Light-dependent ecophysiological costs and benefits of calcification in the coccolithophore Emiliania huxleyi
Creators: name: Grubb, Austin Ryan
Publisher: name: No Publisher Supplied
Year: 2024

description: Biogenic calcification has been widespread throughout marine organisms since the Cambrian era. In the modern ocean, coccolithophores are the most abundant calcifying algae. Calcification in coccolithophores uses bicarbonate to produce calcite plates known as coccoliths and carbon dioxide is released as a byproduct. Coccoliths facilitate carbon export to depth by ballasting organic matter. In addition, they scatter light to such a degree that blooms of the most widespread coccolithophore species, Emiliania huxleyi, can be viewed from Earth-observing satellites. Despite the considerable amount of attention that calcification has garnered over the years, there are still degrees of uncertainty surrounding the mechanisms and purpose of calcification. There are several hypotheses that attempt to address the ecophysiological role of calcification, from protecting cells from grazers and viral infection, to enhancing photosynthesis by modulating the amount of light entering a cell, and/or by increasing intracellular carbon concentrations. Furthermore, uncertainty remains around the cellular costs and benefits associated with calcification and how they co-scale with light and photosynthesis. The aim of my PhD thesis is to better characterize the ecophysiological costs and benefits of calcification across irradiance.My dissertation is divided into three chapters, each focusing on different aspects of this objective. After a brief introduction:Chapter 1 demonstrates how calcification enhances photosynthesis. This work uses genetically-related calcified and non-calcified (‘naked’) phenotypes of E. huxleyi grown across different light intensities (25-2000 µmol photons m-2 s-1). I investigated the impact that light intensity has on calcification and the light and dark reactions of photosynthesis using flow cytometric methods, PIC and POC analysis, fluorescence-based biophysical measurements of photosynthesis, quantitative immunoblots, 14C incubations, and stable isotope analysis. I show that coccoliths serve a photoprotective role and that calcification increases the bicarbonate available for organic carbon fixation. I also demonstrate that coccolith shedding rates are modulated by irradiance. Chapter 2 investigates the molecular mechanisms supporting calcification and assesses what impact calcification has on the expression of metabolic pathways. Through differential gene expression analyses, I identify an alpha carbonic anhydrase that plays a role in calcification. In addition, I find that calcification is associated with downregulation of antioxidant and photorespiration pathways. Expression patterns of ion transporters and a vacuolar H+-ATPase suggest that regulation of ion transport and activity in calcifying E. huxleyi may not occur at the transcript level. Chapter 3 characterizes how the intracellular partitioning of carbon is impacted by calcification. The results from Chapters 1 and 2 demonstrated that calcification increases the carbon supplied for organic carbon fixation. Using empirical data and a numerical model, I demonstrate that calcification is part of a multipronged carbon concentrating mechanism in E. huxleyi. My findings also suggest that calcification is associated with higher rates of protein synthesis and turnover. Lastly, increased lipid quotas associated with calcification suggest that lipids may play a role in buoyancy maintenance.