Cryptomonad

Cryptomonads
Rhodomonas salina
Scientific classification
Domain:	Eukaryota
Kingdom:	Chromalveolata
(unranked):	Hacrobia
Phylum:	Cryptophyta Cavalier-Smith, 1986
Class:	Cryptophyceae West in West & Fritsch, 1927
Typical genera
Order Cryptomonadales    Campylomonas    Chilomonas    Chroomonas    Cryptomonas    Falcomonas    Geminigera    Guillardia    Hemiselmis    Plagioselmis    Proteomonas    Storeatula    Rhodomonas    Teleaulax Order Goniomonadales    Goniomonas
Synonyms
Cryptomonadea Stein, 1878[1] Cryptomonada Senn, 1900 Cryptomonadinae Pascher, 1913 Cryptomonadophyceae Pascher ex Schoenichem, 1925

The cryptomonads (or cryptophytes)^[2] are a group of algae,^[3] 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.^[4]

Characteristics

Cryptomonads are distinguished by the presence of characteristic extrusomes called ejectisomes or ejectosomes, which consist of two connected spiral ribbons held under tension.^[5] 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.^[6][7]

Cryptomonads have one or two chloroplasts, 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.^[8] 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;^[9] 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 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 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.^[10]

Kathablepharids

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

References