Cryptomonads (Cryptophyta): small flagellate algae with complex plastids

The systematic classification of the Cryptophyceae into different genera was mainly based on morphological characteristics and pigmentation.

1. One of the most important features here is the periplast. The periplast is a layered cell envelope -- cryptophyceae do not form a cell wall -- consisting of an inner and an outer periplast component made of proteins. Between them is the cell's plasma membrane. Both periplast components show very fine structuring. For example, the inner periplast component may consist of polygonal plates, overlapping rectangular plates, or a continuous layer. The outer periplast component may also be composed of plates or of rosette scales and fine fibrils.
2. All Cryptophyceae have a cell invagination, which is lined with explosive organelles, the so-called ejecto- or ejectisomes (see extrusome). The opening of this cell invagination can either be small with a blind ending sac behind it (pharynx) or elongated following the cell invagination (furrow). Combinations of furrow and pharynx are also possible.
3. In some genera, the nucleomorph is not freely located in the periplastid space, but is embedded in the pyrenoid matrix (spatially separated by the two inner envelope membranes of the plastid).
4. Of the phycobilisomes, the light-collecting complexes of red algae (and glaucocystophyceae and cyanobacteria), which originally contained three different blue or red pigments, only phycoerythrin remained in the Cryptophyceae. In the Cryptophyceae, however, seven different types of phycoerythrin evolved from the original red phycoerythrin, four of which are blue in color and are therefore called phycocyanins, although they are not directly related to the true phycocyanins from the phycobilisomes.
5. The microtubular flagellar roots, with which the flagella are anchored in the cells, also show differences.

From the combination of the different characteristics

• Structure of the periplast,
• Shape of the cell invagination,
• Position of the nucleomorph,
• Pigment type and
• Structure of the flagellar root apparatus

result in the different genera.

However, research into the relationships within the Cryptophyceae using methods of molecular phylogenetic analysis (= creation of phylogenetic trees based on DNA sequences), revealed a much more complex picture. Cryptophyceae are probably dimorphic, i.e. they can form two different cell types. Therefore, two cell forms of one genus were probably mistaken for two different genera several times. Dimorphism has been reliably demonstrated in the genera Proteomonas and Cryptomonas . Even a leucoplast, a colorless plastid that has lost the ability to photosynthesize, is not a sure characteristic of a distinct genus. The former genus Chilomonas proved to be a colorless Cryptomonas, of which, moreover, at least three different evolutionary lineages exist within Cryptomonas. The remaining genera of the Cryptophyceae probably also require a revision of their systematics.

Genera of Cryptophyceae according to preliminary research: Chroomonas, Cryptomonas (contains the formerly independent genera Campylomonas and Chilomonas), Geminigera, Goniomonas, Guillardia, Hanusia, Hemiselmis, Komma, Plagioselmis, Proteomonas, Rhinomonas, Rhodomonas (Pyrenomonas), Teleaulax.