export of fixed carbon into the deep ocean, providing a sink for atmospheric CO2. However, when diatoms are stressed by high light, they often produce several organic compounds that may be leaked to the surrounding seawater and respired back to CO2 by marine bacteria. In this dissertation, the environmental triggers of metabolic pathways associated with high light responses in diatoms are investigated. Special attention is focused on the pathway of photo respiration and its association with other carbon and nitrogen utilizing pathways. The effect of light on photorespiration is determined using both a molecular marker for a photorespiratory gene and release of the photorespiratory-specific compound glycolate from cells of Thalassiosira weissflogii. Cells acclimated to high light levels were found to have an upregulated photorespiratory pathway that recycles the glycolate produced in the initial steps of photorespiration. Cells acclimated to low light and shifted to high light produced more glycolate than could be metabolized in the photorespiratory pathway resulting in release of glycolate from the cell. This implies that cells in mixing environments may release more of their fixed carbon as glycolate than cells in stratified waters. The synergistic effects of light, temperature and nitrogen source on the cycling of both carbon and nitrogen within the cell during photorespiration were also investigated. The newly sequenced genome of Thalassiosira pseudonana was used to develop primers to five genes involved in carbon and nitrogen utilization. Transcript accumulation for these genes was measured and the influence of light and temperature on photorespiration, nitrate utilization and the Calvin cycle was determined. The results suggest complementary roles for photorespiration and nitrate utilization in energy dissipation. A unifying model is presented to explain the patterns of transcript accumulation in terms of the dynamics of carbon and nitrogen flow through these pathways and energy balance within the cell. The environments likely to promote photorespiration in diatoms and lead to organic carbon release are discussed.