Cryptomonad

Cryptomonads
Rhodomonas salina
Scientific classification
Domain:	Eukaryota
Kingdom:	Chromalveolata
(unranked):	Hacrobia
Phylum:	Cryptophyta
Class:	Cryptophyceae
Typical genera
Order Cryptomonadales    Campylomonas    Chilomonas    Chroomonas    Cryptomonas    Falcomonas    Geminigera    Guillardia    Hemiselmis    Plagioselmis    Proteomonas    Storeatula    Rhodomonas    Teleaulax Order Goniomonadales    Goniomonas

The cryptomonads (or cryptophytes) are a group of algae,^[1] 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.

Some may exhibit mixotrophy.^[2]

Characteristics

Cryptomonads are distinguished by the presence of characteristic extrusomes called ejectisomes or ejectosomes, which consist of two connected spiral ribbons held under tension.^[3] 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 ejectisomes, visible under the light microscope, are associated with the pocket; smaller ones occur underneath the periplast, the cryptophyte-specific cell surrounding.^[4][5]

Cryptomonads have one or two plastids, except for Chilomonas, which has leucoplasts and Goniomonas (formerly Cyathomonas) which lacks plastids entirely. 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.^[6]

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 resting stages with rigid cell walls (cysts) 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.

Classification

Originally the cryptomonads were considered close relatives of the dinoflagellates because of their (seemingly) similar pigmentation. Later botanists treated them as a separate division, Cryptophyta, while zoologists treated them as the flagellate order Cryptomonadida. 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 kingdom Chromalveolata and to form together with the Haptophyta the group Hacrobia.

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.^[7]

Katablepharids

Main article: Katablepharid

The katablepharids, a group of heterotrophic flagellates, have been considered as part of the Cryptophyta since katablepharids were described in 1939.

References

1. ^ Khan H, Archibald JM (May 2008). "Lateral transfer of introns in the cryptophyte plastid genome". Nucleic Acids Res. 36 (9): 3043–53. doi:10.1093/nar/gkn095. PMC 2396441. PMID 18397952. http://nar.oxfordjournals.org/cgi/pmidlookup?view=long&pmid=18397952. 
2. ^ "Cryptophyta - the cryptomonads". http://www.life.umd.edu/labs/delwiche/PSlife/lectures/Cryptophyta.html/wwwtax.cgi?mode=Info&id=412159&lvl=1. Retrieved 2009-06-02. 
3. ^ Graham LE, Graham JM, Wilcox LW (2009) Algae. 2nd Edition. Benjamin Cummings (Pearson), San Francisco, CA
4. ^ Morrall S, Greenwood AD (1980) A comparison of the periodic sub-structures of the trichocysts of the Cryptophyceae and Prasinophyceae. BioSystems 12:71–83
5. ^ Grim JN, Staehelin LA (1984) The ejectisomes of the flagellate Chilomonas paramecium - Visualization by freeze-fracture and isolation techniques. Journal of Protozoology 31: 259-267
6. ^ Douglas S, Zauner S, Fraunholz M, Beaton M, Penny S, Deng LT, Wu XN, Reith M, Cavalier-Smith T, Maier UG (2002) The highly reduced genome of an enslaved algal nucleus. Nature 410: 1091-1096
7. ^ "Cryptomonads". http://tolweb.org/Cryptomonads/2396. Retrieved 2009-06-24. 

External links

• Tree of Life: Cryptomonads
• Phylum Cryptophyta at AlgaeBase