Thesis presented November 30, 2020

Abstract:
Sunlight is the inextinguishable primary energy source on which all living beings eventually rely on for their metabolic needs. In spite of the many advantages of light utilization, it conveys heavy, inherent burdens: intermittency and variability. In order to survive, photosynthetic organisms must constantly reshape their light harvesting machinery, in particular by inducing non-photochemical quenching (NPQ) mechanisms. Controlling the balance between photosynthesis and photoprotection is achieved in higher plants and green algae by a fine interplay between the electrical and the osmotic components of the thylakoid proton motive force. Ion homeostasis has therefore a key role in photosynthesis. Fine-tuning the composition of the proton motive force by specific ion exchange is a means for photosynthetic organisms to regulate NPQ induction efficiently, i.e. minimizing energy leaks and preserving ATP synthesis.
Diatoms have a foremost ecological role in the global oceans, both as key contributors of the biogeochemical carbon pump and as feedstock for marine life. In diatoms, the molecular of NPQ are just starting to be identified. We have demonstrated, using the model organism Phaeodactylum tricornutum, that NPQ is univocally controlled by the pH component of the proton motive force. We were thus able to refine the current model for NPQ dynamics in P. tricornutum, with a preponderant role of the kinetic control of the xanthophyll cycle by pH.
We have identified the K⁺/H⁺ antiporter KEA3 as a major regulator of the proton motive force in diatoms. Using a set of CRISPR-Cas9 constructed mutants, we have determined that this transporter is able to modulate the lumen pH and hence to adapt the NPQ response according to environmental conditions. KEA3 converts the ΔpH component of the proton motive force into ΔΨ, thereby not leading to any energy loss, i.e. maintaining the ATP output.

Keywords:
diatom, photosynthesis, ion homeostasis, photoprotection