Periodic Reporting for period 4 - BEAL (Bioenergetics in microalgae : regulation modes of mitochondrial respiration, photosynthesis, and fermentative pathways, and their interactions in secondary algae)
Reporting period: 2020-12-01 to 2021-11-30
During the course of eukaryote evolution, photosynthesis was propagated from primary eukaryotic algae to non-photosynthetic organisms through multiple secondary endosymbiotic events. ATP is the universal chemical energy carrier in living cells. In photosynthetic eukaryotes, it is produced by two major cellular processes: photosynthesis and respiration taking place in chloroplasts and mitochondria, respectively. Both processes support the production of biomass and govern gas (O2 and CO2) exchanges at earth scale. The regulation modes and interactions of respiration, and photosynthesis are fairly well understood in primary green algae. Conversely, the complex evolutionary history of secondary algae implies a great variety of original regulatory mechanisms that have been barely investigated to date. In the frame of this project, two species were mainly studied: the excavate Euglena gracilis and the dinoflagellates belonging to Symbiodiniaceae. The overall objectives of the project were to characterize the photosynthetic regulation modes by biophysical and biochemical approaches, and investigate interconnections between respiration, and photosynthesis. We first established a consolidated transcriptome of E. gracilis and found that respiratory and photosynthetic complexes of E. gracilis comprise atypical subunits that are responsible for unusual structural domains. We established methodologies to study in vivo bioenergetics parameters of Symbiodiniaceae in symbiosis with corals or jellyfish. In both species, we showed that there is a strong coupling between respiration and photosynthesis, but the nature of coupling may depend on the trophic state in E. gracilis or may vary in its extent among Symbiodiniaceae.
Mitochondria is the essential organelle for the production of cellular ATP in most eukaryotic cells, whether photosynthetic or not. It is widely studied, including in non-photosynthetic parasites such as trypanosomes, as a potential therapeutic target. We determined that many subunits of the respiratory complexes of Trypanosomes are shared with a non-parasitic photosynthetic species, Euglena gracilis. In this respect Euglena ancestor acquired its chloroplast by a secondary endosymbiotic event from a green primary eukaryotic alga. Structural analyzes have further highlighted unusual protein extra-structure associated with various respiratory complexes. Euglena gracilis is an historical microalga, being one the first protist described by microbiologists. Its photosynthesis has been broadly studied during the last century. One striking example is that Euglena was the organism studied by Melvin Calvin when he discovered the biochemical cycle which allow fixation of carbon dioxide into organic matter. In the frame of this project, we established a consolidated transcriptome of E. gracilis, which is a prerequisite to proper genome assembly and state of the art genetic manipulation. We then determined that most of the regulatory mechanisms of photosynthetic electron flow described so far in plants model organisms (Arabidopsis, Chlamydomonas) were not retained in Euglena gracilis. We then showed that original mechanisms (e.g. in terms of antennae composition or in terms of regulation of ATP/NADPH ratio) have been developed. In particular, we found that there is a strong coupling between respiration and photosynthesis. We then studied various strains of dinoflagellates originating not only from reef-building corals but also from soft octocorals, sea anemones, or even from giant clam from distant geographic locations (e.g. Hawai, Panama, Red sea, Japan). We determined the existence of three phenotyping groups with respect to regulatory mechanisms of photosynthetic electron transfer. This may have important ecological consequences due to the critical role of these processes on cellular energetic balance (including interactions between respiration and photosynthesis) and on the photoprotection capacity of cells. We Finally developed methodologies to study bioenergetics parameters in vivo in symbiosis (corals, anemones, jellyfish).
Due the biotechnological and evolutionary interest of E. gracilis, and to the difficulty to sequence its nuclear genome, the consolidated de novo transcriptome meta-assembly can be considered as a major step. Our studies on the structure and composition of bioenergetics complexes in Euglena also will be followed up by characterization of high-resolution structures.
Due to the major importance of Dinoflagellates Symbiodiniaceae representative in coral reef marine ecosystems, the methods that we developed to assess bioenergetics parameters on can now be applied on different symbiotic animals, thus paving the way to the study of symbiotic organisms energetic physiology in the field. In the long term, these developments will contribute to understanding the complexity of cellular energy systems and to achieving the knowledge required to exploit biodiversity from a biotechnological perspective.
Due to the major importance of Dinoflagellates Symbiodiniaceae representative in coral reef marine ecosystems, the methods that we developed to assess bioenergetics parameters on can now be applied on different symbiotic animals, thus paving the way to the study of symbiotic organisms energetic physiology in the field. In the long term, these developments will contribute to understanding the complexity of cellular energy systems and to achieving the knowledge required to exploit biodiversity from a biotechnological perspective.