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Phytoplankton: cellular architecture and energy metabolism

​Drawing upon the 3D reconstruction of the cellular architecture of several phytoplankton families, researchers at the CEA-Irig have demonstrated that the physiological responses of these eukaryotes are associated with certain characteristics of their energy-producing organelles. This work thus provides new perspectives for the production of algal biomass, upstream of biotechnology applications.

Published on 3 June 2021

Phytoplankton play an essential role in sustaining life on earth. By converting CO2, sunlight and nutrients into biomass and oxygen, the unicellular organisms that make up phytoplankton account for about 50% of all primary production (of plant organic matter). They contribute to food webs and to the biological pump, a major component of the marine carbon cycle that permits the fixation of CO2 in the oceans through the sedimentation of microalgae. Understanding the cellular basis of the phytoplankton response to environmental changes could inspire promising new biotechnology developments.

Up until now, phytoplankton have been observed by light microscopy, 3D confocal microscopy and 2D electron microscopy. However, these techniques do not provide images with sufficient resolution for revealing the cellular ultrastructure of phytoplankton.

To address this, several teams from the Irig developed a 3D reconstruction of the cellular architecture of eukaryotes representative of phytoplankton, based on focused ion beam scanning electron microscopy (FIB-SEM).

The researchers could thus identify certain characteristics conserved during the evolution of plankton:

  • the volume of the principal organelles;
  • the volumetric ratio between the two energy-producing organelles (mitochondria and plastids);
  • and their proximity.

These observations also reveal how the cells adapt their physiological responses to various environmental conditions. In particular, there is a strong link between the mitochondria-plastid interaction and the ability to produce algal biomass under mixotrophy (i.e. the synergy between respiration and photosynthesis) in the extremophilic microalga Galdieria sulphuraria, the oleaginous microalga Microchloropsis gaditana, and the model diatom Phaeodactylum tricornutum. These results shed light on the cellular bases underlying the flexibility of phytoplankton energy metabolism.

 This work, supported by the ANR and the ERC, was conducted in collaboration with the Heinrich Heine University in Düsseldorf (Germany), the University of Liège (Belgium), Oxford Brookes University (Great Britain), the Fermentalg company, and the Total petrochemicals group.

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