Abstract :
[en] One of the most diverse and widespread clades of secondary endosymbiotic algae is represented by the stramenopiles clade, which include diatoms. Diatoms constitute a significant part of the total marine phytoplankton and contribute greatly to the global carbon assimilation. However, most of the research has been focused on the study of primary endosymbiosis algae leading to a disparity of knowledge between primary and secondary algae. The relatively recent regain of interest in the study of stramenopiles has led to the discovery of new specific photosynthetic regulation processes, a consequence of secondary endosymbiosis combined with the environmental pressure. In this regard, we have studied the photosynthetic regulation of diatoms under different physiological conditions. Photosynthetic regulation must ensure the right balance between the pathways that produce energy and those that consume it. This implies, among other things, the need to adjust the ATP / NADPH chloroplastic ratio to the requirements of the Calvin-Benson-Bassham cycle and the regulation of the amount of light energy absorbed by photosystems. In this regard, we have analyzed the main pathway that optimizes the ATP / NADPH ratio in diatoms, which consists in a close cooperation between the mitochondria and the chloroplast. This cooperation takes place via the export of reduced equivalents from the chloroplast to the mitochondria and through the import of ATP from the mitochondria to chloroplast. Consequently, the chloroplastic ATP / NADPH ratio increases and optimizes the Calvin-Benson-Bassham cycle activity. Because of this mitochondria importance in photosynthetic regulation in diatoms, we then studied the effects of the mitochondrial respiration inhibition on photosynthetic activity. We have characterized the photosynthetic activity in anoxic condition of two diatoms and compared to that of a secondary endosymbiosis green alga, Euglena gracilis. Firstly, we have shown that despite the lack of mitochondrial activity and in absence of oxygen, the photosynthetic activity is maintained. This implies the presence and the activity of other alternative electron pathways which do not depend on mitochondrial respiration or the presence of oxygen and that fulfill a similar role of the chloroplast-mitochondria interactions, namely by the increase of the ATP / NADPH ratio. Secondly, the regulation pathways that protect photosynthetic apparatus from photodamages also differ from primary algae. These differences in the regulatory processes lead in some species to the establishment of a non-photochemical quenching (NPQ) in the dark. This is the case in the stramenopile Phaeomonas sp. which develops a huge NPQ capacity in the dark. We have demonstrated that the establishment of this NPQ in the dark relies on the activation of deepoxidase, the enzyme responsible for this quenching, which in turn is due to the import of mitochondrial ATP. We then demonstrated that the NPQ regulation in this alga operates mainly via the regulation of epoxidase enzyme. In this work, we have identified new photosynthetic regulation pathways specific to secondary algae which contributes to increase general knowledge about the diversity of photosynthesis and its regulation.
