Euglenozoan kleptoplasty illuminates the early evolution of photoendosymbiosis

Abstract

Kleptoplasts are distinct among photosynthetic organelles in eukaryotes (i.e, plastids) because they are routinely sequestered from prey algal cells and function only temporarily in the new host cell. Therefore, the hosts of kleptoplasts benefit from photosynthesis without constitutive photoendosymbiosis. Here, we report that the euglenozoanRapaza viridishas only kleptoplasts derived from a specific strain of green alga,Tetraselmissp., but no canonical plastids like those found in its sister group, the Euglenophyceae.R. viridisshowed a dynamic change in the accumulation of cytosolic polysaccharides in response to light–dark cycles, and13C isotopic labeling of ambient bicarbonate demonstrated that these polysaccharides originate in situ via photosynthesis; these data indicate that the kleptoplasts ofR. viridisare functionally active. We also identified 247 sequences encoding putative plastid-targeting proteins and 35 sequences of presumed kleptoplast transporters in the transcriptome ofR. viridis. These genes originated in a wide range of algae other thanTetraselmissp., the source of the kleptoplasts, suggesting a long history of repeated horizontal gene transfer events from different algal prey cells. Many of the kleptoplast proteins, as well as the protein-targeting system, inR. viridiswere shared with members of the Euglenophyceae, providing evidence that the early stages in the endosymbiotic origin of euglenophyte plastids also involved kleptoplasty.

Nitrate Assimilation Underlying Kleptoplasty

“…Rapaza viridis depends the photosynthesis of the kleptoplast to obtain organic carbon required for its biological activity (Karnkowska et al 2022). Somewhat unusually, R. viridis does not appear to digest the cytoplasm of Tetraselmis sp., a chloroplast donor ingested by phagocytosis; moreover, it does not exhibit any other heterotrophic activity (no phagocytosis of other objects or osmotrophic growth).…”

“…Interestingly, however, Rv NaRL was not included in this clade, so we could conclude that R. viridis acquired this gene by a horizontal transfer independent of Euglenophyceae. Indeed, the acquisition of various exogenous genes by horizontal transfers to R. viridis was thought to have facilitated the evolution of kleptoplasty in this organism (Karnkowska et al 2022). The same is likely true for the acquisition of Rv NaRL, which was a key factor in allowing R. viridis to evolve into a kleptoplasty-based photoautotroph.…”

“…Cultures of R. viridis were supplied with three times as many cells of the kleptoplast donor Tetraselmis sp. (1:3 ratio) upon transfer to fresh medium every 2 weeks; however, the latter were usually completely consumed within 12 hours, so that the former cultures were essentially maintained in a monospecific state (Karnkowska et al 2022). The experimental media used in the present study were f/2 medium-based compositions containing only nitrate, only ammonium, or neither.…”

“…The present study investigated the utilization of inorganic nitrogen by Rapaza viridis (Yamaguchi et al 2012, Karnkowska et al 2022), a phagotrophic euglenid that exhibits kleptoplasty. The kleptoplasty of R. viridis confers phototrophic physiology to this flagellate derived from a heterotrophic lineage.…”

“…The thus-obtained kleptoplast is exploited as if it were its own actual organelle for photosynthesis. Subsequently, 13 C isotope labeling experiments showed that inorganic carbon was taken up and incorporated into the polysaccharide grains formed in the cytosol of R. viridis , which accumulate only in the light phase and disappear in the dark phase under light–dark cycles during culture (Karnkowska et al 2022). The cells of R. viridis multiply by repeated cell division for approximately one week during the period following kleptoplast acquisition.…”