Cryptomonad

Cryptomonads, also called cryptophytes, are a collection of algae and colorless flagellates, the majority of which contain plastids. Phycologists have traditionally classified them as a division of algae named Cryptophyta. They are frequently found in freshwater, but also live in marine and brackish environments. Individual cells measure roughly 10–50 μm, are flattened, and feature a groove or pocket at the front. Two flagella, slightly different in length, sit at the pocket’s edge. Some species can mixotrophy. The group splits into two classes: the heterotrophic Goniomonadea and the phototrophic Cryptophyceae. These two classes are linked by three shared features: a periplast, ejectisomes that have a secondary scroll, and mitochondrial cristae shaped like flat tubules. Genetic research from as early as 1994 also backed the idea that Goniomonas is the sister group to Cryptophyceae.

A key trait of cryptomonads is their ejectosomes—special extrusomes made of two connected spiral ribbons held under tension. When the cell is disturbed by mechanical, chemical, or light stress, these ejectosomes discharge, pushing the cell in a zigzag path away from the irritation. Large ejectosomes, visible with a light microscope, are linked to the pocket; smaller ones sit under the periplast, the cell’s distinctive outer covering. Apart from the class Goniomonadea, which has no plastids at all, and *Cryptomonas paramecium* (once called *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 a reduced cell nucleus called a nucleomorph lies between the middle two membranes. This setup shows the plastid came from a eukaryotic symbiont, which genetic studies have identified as a red alga. Still, these plastids differ greatly from those of red algae: phycobiliproteins are present but only in the thylakoid lumen, and only as phycoerythrin or phycocyanin. In *Rhodomonas*, the crystal structure has been resolved to 1.63 Å, and the alpha subunit has no connection to any other known phycobiliprotein.

Some cryptomonads, like *Cryptomonas*, can form palmelloid stages but easily slip out of the surrounding mucus to become free-living flagellates again. Certain *Cryptomonas* species can also create immotile microbial cysts—resting stages with rigid cell walls that help them survive tough conditions. The flagella of cryptomonads are inserted parallel to each other and are covered with bipartite hairs called mastigonemes, which are made inside the endoplasmic reticulum and moved to the cell surface. Small scales may also appear on the flagella and cell body. The mitochondria have flat cristae, mitosis is open, and sexual reproduction has been documented.

The first known mention of cryptomonads came from Christian Gottfried Ehrenberg in 1831 while he studied Infusoria. Later, botanists treated them as a separate algae group—class Cryptophyceae or division Cryptophyta—while zoologists classified them as the flagellate protozoa order Cryptomonadina. In some systems, cryptomonads were thought to be close relatives of dinoflagellates due to their seemingly similar pigmentation, and both were grouped as Pyrrhophyta. Cryptomonad chloroplasts are closely related to those of heterokonts and haptophytes, and Cavalier-Smith united these three groups as Chromista. However, the idea that the organisms themselves are closely related was contradicted by major differences in cell organization (ultrastructural identity), suggesting that the three main chromist lineages got their plastids independently and that chromists are polyphyletic. The view that cryptomonads were originally heterotrophic and later acquired chloroplasts is supported by molecular evidence. 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), a group of flagellates that also have ejectisomes. The name Cryptophyta was proposed in 1986 by Cavalier-Smith.

One suggested grouping is: (1) *Cryptomonas*, (2) *Chroomonas*/*Komma* and *Hemiselmis*, (3) *Rhodomonas*/*Rhinomonas*/*Storeatula*, (4) *Guillardia*/*Hanusia*, (5) *Geminigera*/*Plagioselmis*/*Teleaulax*, (6) *Proteomonas sulcata*, (7) *Falcomonas daucoides*.