| Cryptomonads | |
|---|---|
 | |
| Rhodomonas salina | |
Scientific classification | |
| Domain: | Eukaryota |
| Clade: | Diaphoretickes |
| Clade: | CAM |
| Clade: | Pancryptista |
| Phylum: | Cryptista |
| Subphylum: | Rollomonadia |
| Superclass: | Cryptomonada Cavalier-Smith, 2004 stat. nov. 2015[1] |
| Classes & orders[1] | |
Thecryptomonads (orcryptophytes)[2] are a group ofalgae,[3] most of which haveplastids. They are traditionally considered adivision ofalgae amongphycologists, under the name ofCryptophyta.[4] They are common in freshwater, and also occur in marine and brackish habitats. Each cell is around10–50 μm in size and flattened in shape, with an anterior groove or pocket. At the edge of the pocket there are typically two slightly unequalflagella. Some may exhibitmixotrophy.[5] They are classified ascladeCryptomonada, which is divided into two classes: heterotrophicGoniomonadea and phototrophicCryptophyceae. The two groups are united under three shared morphological characteristics: presence of aperiplast,ejectisomes with secondary scroll, and mitochondrialcristae with flat tubules.[6] Genetic studies as early as 1994 also supported the hypothesis thatGoniomonas was sister to Cryptophyceae.[7] A study in 2018 found strong evidence that the common ancestor ofCryptomonada was an autotrophic protist.[8]
Cryptomonads are distinguished by the presence of characteristicextrusomes calledejectosomes, which consist of two connected spiral ribbons held under tension.[9] If the cells are irritated either by mechanical, chemical or light stress, they discharge, propelling the cell in a zig-zag course away from the disturbance. Large ejectosomes, visible under the light microscope, are associated with the pocket; smaller ones occur underneath theperiplast, the cryptophyte-specific cell surrounding.[10][11]
Except for the classGoniomonadea, which lacks plastids entirely,[12] andCryptomonas paramecium (previously calledChilomonas paramecium), which hasleucoplasts, cryptomonads have one or two chloroplasts. These containchlorophyllsa andc, together withphycobiliproteins and other pigments, and vary in color (brown, red to blueish-green). Each is surrounded by four membranes, and there is a reducedcell nucleus called anucleomorph between the middle two. This indicates that the plastid was derived from aeukaryotic symbiont, shown by genetic studies to have been ared alga.[13] However, the plastids are very different from red algal plastids: phycobiliproteins are present but only in the thylakoid lumen and are present only as phycoerythrin orphycocyanin. In the case ofRhodomonas, the crystal structure has been determined to 1.63 Å;[14] and it has been shown that the alpha subunit bears no relation to any other known phycobiliprotein.
A few cryptomonads, such asCryptomonas, can formpalmelloid stages, but readily escape the surrounding mucus to become free-living flagellates again. SomeCryptomonas species may also form immotilemicrobial cysts—resting stages with rigid cell walls to survive unfavorable conditions. Cryptomonad flagella are inserted parallel to one another, and are covered by bipartite hairs calledmastigonemes, formed within theendoplasmic reticulum and transported to the cell surface. Small scales may also be present on the flagella and cell body. Themitochondria have flatcristae, andmitosis is open;sexual reproduction has also been reported.
The first mention of cryptomonads appears to have been made byChristian Gottfried Ehrenberg in 1831,[15] while studyingInfusoria. Later, botanists treated them as a separatealgae group, class Cryptophyceae or division Cryptophyta, while zoologists treated them as theflagellateprotozoa order Cryptomonadina. In some classifications, the cryptomonads were considered close relatives of thedinoflagellates because of their (seemingly) similar pigmentation, being grouped as thePyrrhophyta. Cryptomonad chloroplasts are closely related to those of theheterokonts andhaptophytes, and the three groups were united by Cavalier-Smith as theChromista. However, the case that the organisms themselves are closely related was counter-indicated by the major differences in cell organization (ultrastructural identity), suggesting that the three major lineages assigned to the chromists had acquired plastids independently, and that chromists are polyphyletic. The perspective that cryptomonads are primitively heterotrophic and secondarily acquired chloroplasts, is supported by molecular evidence.[16] Parfrey et al. and Burki et al. placed Cryptophyceae as a sister clade to theGreen Algae,[17] or green algae plusglaucophytes.[18] The sister group to the cryptomonads is likely the kathablepharids (also referred to as katablepharids), a group of flagellates that also have ejectisomes.[19]
One suggested grouping is as follows: (1)Cryptomonas, (2)Chroomonas/Komma andHemiselmis, (3)Rhodomonas/Rhinomonas/Storeatula, (4)Guillardia/Hanusia, (5)Geminigera/Plagioselmis/Teleaulax, (6)Proteomonas sulcata, (7)Falcomonas daucoides.[20]
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