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Identification of a new actor of light acclimation in marine microalgae

Diatoms are remarkable for dissipating light energy when it exceeds the maximum photosynthetic electron transfer capacity, thereby avoiding the harmful effects of excess light. IRIG researchers have elucidated the role of the KEA3 protein in the model alga Phaeodactylum tricornutum and explain the strong presence of diatoms in diverse environments, where light acclimation is often a major determinant of growth and survival.

Published on 24 February 2022
The process of photosynthesis used by diatoms, among others, is a real biochemical prowess because it allows the conversion of electromagnetic energy - carried by photons - into chemical energy that can be directly used by the cells. Although light is a free and abundant source of energy, it is nevertheless intrinsically variable in intensity and quality, and this characteristic can cause irreparable damage to the photosynthetic apparatus. Photosynthetic organisms have developed protective mechanisms to dissipate excess light energy, namely the Non-Photochemical Quenching (NPQ) process. Despite the ecological importance of diatoms, the determinants of NPQ regulation are still poorly understood.

Diatoms are a remarkably diverse family of marine microorganisms, capable of living in fresh and salt water as well as on ice. Their abundance in the oceans gives them an essential role in the functioning of marine ecosystems, notably as carbon sinks and oxygen producers. Their ecological success is due, among other factors, to the exceptional flexibility of their photosynthetic apparatus, which allows them to adapt to changing light conditions.
Using a spectroscopy approach, IRIG researchers demonstrated that in diatoms there is a direct coupling between NPQ and the ΔpH component of the proton-motive force (PMF) generated by photosynthetic activity. In the model alga Phaeodactylum tricornutum, they identified the existence of a proton/potassium antiport called KEA3. Being an important regulator of the PMF, this antiport is essential for NPQ to be established under normal conditions. By combining genetics and photophysiology, the researchers observed that the antiport KEA3 is responsible for adapting the NPQ response to environmental conditions. Because KEA3 is an ion exchanger, it is able to convert the ΔpH component of PMF to ΔΨ without energy loss. Thus, the PMF, and ATP production, is maintained while providing good protection to the photosynthetic apparatus through the establishment of NPQ.
Although evolutionarily related to the KEA1-3 family found in plants, the diatom KEA3 protein contains a motif capable of binding calcium. The researchers have shown that this domain controls KEA3 activity in diatoms, providing a possible link between intracellular Ca2+ concentration and responses to rapid (minutes-long) or slow (e.g. daily) changes in the light environment..

Overall, the elucidation of the NPQ regulatory circuitry in diatoms as well as the role of the KEA3 protein may help explain the prosperity of the diatom family in diverse environments, where light acclimation is often a major determinant of growth and survival.

Localization of the fused KEA3 protein in Phaeodactylum tricornutum.
1 - light microscopy image of the diatom;
2 - auto-fluorescence of chlorophyll;
3 - fluorescence of the fused KEA3 protein;
4 - fusion of the images;
5 - fusion of the images on the light microscopy.
The proton-motive force corresponds to a proton gradient across a biological membrane that is both electrical (ΔΨ) and osmotic (ΔpH) in nature. This gradient is used by the transmembrane complex ATP synthase in chloroplasts and mitochondria to produce ATP, an energy molecule that is essential for carrying out the rest of the biochemical processes in the cell.
An antiport protein is a protein that allows the exchange of two molecules across the cell membrane.

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