Cryptomonad

Cryptomonads, also called cryptophytes, are a group of algae and colorless flagellates, most of which contain plastids. Phycologists have traditionally treated them as a distinct algal division, Cryptophyta. They are widespread in freshwater and also turn up in marine and brackish environments. Individual cells measure roughly 10–50 μm, are flattened, and feature a groove or pocket at the front. Two slightly unequal flagella sit at the edge of this pocket. Some species can mixotrophically combine photosynthesis with ingestion of other organisms. The group splits into two classes: the heterotrophic Goniomonadea and the phototrophic Cryptophyceae. These two classes share three structural traits: a periplast (a specialized cell covering), ejectisomes with a secondary scroll, and mitochondria with flat, tubelike cristae.

A key feature of cryptomonads is their ejectosomes—extrusomes made of two connected spiral ribbons held under tension. When the cell is stressed mechanically, chemically, or by light, these ejectosomes discharge, sending the cell on a zigzag path away from the disturbance. Large ejectosomes, visible with a light microscope, are linked to the pocket; smaller ones lie beneath the periplast. Except for the entirely plastid-free Goniomonadea and *Cryptomonas paramecium* (formerly *Chilomonas paramecium*), which has leucoplasts, cryptomonads possess one or two chloroplasts. These chloroplasts contain chlorophylls a and c, along with phycobiliproteins and other pigments, giving them colors ranging from brown and red to blueish-green. Each chloroplast is wrapped in four membranes, and between the middle two membranes sits a reduced nucleus called a nucleomorph. This arrangement indicates the plastid came from a eukaryotic symbiont—genetic evidence points to a red alga. However, cryptomonad plastids differ markedly from typical red algal plastids: their phycobiliproteins are confined to the thylakoid lumen and exist only as phycoerythrin or phycocyanin. In *Rhodomonas*, the crystal structure of phycoerythrin has been resolved to 1.63 Å, and its alpha subunit shows no similarity to any other known phycobiliprotein.

Some cryptomonads, like *Cryptomonas*, can form palmelloid stages—clumps embedded in mucus—but they readily break free to become free-living flagellates again. Certain *Cryptomonas* species also produce immotile microbial cysts with rigid cell walls, allowing them to survive unfavorable conditions. The flagella are inserted parallel to each other and are covered with bipartite hairs called mastigonemes, which are made inside the endoplasmic reticulum and transported to the cell surface. Small scales may also appear on the flagella and cell body. Mitochondria have flat cristae, mitosis is open, and sexual reproduction has been documented.

The first recorded mention of cryptomonads came from Christian Gottfried Ehrenberg in 1831, while he was studying infusoria. Botanists later classified them as a separate algal group (class Cryptophyceae or division Cryptophyta), while zoologists placed them among the flagellate protozoa as order Cryptomonadina. Some early classifications grouped cryptomonads with dinoflagellates as Pyrrhophyta, based on their seemingly similar pigmentation. Cryptomonad chloroplasts are actually closely related to those of heterokonts and haptophytes, leading Cavalier-Smith to unite the three groups as Chromista. But major differences in cell organization suggest these three lineages acquired plastids independently, making chromists polyphyletic. Molecular evidence supports the idea that cryptomonads were originally heterotrophic and later gained chloroplasts. Parfrey et al. and Burki et al. placed Cryptophyceae as a sister clade to green algae, or to green algae plus glaucophytes. The likely sister group to cryptomonads is the kathablepharids (also called katablepharids), flagellates that also have ejectisomes. Cavalier-Smith proposed the name Cryptophyta in 1986. One suggested classification groups the genera as follows: (1) *Cryptomonas*, (2) *Chroomonas*/*Komma* and *Hemiselmis*, (3) *Rhodomonas*/*Rhinomonas*/*Storeatula*, (4) *Guillardia*/*Hanusia*, (5) *Geminigera*/*Plagioselmis*/*Teleaulax*, (6) *Proteomonas sulcata*, (7) *Falcomonas daucoides*.