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Rapaza viridis (Latin for 'green grasper') is a species of single-celled flagellate within the Euglenophyceae, a group of algae. It is the only species within the genus Rapaza, family Rapazidae and order Rapazida. It was discovered in a tide pool in British Columbia and described in 2012.
Rapaza viridis is the first known mixotroph (an organism that combines photosynthesis and ingestion of food) and kleptoplastic species within the phylum Euglenozoa. It eats microalgae by engulfing them—a process called phagocytosis—and then uses the from these algae to perform photosynthesis, altering the chloroplasts' structure in the process. In particular, Rapaza viridis can only feed on Tetraselmis cells native to their original environment, and will reject any other prey.
Due to its unique mode of nutrition and phylogenetic position, Rapaza viridis is considered an evolutionary step between and with permanent chloroplasts. Scientists consider that the common ancestor of all Euglenophyceae (a group of algae) was similar to R. viridis. It likely stole chloroplasts from its prey—just like R. viridis—a behavior supported by the discovery of genes in Euglenophyceae that came from different types of algae through a process called horizontal gene transfer. After the divergence of R. viridis, the remaining Euglenophyceae acquired permanent from Pyramimonas.
The genus was defined as including flexible mixotrophic euglenids with two unequal flagella, a minimum of one chloroplast with three membranes and penetrated by stacks of , a robust stigma, a paraflagellar swelling, and a feeding pocket supported by . 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.
In 2016, American protozoologist Thomas Cavalier-Smith assigned this genus to several monotypic higher-level taxa: family Rapazidae, order Rapazida and subclass Rapazia within the class Euglenophyceae, leaving the remaining euglenophyceans (Euglenales and Eutreptiales) under a new subclass Euglenophycidae. 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. 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. As of 2021, only Rapazidae and Rapazida are accepted taxa.
Upon exposition to the native Tetraselmis strain, R. viridis cells enter a feeding frenzy: they capture algae with the anterior part of the cell and drag the prey, either swimming backward in a spiral pattern or rotating rapidly. The euglenid can gradually peel away the theca (cell covering) of Tetraselmis through repeated peristaltic euglenoid movement (or metaboly), and then engulf the naked prey cell, or engulf the cell with an intact theca and afterwards discharge the theca. The entire process takes between 5 and 40 minutes, but a single R. viridis individual can contain several ingested Tetraselmis cells.
However, subsequent studies revealed that R. viridis does not have canonical plastids. Instead, it extracts and temporarily retains the chloroplasts of its prey for its own use, a process known as kleptoplasty ('stealing of plastids'). After phagocytosis of the algal prey, its cell membrane is digested and the plastids are separated from the other cellular components, which are later excreted from the host cell. 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).
Rapaza viridis needs a regular influx of kleptoplasts, obtained through the phagocytosis of its prey. Without acquiring new kleptoplasts, the cells cannot survive for more than 35 days. During starvation, the remaining kleptoplasts are gradually degraded, and are formed to recycle intracellular substances.
Rapaza viridis is the first case of kleptoplasty within Euglenozoa. Particularly, its chloroplasts are obtained from the green alga Tetraselmis. Transcriptomic and genomic analyses revealed that there are genes encoded in the cell nucleus of R. viridis and other Euglenophyceae for plastid-targeted proteins acquired from chloroplasts of many different algae (including algae from the "red lineage", i.e. red algae and chromalveolate algae) through multiple ancient events of horizontal gene transfer. 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. This common ancestor horizontally acquired the protein targeting system from many algae after prolonged coexistence (from both kleptoplasty and predation). This targeting system could have been involved in the establishment of permanent plastids in the remaining Euglenophyceae, which originated from the green alga Pyramimonas. Additionally, Tetraselmis-derived genes are abundant in other Euglenophyceae, while Pyramimonas-derived genes are minor in Rapaza, meaning that the close association with Pyramimonas began after the divergence of Rapaza.
In addition to kleptoplast-targeted proteins, Rapaza viridis obtained a nucleus-coded nitrate reductase through horizontal gene transfer from ancient algal prey. Nitrate reductases are a key component of phototrophic organisms, since it allows for the assimilation of inorganic nitrogen, which heterotrophic organisms are not capable of. This enzyme, known as RvNaRL, is a crucial step of metabolic integration in the early stages of secondary endosymbiosis towards permanent phototrophy.
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