- WP1: Intracellular carbonates and pH regulation as key permissive traits for the origin of plastids.
We isolated new Gloeomargaritales, including a new thermophilic one, and identified their growth conditions.
We characterized a new protein, calcyanin, involved in cellular calcium homeostasis and intracellular carbonate precipitation in Gloeomargaritales.
- WP2: Increasing the sampling of plastid-related cyanobacteria and their genomes.
We isolated the first thermophilic species, Gloeomargarita ahousahtiae, and characterized its phenotype and genome sequence. We also sequenced the genome of co-isolated parasitic bacteria.
We also assembled 11 new Gloeomargaritales genome sequences from various metagenomes. Their analysis indicates that the ancestral Gloeomargaritales were freshwater thermophilic species that synthesized intracellular carbonates. This, combined with phylogenetic analysis of photosynthetic eukaryotes, suggests that these eukaryotes also originated in moderately hot freshwater environments, instead of marine environments as previously thought.
- WP3: Reconstructing endosymbiotic gene transfer patterns in primary and secondary plastid endosymbiosis.
We sequenced the genomes of seven unicellular red algae to study the origin of red secondary plastids and the associated endosymbiotic gene transfers (EGTs). We also isolated five new non-photosynthetic species closely related to two major clades of secondary algae (chlorarachniophytes, with green plastids, and ochrophytes, with red plastids).
We analyzed the EGT events linked to the primary and secondary endosymbioses using our enriched collection of genome sequences. We detected ~900 EGT candidates, significantly increasing the number of previously detected EGT and allowing to better characterize the impact of plastid endosymbiosis on the host genomes. We discovered cryptic red algal endosymbioses in algae carrying green secondary plastids (chlorarachniophytes and euglenids), which appear to have facilitated the later acquisition of green plastids.
We characterized new protist species that feed on unicellular algae and that are closely related to the large photosynthetic clade of the Ochrophyta (containing well-known algae such as diatoms and kelp). These new species are excellent host models to understand the first steps in secondary plastid endosymbioses.
- WP4: Dating primary and secondary plastid endosymbioses with improved calibration points.
We have worked on species from groups with reliable fossil records to include new calibration points in molecular dating analyses. We sequenced the genomes of two coralline red algae and one dasycladales green alga and included them in a large plastid genome dataset of primary and secondary photosynthetic eukaryotes. Our molecular dating results validate previous inferences based on nuclear genome data but show better precision narrower for the inferred dates and higher stability in the presence or absence of specific fossil calibrations. The ancestors of the groups containing secondary red algal plastids appear to have coexisted during the early Neoproteorozoic (more than 1 Ga), making possible the complex series of endosymbiotic events that gave rise to these lineages.
Our main results have been published in peer-reviewed journals and are available on open-access platforms (Europe PMC
https://europepmc.org/(s’ouvre dans une nouvelle fenêtre) and HAL
https://cnrs.hal.science/(s’ouvre dans une nouvelle fenêtre)). We also communicated them to the scientific community through international scientific meetings, and to the general public via public lectures and articles in the popular press -particularly those covering the expeditions in search of new cyanobacterial and algal species.
