CO2_CMΨ

Coordination institution : CNRS

Project leader : Hélène Launay
Project duration : 60 months | January 1st  2025 → December 31th 2029

Grant : 1 116 195 €

 

Insitutional partnership :  CEA 

 

 

The bioconversion of CO₂ by RuBisCO reaches 10¹⁴ kilograms per year on Earth and fuels all carbon fluxes within trophic networks. The carboxylation activity of RuBisCO on ribulose‑1,5‑bisphosphate (RuBP) competes with its oxygenation activity, and the O₂/CO₂ ratio varies greatly across space and time in aquatic ecosystems. CO₂ diffuses less efficiently in water than O₂, and most dissolved inorganic carbon (DIC) is present as bicarbonate. Aquatic photosynthetic microorganisms therefore possess CO₂‑concentrating mechanisms (CCMs) that channel a flux of CO₂ toward the RuBisCO active site, favouring carboxylation over oxygenation.

CCMs have two defining features:

• Bicarbonate transporters and carbonic anhydrases enable microorganisms to mobilise dissolved bicarbonate. A second key feature is that RuBisCO is not homogeneously distributed within the cytoplasm of cyanobacteria or within the plastids of photosynthetic eukaryotes. Instead, RuBisCO enzymes are packed at extremely high concentrations inside biomolecular condensates: carboxysomes, which are semi‑crystalline structures in cyanobacteria, or pyrenoids, which are semi‑rigid in diatoms and liquid‑like in green microalgae. These condensates differ in molecular composition and exhibit distinct physicochemical properties, yet they assemble enzymatic complexes with similar functions.
• These biocondensates have evolved convergently, and the prevailing hypothesis is that such supramolecular organisations enhance RuBP carboxylation at the expense of oxygenation, providing a major advantage to photosynthetic microorganisms.

The objective of the project is to test this hypothesis across multiple scales—culture, cellular, and molecular—and in three model organisms: the heterocystous cyanobacterium Anabaena PCC 7120, the centric diatom Phaeodactylum tricornutum, and the green microalga Chlamydomonas reinhardtii. Understanding how these supramolecular organisations that enable CO₂ concentration and efficient fixation have evolved convergently is essential for explaining their ecological performance.

The project aims to investigate, at different scales, the physicochemical properties of these micro‑compartments and their contribution to CO₂ acquisition and fixation. At the molecular scale, in‑vitro reconstitutions of these assemblies will allow comparison of their physicochemical properties, particularly how CO₂‑fixation metabolites diffuse within these environments. At the cellular scale, monitoring the biogenesis and dissociation of these micro‑compartments—together with measurements of CO₂‑fixation efficiency in different photosynthetic organisms—will make it possible to compare their dependence on CCMs.

A distinctive feature of the project is the use of identical experimental approaches across three species that are ecophysiologically and biotechnologically relevant, yet phylogenetically distant: the heterocystous cyanobacterium Anabaena PCC 7120, the centric diatom P. tricornutum, and the green alga C. reinhardtii. Identifying these molecular determinants will help enhance CO₂ fixation by aquatic unicellular organisms and inspire bio‑based CO₂‑capture solutions.