2.14: Cryptomonads - Unicellular Photosynthetic Algae
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As the name implies, cryptophytes (crypto = hidden) are unicellular algae that are often hidden. This is a consequence of their relatively small size (10-30 um), the fact that they often occur in deeper waters, and the fact that they are often difficult to collect in an intact condition. However, they are significant contributors to aquatic food chains, both marine and fresh water, and have interesting features that relate to their evolution.
Taxonomy and phylogeny
While consistent structural features unify the cryptomonad group, their placement relative to other living things is problematical. Although they have similar pigmentation (chlorophylls a and c and phycobillins) with the dinoflagellates, this may be the result of ancestral forms of both groups separately ingesting the same eukaryotic algae (a red algae) in a secondary endosymbiosis manner.
Cryptomonads have a distinctive structure. They are unicellular and have two flagella with an anterior groove. Their chloroplasts have four membranes, reflecting secondary endosymbiosis, i.e., that a eukaryote ingest ed another eukaryote, in this case one with a chloroplast (see Diatoms). Because of pigmentation, the second endosymbiotic event is thought to have involved a red algae being ingested by a unicellular heterotroph. This pattern is also thought to be the case for dinoflagellates and diatoms. Cryptomonads possess unusual structures called ejectisomes that can be discharged when the alga is disturbed, triggering movement that may deter a herbivore. Reflecting their secondary endosymbiotic origin, they have DNA in four locations: a nucleus, the mitochondrion, the chloroplast and in a structure called a nucleomorph, thought to be a remnant of the nucleus present in the cell of the second endosymbiotic event. They have no cell wall but do have a proteinaceous layer just inside the plasma membrane similar to the pellicle found in dinoflagellates. As might be expected for an organism lacking a cell wall, they possess contractile vacuoles to maintain water levels.
Sex and reproduction
There is some evidence for sexual reproduction in at least one species but primarily they reproduce asexually by mitosis.
Matter and energy
Almost all cryptomonads possess photosynthetic pigments and are photosynthetic autotrophs, acquiring carbon and 16 other elements in inorganic form from their environment. However, cryptomonads do require B vitamins, reflecting their heterotrophic ancestry. And a few species lack photosynthetic pigments and are heterotrophs, obtaining food by phagocytosis (invagination of the cell membrane to engulf a food particle). Some photosynthetic forms are also capable of phagocytosis, indicating mixotrophy (being both an autotroph and heterotroph ).
Because red light penetrates deeper in the water column and because cryptomonads possess phycobiliproteins pigments that can utilize red light, cryptomonads can photosynthesize at greater depths than other algae, and cryptomonads are often found at greater depths than other algae. Although they are not a particularly diverse group, they appear to be quite important in several habitats, typically cooler ones, both marine and fresh water, serving as the base of food chains.