More than 1 Ga ago, an ancient eukaryote became photosynthetic by engulfing a cyanobacterium that evolved into the first plastid. The identity of that cyanobacterium was a mystery until we recently found strong phylogenomic evidence that it belonged to the Gloeomargaritales, a newly discovered deep-branching cyanobacterial group. Yet, why this original endosymbiosis was so successful remains unknown. We hypothesize that it was due to the unusual capacity of Gloeomargaritales to produce intracellular carbonates which allowed buffering the pH increase concomitant with photosynthetic C fixation before the host evolved active pH regulation.
With PLAST-EVOL, we plan to test this hypothesis and gain insight into the origin and subsequent evolution of plastids by:
1) characterizing the physiology of Gloeomargarita lithophora, the only Gloeomargaritales species isolated so far;
2) obtaining new genomic data from diverse Gloeomargaritales by culture, metagenomic and single-cell approaches;
3) studying the impact of cyanobacterial genes endosymbiotically transferred to the host algal and plant nuclear genomes; and
4) retracing the evolutionary history of these genes in secondary and tertiary endosymbioses.