Quantifying the response of diatom nitrogen metabolism to environmental change

Diatoms are microscopic unicellular organisms that fix as much carbon annually as all terrestrial rainforests combined and, through nitrogen uptake and carbon drawdown, link global carbon and nitrogen cycles. As the most diverse group of marine phytoplankton, species-specific metabolic differences influence diatom distributions and abundance, and impact nitrogen availability in the marine environment. This thesis investigates the molecular underpinnings of diatom metabolism by focusing on the importance of nitrogen in controlling diatom growth in the marine environment. First, the role of the urea cycle in diatom nitrogen metabolism was investigated by monitoring changes in transcript abundance patterns for key nitrogen metabolism genes in Thalassiosira pseudonana grown under different nitrogen sources and light intensities on a light:dark cycle. Findings integrate the urea cycle into diatom metabolism, through glutamine utilization and urea production, and demonstrate the effects of light and nitrogen source on the flow of nitrogen in a diatom cell. Second, the responses of three evolutionarily diverse model diatoms (Thalassiosira pseudonana, Fragilariopsis cylindrus and Pseudo-nitzschia multiseries) to the environmentally relevant condition of nitrate starvation were examined to test the hypothesis that changes in nitrogen availability elicit uniform metabolic responses across all diatoms. Greater transcriptional and functional similarities between the two pennate diatoms compared to the bipolar centric diatom form fundamental differences in how each group has evolved to respond to its environment. Each diatom appears to maintain its own cell-wide response to nitrogen availability that is activated via finely tuned transcriptional regulation. Third, to understand how these diverse transcriptional responses affect diatom community composition, metatranscriptional patterns were determined for diatom communities sampled along Line P in the Northeast Pacific Ocean. Phylogenetic analyses revealed that individual members of the diatom community responded to the same bulk chemical environment in vastly different ways. In the field, these species-specific differences likely play an important role in determining why one diatom species blooms over another. These complex transcript abundance patterns provide insight into the molecular mechanisms that underlie diatom distributions in the marine environment; future studies can use these findings as a platform for detecting and predicting the metabolic responses of individual species and taxonomic groups within a natural diatom assemblage.