Thèse Repousser les Limites de la Photosynthèse Etude des Mécanismes de Photoprotection chez les Cyanobactéries Contenant de la Chlorophylle-F H/F - Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Biosphera - Biologie, Société, Ecologie & Environnement, Ressources, Agriculture & Alimentation École doctorale : Sciences du Végétal : du gène à l'écosystème Laboratoire de recherche : I2BC - Institut de Biologie Intégrative de la Cellule Direction de la thèse : Corinne RIVASSEAU Début de la thèse : 2026-10-01 Date limite de candidature : 2026-05-06T23:59:59 Ce projet explore comment la photosynthèse peut être étendue au-delà de ses limites habituelles en étudiant les mécanismes de photoprotection chez les cyanobactéries contenant de la chlorophyllef (Chl-f). Le projet porte sur Chroococcidiopsis thermalis, une cyanobactérie capable de croître sous lumière blanche ou rouge lointain en utilisant uniquement la Chl-a ou une combinaison de Chl-a et de Chl-f, contrairement à la plupart des plantes et des microalgues qui utilisent la Chl-a et la Chl-b. Nous étudierons les mécanismes de photoprotection en lumière rouge lointain en nous concentrant particulièrement sur l'antenne photosynthétique des cyanobactéries et sur la protéine caroténoïde orange (OCP), une protéine photoprotectrice clé chez les cyanobactéries. Dans un premier temps, nous étudierons les mécanismes de photoprotection sous lumière rouge lointain au niveau moléculaire. Ensuite, nous examinerons comment le système d'antenne photosynthétique est remodelé pendant la photoprotection sous lumière rouge lointain.
L'étudiant/étudiante mettra en oeuvre des techniques de biochimie, de spectroscopie de fluorescence avancées, de spectrométrie de masse et de cryo-microscopie électronique.
Ces travaux pourraient jeter les bases du développement de plantes cultivées contenant de la Chl-f, ce qui pourrait accroître la productivité agricole et les rendements des cultures.
Photosynthesis is the foundation of life on Earth. It is constrained to the visible part of the light spectrum by the chlorophyll-a molecules that can absorb only in this range. The discovery of chlorophyll-f challenged this paradigm demonstrating the ability to perform oxygenic photosynthesis using less energetic photons. This breakthrough expands the energy limits of photosynthesis and opens up possibilities for enhancing light-use efficiency in both natural and engineered systems.

Ultimately, this research could lay the foundation for engineering crop plants with Chl-f, potentially increasing agricultural productivity and crop yields.
This project aims to uncover the mechanisms of photoprotection in Chl-f-containing cyanobacteria, with the broader goal of improving light-use efficiency. This investigation is structured at two complementary levels. First, at the mechanistic level (Task 1), it combines (i) a comprehensive, unbiased approach to identify the proteins involved in photoprotection, and (ii) a targeted analysis of the orange carotenoid protein (OCP), a key cyanobacterial photoprotective protein responsible for energy quenching. Second, at the organizational level (Task 2), the work seeks to elucidate the structure of the light-harvesting antennae and to assess their potential remodeling during adaptation to far-red light. Task 1: Investigating the photoprotection strategies in far-red light and OCP-Mediated Quenching in Chroococcidiopsis thermalis
Background: In this project, we will use the cyanobacterium Chroococcidiopsis thermalis (hereafter C. thermalis), which is capable of growing under different light conditions using either Chl-a alone or a mixture of Chl-a and Chl-f. A BLAST analysis has identified a gene homologous to the OCP gene in C. thermalis, suggesting that OCP-mediated photoprotection may be functional in this species. However, other photoprotection strategies could take place in far-red light.
Methods and work programme of Task 1: To test the OCP-mediated photoprotection, we will grow C. thermalis under both white light and far-red light conditions and measure chlorophyll fluorescence. A reduction in fluorescence would indicate active OCP-mediated energy quenching, in analogy with other known cyanobacteria models. Additionally, we will perform immunoblotting to confirm the presence of the OCP protein in C. thermalis.
Others molecular actors may be involved in photoprotection in these conditions. We will extract the proteome from C. thermalis grown in white light and far-red light in standard growth conditions and high light (photoprotection conditions) and we will identify the proteome composition using a state-of-the-art microLC shotgun methodology coupled with tandem mass spectrometry. This method will highlight new proteins involved in photoprotection in far-red light, and the role of the most promising ones will be investigated.

Task 2: Remodelling of the photosynthetic apparatus under far-red light
Background: Photosynthetic efficiency relies on antenna proteins that capture and transfer light energy to the reaction centers. In cyanobacteria, the primary antenna complexes are phycobilisomes (PBs), a large protein-pigment assemblies optimized for light harvesting. To enable Chl-f-driven photochemistry, both Chl-f-containing Photosystem II and Photosystem I must be coupled with modified PBs capable of harvesting far-red light. Several cyanobacterial species have been shown to possess a specific operon encoding far-red-absorbing phycobiliproteins (far-red-PBs), which are incorporated into specialized PBs under infrared light conditions.
In this task, we aim to investigate how the photosynthetic antenna system is remodeled during the transition from white light to far-red light and describe how OCP or other antenna quenchers bind far-red-PBs. This will be carried out through a time-course experiment.
Methods and work programme: PBs will be isolated from C. thermalis cells grown under white light and at various time points after shifting to far-red light. The first time and the last time point will be expose to high light to induce photoprotection mechanisms. PBs will be purified using sucrose gradient ultracentrifugation. Each gradient fraction will be analyzed by fluorescence emission spectroscopy at 77K to assess pigment composition, especially changes in chromophore types. The protein components of PBs will be characterized by SDS-PAGE followed by mass spectrometry to identify changes in PB subunit composition over time. Cryo-electron microscopy will be used to resolve the structural organization of PBs at different time points during the light transition, providing insight into their remodeling dynamics. We will attempt to detect OCP association with PBs using immunoblotting and PBs quenching by RT fluorescence.

Nous recherchons un étudiant motivé ayant une formation en chimie ou biologie avec un intérêt pour la photosynthèse.