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|Scientific classification |
(incertae sedis within Eukaryota)
Margulis & Schwartz 1998 ex Cavalier-Smith 2004 em. Cavalier-Smith 2015
|Classes & Orders|
The cryptomonads (or cryptophytes) are a group of algae, most of which have plastids. They are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10–50 μm in size and flattened in shape, with an anterior groove or pocket. At the edge of the pocket there are typically two slightly unequal flagella.
Cryptomonads are distinguished by the presence of characteristic extrusomes called ejectosomes or ejectisomes, which consist of two connected spiral ribbons held under tension. 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 the periplast, the cryptophyte-specific cell surrounding.
Except for Chilomonas, which has leucoplasts, and Goniomonas (formerly Cyathomonas) which lacks plastids entirely, cryptomonads have one or two chloroplasts. These contain chlorophylls a and c, together with phycobiliproteins and other pigments, and vary in color (brown, red to blueish-green). Each is surrounded by four membranes, and there is a reduced cell nucleus called a nucleomorph between the middle two. This indicates that the plastid was derived from a eukaryotic symbiont, shown by genetic studies to have been a red alga. 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 or phycocyanin. In the case of "Rhodomonas" the crystal structure has been determined to 1.63Å; and it has been shown that the alpha subunit bears no relation to any other known phycobiliprotein.
A few cryptomonads, such as Cryptomonas, can form palmelloid stages, but readily escape the surrounding mucus to become free-living flagellates again. Some Cryptomonas species may also form immotile microbial 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 called mastigonemes, formed within the endoplasmic reticulum and transported to the cell surface. Small scales may also be present on the flagella and cell body. The mitochondria have flat cristae, and mitosis is open; sexual reproduction has also been reported.
The first mention of cryptomonads appears to have been made by Christian Gottfried Ehrenberg in 1831, while studying Infusoria. Later, botanists treated them as a separate algae group, class Cryptophyceae or division Cryptophyta, while zoologists treated them as the flagellate protozoa order Cryptomonadina. In some classifications, the cryptomonads were considered close relatives of the dinoflagellates because of their (seemingly) similar pigmentation, being grouped as the Pyrrhophyta. There is considerable evidence that cryptomonad chloroplasts are closely related to those of the heterokonts and haptophytes, and the three groups are sometimes united as the Chromista. However, the case that the organisms themselves are closely related is not very strong, and they may have acquired plastids independently. Currently they are discussed to be members of the clade Diaphoretickes and to form together with the Haptophyta the group Hacrobia. Parfrey et al. and Burki et al. placed Cryptophyceae as a sister clade to the Green Algae.
One suggested grouping is 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.
- Superclass Cryptomonada Cavalier-Smith 2004 sta. n.
- Class Goniomonadea Cavalier-Smith 1993 [Cyathomonadea Cavalier-Smith 1989]
- Class CryptophyceaeFritsch 1937 [Cryptomonadea Stein 1878 emend. Schoenichen 1925]
- Order Tetragonidiales Kristiansen 1992
- Order Pyrenomonadales Novarino & Lucas 1993
- Order Cryptomonadales Pascher 1913
- Family ?Butschliellaceae Skvortzov 1968
- Family ?Cryptochrysidaceae Pascher 1931
- Genus Cryptochrysis Pascher 1911
- Family ?Hilleaceae Pascher 1967
- Family ?Pleuromastigaceae Bourrelly ex Silva 1980
- Genus Proteomonas Hill & Wetherbee 1986
- Family Chroomonadaceae Clay, Cugrens & Lee 1999 [Hemiselmidaceae Butcher 1967; Senniaceae Skuja 1948; Planonephraceae Christensen 1967]
- Family Baffinellaceae Daugbjerg & Norlin 2018
- Genus Baffinella Norlin & Daugbjerg 2018
- Family Geminigeraceae Clay, Cugrens & Lee 1999
- clade Guillardia group
- Family Cryptomonadaceae Ehrenberg 1831 [Campylomonadaceae Clay, Kugrens & Lee 1999]
- Genus Cryptomonas Ehrenberg 1832 [Campylomonas Hill 1991]
- Genus Cryptella Pascher 1929
- Genus Cryptochloris Schiller 1925
- Genus Cyanomastix Lackey 1936
- Genus Isoselmis Butcher 1967
- Genus Kisselevia Skvortzov 1969
- Genus Meyeriella Skvortzov 1968
- Genus Olivamonas Skvortzov 1969
- Genus Protocryptomonas Skvortzov 1969 ex Bicudo 1989
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- Novarino, G. (2012). "Cryptomonad taxonomy in the 21st century: The first 200 years". Phycological Reports: Current Advances in Algal Taxonomy and its Applications: Phylogenetic, Ecological and Applied Perspective: 19–52. Retrieved 2018-10-16.
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- Burki, Fabien; Kaplan, Maia; Tikhonenkov, Denis V.; Zlatogursky, Vasily; Minh, Bui Quang; Radaykina, Liudmila V.; Smirnov, Alexey; Mylnikov, Alexander P.; Keeling, Patrick J. (2016-01-27). "Untangling the early diversification of eukaryotes: a phylogenomic study of the evolutionary origins of Centrohelida, Haptophyta and Cryptista". Proc. R. Soc. B. 283 (1823): 20152802. doi:10.1098/rspb.2015.2802. ISSN 0962-8452. PMC 4795036. PMID 26817772.
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- Daugbjerg, Niels; Norlin, Andreas; Lovejoy, Connie (2018-07-25). "Baffinella frigidus gen. et sp. nov. (Baffinellaceae fam. nov., Cryptophyceae) from Baffin Bay: Morphology, pigment profile, phylogeny, and growth rate response to three abiotic factors" (PDF). J. Phycol. 54 (5): 665–680. doi:10.1111/jpy.12766. ISSN 1529-8817. PMID 30043990.