ETUDE DE LA BIOGÉNÈSE ET DE L'ACTIVITÉ DES PLASTES CHEZ LES DIATOMÉES MARINES. // STUDY OF PLASTID BIOGENESIS AND ACTIVITY IN MARINE DIATOMS

Diatoms are stramenopiles unicellular microalgae that represent a major class of phytoplankton in both marine and fresh water environments (Pierella Karlusich et al., 2020) and contribute 20% of the annual global carbon fixation. Evolutionarily, they are rather distant from Archaeplastida, which originate from a primary endosymbiosis of a cyanobacterium into a heterotrophic protist. At variance, diatoms result from the capture of a red alga by an eukaryote that had previously captured a green alga (Falciatore et al, 2020). Consequently, diatom plastids significantly differ from those of Archaeplastida: they are surrounded by four membranes instead of two, contain stacks of three thylakoids that run through the entire organelle, and use specific pigments for light harvesting and photoprotection (chlorophyll a and c, beta-carotene, fucoxanthin and diadinoxanthin /diatoxanthin). We only begin to elucidate the sophisticated acclimation mechanisms allowing diatoms to maintain an efficient photosynthesis under highly variable environments (Bailleul et al., 2010; Bailleul et al., 2015) and explaining their ecological success. However, almost nothing is known about plastid biogenesis nor regulation of photosynthesis in diatoms (or any other photosynthetic eukaryotes derived from secondary endosymbiosis). Studies of photosynthetic mutants (mainly in Chlamydomonas and Arabidopsis) have been instrumental to decipher the rules of chloroplast biogenesis and gene expression in the green lineage. In a similar way, the host laboratory has developed Cyclotella cryptica as a novel model system for the study of diatom photosynthesis. This diatom combines the availability of genomic and genetic resources with the ability to grow both heterotrophically (using glucose as a source of reduced carbon) and autotrophically, an essential feature to identify and manipulate genes essential for photosynthesis. We also established recently an efficient protocol to routinely transform C. cryptica by microparticle bombardment, which has allowed the mutagenesis of nuclear genes with Cas9/sgRNA ribonucleoproteins, as well as plastid genome transformation through homologous recombination of transforming DNA. Based on these important results and tools, the proposed PhD project aims to:

i) Characterize functionally and molecularly the phenotype of already obtained photosynthetic mutants, i.e., ATP synthase Crispr/cas9 mutants of the ATPC nuclear gene, encoding the gamma subunit of plastidial ATP synthase (Jensen et al, 2025). Surprisingly, and although they are able of heterotrophic growth in the light in the presence of glucose, the mutants are unable to grow in the dark even in the presence of glucose, at variance with their counterparts in green algae. They thus could provide an entry point to study the remarkable resilience of diatoms to extreme environmental conditions, which are able to survive under prolonged darkness (month) during the polar night, under sea-ice, or buried under sediments.
ii) Isolate new mutants affecting the other photosynthetic complexes that have not been inactivated yet (Photosystem II, cytochrome b6f complex and Photosystem I). Both nucleus- and plastid-encoded genes will be targeted. Mutant lines (knockout, overexpressor and tagged) will be generated and analyzed for alterations of photosynthesis using a combination of molecular, biochemical approaches as well as, state-of-the- art methods of fluorescence and absorption spectroscopy, well mastered by the host laboratory. This will contribute to generate a catalogue of diatom photosynthesis mutant signatures, associated to specific phenotypes.
iii) To use the resources generated in i and ii) to start characterizing the mechanisms controlling assembly of photosynthetic complexes.
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Diatoms are stramenopiles unicellular microalgae that represent a major class of phytoplankton in both marine and fresh water environments (Pierella Karlusich et al., 2020) and contribute 20% of the annual global carbon fixation. Evolutionarily, they are rather distant from Archaeplastida, which originate from a primary endosymbiosis of a cyanobacterium into a heterotrophic protist. At variance, diatoms result from the capture of a red alga by an eukaryote that had previously captured a green alga (Falciatore et al, 2020). Consequently, diatom plastids significantly differ from those of Archaeplastida: they are surrounded by four membranes instead of two, contain stacks of three thylakoids that run through the entire organelle, and use specific pigments for light harvesting and photoprotection (chlorophyll a and c, beta-carotene, fucoxanthin and diadinoxanthin /diatoxanthin). We only begin to elucidate the sophisticated acclimation mechanisms allowing diatoms to maintain an efficient photosynthesis under highly variable environments (Bailleul et al., 2010; Bailleul et al., 2015) and explaining their ecological success. However, almost nothing is known about plastid biogenesis nor regulation of photosynthesis in diatoms (or any other photosynthetic eukaryotes derived from secondary endosymbiosis). Studies of photosynthetic mutants (mainly in Chlamydomonas and Arabidopsis) have been instrumental to decipher the rules of chloroplast biogenesis and gene expression in the green lineage. In a similar way, the host laboratory has developed Cyclotella cryptica as a novel model system for the study of diatom photosynthesis. This diatom combines the availability of genomic and genetic resources with the ability to grow both heterotrophically (using glucose as a source of reduced carbon) and autotrophically, an essential feature to identify and manipulate genes essential for photosynthesis. We also established recently an efficient protocol to routinely transform C. cryptica by microparticle bombardment, which has allowed the mutagenesis of nuclear genes with Cas9/sgRNA ribonucleoproteins, as well as plastid genome transformation through homologous recombination of transforming DNA. Based on these important results and tools, the proposed PhD project aims to:
i) Characterize functionally and molecularly the phenotype of already obtained photosynthetic mutants, i.e., ATP synthase Crispr/cas9 mutants of the ATPC nuclear gene, encoding the gamma subunit of plastidial ATP synthase (Jensen et al, 2025). Surprisingly, and although they are able of heterotrophic growth in the light in the presence of glucose, the mutants are unable to grow in the dark even in the presence of glucose, at variance with their counterparts in green algae. They thus could provide an entry point to study the remarkable resilience of diatoms to extreme environmental conditions, which are able to survive under prolonged darkness (month) during the polar night, under sea-ice, or buried under sediments.
ii) Isolate new mutants affecting the other photosynthetic complexes that have not been inactivated yet (Photosystem II, cytochrome b6f complex and Photosystem I). Both nucleus- and plastid-encoded genes will be targeted. Mutant lines (knockout, overexpressor and tagged) will be generated and analyzed for alterations of photosynthesis using a combination of molecular, biochemical approaches as well as, state-of-the- art methods of fluorescence and absorption spectroscopy, well mastered by the host laboratory. This will contribute to generate a catalogue of diatom photosynthesis mutant signatures, associated to specific phenotypes.
iii) To use the resources generated in i and ii) to start characterizing the mechanisms controlling assembly of photosynthetic complexes.
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Début de la thèse : 01/10/2026
WEB : http://www.ibpc.fr/UMR7141/en/home/

Public funding alone (i.e. government, region, European, international organization research grant)

Concours pour un contrat doctoral - SU

Sorbonne Université SIS (Sciences, Ingénierie, Santé)

515 Complexité du vivant

The proposed PhD, is based on multidisciplinary approaches and the candidate should have a strong training in molecular genetics and be interested by both genetic (classic and molecular), biochemical, functional (biophysical screens of photosynthetic mutants) and bioinformatics approaches.
The proposed PhD, is based on multidisciplinary approaches and the candidate should have a strong training in molecular genetics and be interested by both genetic (classic and molecular), biochemical, functional (biophysical screens of photosynthetic mutants) and bioinformatics approaches.