The Photosynthetic Adventure of Paulinella Spp.

Paulinella photosynthetic species are unicellular, silica shell-forming amoebas classified into the supergroup Rhizaria. They crawl at the bottom of freshwater and brackish environments with the help of filose pseudopodia. These protists have drawn the attention of the scientific community because of two photosynthetic bodies, called chromatophores, that fill up their cells permitting fully photoautotrophic existence. Paulinella chromatophores, similarly to primary plastids of the Archaeplastida supergroup (including glaucophytes, red algae as well as green algae and land plants), evolved from free-living cyanobacteria in the process of endosymbiosis. Interestingly, these both cyanobacterial acquisitions occurred independently, thereby undermining the paradigm of the rarity of endosymbiotic events. Chromatophores were derived from α-cyanobacteria relatively recently 60-140 million years ago, whereas primary plastids originated from β-cyanobacteria more than 1.5 billion years ago. Since their acquisition, chromatophore genomes have undergone substantial reduction but not to the extent of primary plastid genomes. Consequently, they have also developed mechanisms for transport of metabolites and nuclear-encoded proteins along with appropriate targeting signals. Therefore, chromatophores of Paulinella photosynthetic species, similarly to primary plastids, are true cellular organelles. They not only show that endosymbiotic events might not be so rare but also make a perfect model for studying the process of organellogenesis. In this chapter, we summarize the current knowledge and retrace the fascinating adventure of Paulinella species on their way to become photoautotrophic organisms.