Cryptophyceae

The Cryptophyceae (from Old Greek Κρύπτος secret and Φύκιον alga) are a class of unicellular, microscopic algae found in fresh and marine water. Cryptophyceae move through the water by means of two flagella and may be reddish, bluish, or brownish in color. Some Cryptophyceae form thick-walled and spherical permanent stages to survive unfavourable environmental conditions. As an ecologically very important group of algae, the Cryptophyceae serve as food for many protists. Since the Cryptophyceae include colorless and photosynthetically active genera or species, there are botanical and zoological classifications. Hence, they can also be zoologically referred to as cryptomonads. However, from their evolutionary history and resulting relationships, they systematically belong neither to animals nor to plants (according to the current state of knowledge, the kingdom Plantae sensu Archaeplastida includes glaucophytes, red algae and viridiplantae with green algae and land plants). The closest relatives of the Cryptophyceae are the mostly colorless and phagotrophic Catablepharidophyta. Together they form the taxon Cryptophyta.

Most Cryptophyceae (from Old Greek Κρύπτο, "crypto" secret and φύκιον "phykion" alga), with the exception of the genus Goniomonas, have two cell nuclei of different evolutionary origin and therefore became of interest to evolutionary biologists. Here, the different cell compartments are interlocked like a matryoshka. The outermost compartment contains the actual cell nucleus and the cytoplasm, which also contains the mitochondria. The next smaller so-called periplasmic compartment contains the second greatly reduced nucleus (nucleomorph), and starch granules. The innermost compartment is the actual photosynthetic organelle, the plastid. Since mitochondria and plastids also have their own genomes (mitogenome, plastome), a cryptophycean cell contains a total of four genomes.

The multiplicity of genomes is explained by a secondary endosymbiosis in which a phagotrophic eukaryote ingested a photosynthetically active eukaryote. Normally, ingested organisms are digested. However, in endosymbiosis, the ingested cell is retained and over time transforms into a dependent organelle. However, only in the Chlorarachniophyta and the unrelated Cryptophyceae was the nucleus of the ingested alga not lost.

The marine cryptophycete Guillardia theta was selected as a model organism to sequence the nucleomorph and plastid genomes. Both genome sequencings revealed that the plastid of the Cryptophyceae must have originally been a red alga. Further evidence for this theory includes starch synthesis and 80S ribosomes in the periplastid space (the former cytoplasm of the ingested red alga) and the four envelope membranes surrounding the plastid. All plastids originating from a primary endosymbiosis (the chloroplasts of green algae and land plants, the cyanelles of glaucocystophytes and the rhodoplasts of red algae) are delimited by only two envelope membranes and not by three or four (= complex plastids).

Systematics of the Cryptophyceae

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.

Illustration with different representatives of the Cryptophyceae: Cryptomonas, Chroomonas, Hemiselmis, Chilomonas.