Rapaza viridis is the first known mixotroph (an organism that combines photosynthesis and ingestion of food) and kleptoplastic species within the phylum Euglenozoa.

Due to its unique mode of nutrition and phylogenetic position, Rapaza viridis is considered an evolutionary step between phagotrophs and phototrophs with permanent chloroplasts.

[1] The genus Rapaza was circumscribed in 2012 by protistologists Aika Yamaguchi, Naoji Yubuki and Brian S. Leander, on a study published in the journal BMC Evolutionary Biology.

It was created to describe a population of euglenids isolated in 2010 from marine water samples collected at a tide pool in Pachena Beach, British Columbia, Canada.

After cultivation, various growth experiments and molecular phylogenetics, the microorganisms were shown to belong to the phototrophic euglenids (Euglenophyceae) and were described as the species Rapaza viridis.

[1] The genus was defined as including flexible mixotrophic euglenids with two unequal flagella, a minimum of one chloroplast with three membranes and pyrenoids penetrated by stacks of thylakoids, a robust stigma, a paraflagellar swelling, and a feeding pocket supported by microtubules.

The species was further defined by the length and width measurements of the cells and flagella, the presence of paramylon grains in the cytoplasm, 16 pellicle strips, four rows of microtubules supporting the feeding pocket, and Tetraselmis as its preferred prey.

He defined these three taxa as containing phagotrophic photosynthetic eukaryote-eating (eukaryovorous) euglenids that swim in the water column instead of gliding on the substrate, and present four rows of microtubules supporting the feeding pocket instead of one as in Euglenophycidae.

[2] His classification scheme was neglected by other authors in favour of treating the entirety of Euglenida (Euglenophyceae plus a variety of heterotrophic flagellates) as a class, and deprecating the use of Rapazia as a subclass.

[5] In the same sample where the species was discovered, the microorganism consumed native Tetraselmis algae and grew to distinctly larger and brighter cells in their presence, digesting them completely in the course of around 12 hours.

Then, the stolen plastids ('kleptoplasts') are transformed until they resemble canonical plastids: they are divided into smaller fragments by fission, the green algal pyrenoid surrounded by starch disappears, smaller pyrenoids penetrated by thylakoids are formed, the starch grains gradually disappear, and a three-membrane envelope is displayed (two membranes from the original chloroplast and one membrane belonging to the food vacuole).

Due to these discoveries, the leading hypothesis is that the last common ancestor of all Euglenophyceae was not a phototroph, but an alga-eating phagotroph without permanent plastids that could have exhibited kleptoplasty, much like Rapaza viridis.

[4] In addition to kleptoplast-targeted proteins, Rapaza viridis obtained a nucleus-coded nitrate reductase through horizontal gene transfer from ancient algal prey.

This enzyme, known as RvNaRL, is a crucial step of metabolic integration in the early stages of secondary endosymbiosis towards permanent phototrophy.