Abstract:

The rising concentration of atmospheric carbon dioxide (CO2), primarily resulting from anthropogenic CO2 emissions, has driven global climate change and ocean acidification (OA). Oceans have absorbed over 40% of anthropogenic CO2 emissions, leading to a decline of approximately 0.11 pH units in seawater since the Industrial Revelation. Diatoms, a major group of marine phytoplankton, contribute approximately 40% of marine primary productivity. Understanding the effects of OA on diatoms is therefore essential for predicting marine ecosystem responses and the efficiency of the biological carbon pump. In this thesis, we investigate the impacts of OA on elemental accumulation in diatom, with a particular focus on calcium (Ca) homeostasis, nutrients sensing, cadmium (Cd) tolerance, and silicon (Si) metabolism. Through integrated molecular and materials science approaches, we demonstrated that OA significantly disrupts Ca2+ homeostasis and signaling in the marine diatom Phaeodactylum tricornutum. Under nutrient-limited and elevated partial pressure of (pCO2) conditions, OA was found to impaired phosphorus sensing, accelerating population decline. Furthermore, elevated anthropogenic CO2 levels reduced Cd accumulation and mitigated Cd toxicity in diatoms. Using the model diatom species Thalassiosira pseudonana, we show that OA impairs silicification by disrupting active silicic acid transport and reduces sinking velocities via decreased frustule silicification, potentially altering silicon export and carbon sequestration. By simulating future high-CO2 conditions, this thesis demonstrates that OA compromises diatom nutrient sensing and elemental accumulation. These findings provide critical insights into diatom responses to climate change and highlight important implications for marine biogeochemical cycles. 

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