UC San Diego

Diatoms are among the most successful group of phytoplankton in the oceans and produce up to one-fifth of earth’s primary production. Their high productivity and ability to form dense blooms have a proportionately large impact on the ecology and biogeochemistry of the oceans. Diatoms possess unique traits that collectively help them outcompete and out-survive other phytoplankton which include a cell wall made of biosilica called the frustule, a robust carbon concentrating mechanism (CCM) for photosynthesis, and large vacuoles that allows them to store high amounts intracellular nutrients. While these strategies is generally recognized, the cellular and molecular mechanisms for these traits are not well understood. Leveraging modern tools in molecular biology and subcellular imaging, this study investigates the function of the evolutionarily conserved proton pump and pH regulation enzyme V-type H+ATPase (VHA) in the physiology of diatoms. The VHA proton pump was found to localize in the membranes of three distinct organelles in the marine diatom Thalassiosira pseudonana including silica deposition vesicles (SDVs), chloroplasts, and vacuoles. In the SDV, VHA promotes silica biomineralization by maintain acidic pH and helps traffic the structural proteins which guide frustule morphology. This investigation also revealed that vacuole play a role in translocating VHA and donating membrane to the SDV. In chloroplasts, VHA was found to support a carbon concentrating mechanism (CCM) to enhance photosynthesis. The activity of VHA in the CCM was found to respond dynamically to irradiance and DIC, and was also confirmed in the coccolithophore Emiliania huxleyi, but not in the chlorophyte Chlorella. Collectively, these results suggest that a VHA mediated CCM maybe a conserved mechanism of secondary endosymbiosis of red algal symbionts. The VHA proton pump was prominently found in diatom vacuoles at various stages of the cell. Inhibition of VHA increased pH in the vacuole, suggesting it actively pumps protons to support the various functions of the vacuole including nutrient storage and generating buoyancy for the cell. In combination, these results show a vital role for pH regulation by VHA in multiple aspects of diatom biology which are key to their competitive success.