Monospecific genus of predatorial algae
Rapaza viridis (Latin for 'green grasper') is a species of single-celled algae within the Euglenophyceae. It is the only species within the genus Rapaza, family Rapazidae and order Rapazida. It was discovered in a tide pool in British Columbia along with its prey, Tetraselmis. It is the first example of a mixotrophic euglenid, and requires both photosynthesis and preying on microalgae to survive. Due to its unique mode of nutrition and phylogenetic position, it is considered an evolutionary step between phagotrophs and phototrophs.
Etymology
The genus name Rapaza comes from Latin rapax 'seizing' and 'grasping', in reference to the feeding behavior of the cells. The species epithet viridis, meaning 'green', references the color of the chloroplasts and algal prey cells in the process of being digested. Together, the binomial name means 'green grasper' in Latin.[1]
Taxonomy
The genus Rapaza was described 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 accommodate a species of euglenid, Rapaza viridis, isolated in 2010 from marine water samples collected at a tide pool in Pachena Beach, British Columbia, Canada. After cultivation, various growth experiments and DNA analysis, the microorganism was shown to belong to the phototrophic euglenids (Euglenophyceae). The new species had a functioning chloroplast but also exhibited phagotrophy, making it the first and only example of mixotrophic euglenids.[1]
In 2016, protozoologist Thomas Cavalier-Smith assigned this genus to several monotypic higher-level taxa: family Rapazidae, order Rapazida and subclass Rapazia within the then-accepted class Euglenophyceae.[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. As of 2021, only Rapazidae and Rapazida are accepted taxa.[3]
Description
Rapaza viridis is a unicellular flagellate, a type of protist that is capable of swimming through two flagella unequal in length and in movement. The longer flagellum is around twice the length of the shorter flagellum, but has the same thickness. The cells are slender with a tapered posterior end, measuring approximately 10–40 μm long and 3–15 μm wide. Like other euglenids, cells are surrounded by a pellicle composed of 16 protein strips arranged helically below the cell membrane. As in other spirocutes (i.e. flexible euglenids), cells of R. viridis are capable of 'metaboly' or 'euglenoid movement', which allows for active peristaltic deformation of the cell shape. In addition, it is a species of algae due to the presence of at least one disc-shaped chloroplast surrounded by three membranes and containing thylakoids organized in stacks of two. Pyrenoids are also present within the chloroplasts. Its feeding apparatus consists of one rod built of four rows of microtubules and a feeding pocket. There is a stigma composed of 1 to over 10 pigmented particles. The cytoplasm contains ellipsoid paramylon grains.[1][3]
Nutrition
LM of
R. viridis cells that were starved for 7 days (left) and cells that were fed 24 hours earlier (right). Arrow:
stigma, double arrowhead: chloroplast. Scale bar: 10 μm.
Rapaza viridis is an obligate mixotroph that is capable of both phagotrophy and phototrophy.[4] It is a eukaryovorous species which requires feeding on microalgae to survive, but also requires a source of light to remain alive through photosynthesis.[1]
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. When starved from the algae, cells of R. viridis became smaller and colorless, retaining at least one healthy chloroplast within its cytoplasm. During growth experiments, cells of R. viridis were exposed to a variety of different algae (e.g., Navicula, Pycnococcus, Dunaliella, Scrippsiella and non-native strains of Tetraselmis) while starved from the Tetraselmis strain that the species was found with. However, the mixotroph rejected all other preys, and could not survive for longer than 35 days without being exposed to that specific algal strain. On the other hand, even under constant supply of that strain, the species could not survive for more than a week in the absence of a light source for photosynthesis.[1]
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 it swimming backward in a spiral pattern or rotate rapidly. The euglenid can gradually peel away the cell wall 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 cell.[1]
Evolution
According to phylogenetic analyses, Rapaza viridis is the sister group to all other Euglenophyceae, making it an intriguing candidate for the study of euglenid evolution as the earliest-diverging lineage after the endosymbiotic event where the euglenophycean chloroplast was acquired. This phylogenetic position is consistent with its place as an evolutionary step between the completely phagotrophic peranemids and the phototrophic Euglenophyceae, because mixotrophy is considered the transitional state during the establishment of the endosymbiotic pray cell and the phagotrophic host cell.[1]
In addition, R. viridis is the first example of a eukaryovorous euglenid that is capable of swimming in the water column. This swimming pattern is only seen elsewhere in the Eutreptiales, which are completely phototrophic, while phagotrophic euglenids are only capable of gliding along the substrate. This, again, is consistent with the phylogenetic position as the living representative of an important transitional stage in euglenid evolution.[1]
