Cryptomonads are a group of unicellular, biflagellate protists occurring in a variety of aquatic habitats. Several nonpigmented forms exist but the vast majority bear plastids containing chlorophylls a and c, and accessory pigments known as phycobiliproteins. Plastids of cryptomonads are derived from a secondary endosymbiotic event involving the engulfment of a unicellular red alga, its retention as an endosymbiont, and its conversion into a photosynthetic organelle. Unique features of cryptomonads include the cell covering termed a periplast, the surface architecture consisting of a furrow and/or gullet, extrusive organelles called ejectisomes, and the presence of a reduced eukaryotic genome known as the nucleomorph. Accurate identification of cryptomonad species using light microscopy is problematic owing to the range of morphological plasticity and the presence of two distinct morphotypes in certain species. In addition to traditional morphological characters, proper diagnosis of species cannot exclude molecular data.

Key Concepts:

  • Accurate identification and estimation of cryptomonads from field samples requires counting cells in the living state.

  • Proper characterisation of cryptomonad periplast plates requires use of quick‐freezing and freeze‐fracture techniques.

  • Accumulating karyotypic data continues to provide insight into the genetic variability of nucleomorph genomes, which may prove systematically significant.

  • Cryptomonad biliproteins all derive from an ancestral phycoerythrin, which has since evolved into at least seven spectral types.

  • Morphological species concepts are problematic given the increasing awareness of morphological plasticity and dimorphisms among cryptomonads.

  • Resolution of the large‐scale phylogentic affinities of cryptomonads remains tenuous and additional molecular analyses are required.

Keywords: algae; algal systematics; cryptomonad phylogeny; cryptomonads; Cryptophyceae; nucleomorph; phycobiliproteins; protists; secondary endosymbiosis

Figure 1.

Illustration of a typical cell of Goniomonas including ultrastructural detail. Members of this genus are primitively colourless and lack a periplastidial complex. DB, digested bacterium; F, furrow; FV, food vacuole; I, infundibulum; LE, large ejectisome; LF, long flagellum; M, mitochondrion; N, nucleus; RP, rectangular plate; S, stoma; SF, short flagellum; and V, vestibulum.

Figure 2.

Illustration of a typical cell of Pyrenomonas including ultrastructural details. Note the pyrenoid embedded in the nucleomorph. C, chloroplast; F, furrow; G, gullet; ISP, inner square plate; LE, large ejectisome; LF, long flagellum; M, mitochondrion; N, nucleus; Nm, nucleomorph; P, pyrenoid; SE, small ejectisome; SF, short flagellum; and V, vestibulum.

Figure 3.

Illustration of typical cells of the Cryptomonas cryptomorph and campylomorph including ultrastructural details. CF, complex furrow; EP, ejectisome pore; F, furrow; FP, fibrous furrow plate; G, gullet; IPS, inner periplast sheet; IOP, inner oval plate; LE, large ejectisome; LF, long flagellum; M, mitochondrion; N, nucleus; Nm, nucleomorph; P, pyrenoid; S, stoma; SE, small ejectisome; SF, short flagellum; SG, starch grain; SP, scalariform plate; V, vestibulum; and VL, vestibular ligule.

Figure 4.

Illustration of a typical cell of Chroomonas including ultrastructural detail. Note the eyespot which is present in some species of Chroomonas. C, chloroplast; E, eyespot; G, gullet; IRP, inner rectangular plate; LE, large ejectisome; LF, long flagellum; M, mitochondrion; N, nucleus; Nm, nucleomorph; P w/T, pyrenoid with traversing thylakoids; SE, small ejectisome; SF, short flagellum; SG, starch grain; and V, vestibulum.

Figure 5.

Formation of the cryptomonad chloroplast endoplasmic reticulum (CER) and periplastidial complex by secondary endosymbiosis. The resultant cryptomonad is a composite organism that possesses four genomes. RER, rough endoplasmic reticulum.



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Further Reading

Cavalier‐Smith T (1986) The Kingdom Chromista: origin and systematics. In: Round FE and Chapman DJ (eds) Progress in Phycological Research, vol. 4, pp. 309–347. Bristol: Biopress.

Gillot M (1990) Phylum Cryptophyta (Cryptomonads). In: Margulis L, Corliss JO, Melkonian M and Chapman DJ (eds) Handbook of Protoctista, pp. 139–151. Boston, MA: Jones & Bartlett.

Hill DRA (1991a) Diversity of heterotrophic cryptomonads. In: Patterson DJ and Larsen J (eds) The Biology of Free‐Living Heterotrophic Flagellates. Systematics Association Special Volume 45, pp. 235–240. Oxford: Clarendon Press.

Hill DRA (1991b) A revised circumscription of Cryptomonas (Cryptophyceae) based on examination of Australian strains. Phycologia 30: 170–188.

Kugrens P and Clay BL (2010) Cryptophyta. In: Algae: Source to Treatment, pp. 187–205. Denver, Colorado: American Water Works Association.

Kugrens P and Clay BL (2003) Cryptomonads. In: Wehr JD and Sheath RG (eds) Freshwater Algae of North America, pp. 715–756. Amsterdam, Boston: Academic Press.

Kugrens P and Lee RE (1991) Organization of cryptomonads. In: Patterson DJ and Larsen J (eds) The Biology of Free‐Living Heterotrophic Flagellates. Systematics Association Special Volume 45, pp. 219–233. Oxford: Clarendon Press.

Kugrens P, Lee RE and Hill DRA (2000) Order Cryptomonadida Senn, 1900. In: Lee JJ, Leedale GF and Bradbury P (eds) An Illustrated Guide to the Protozoa, pp. 1111–1125. Lawrence, Kansas: Society of Protozoologists.

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Clay, Brec L(May 2011) Cryptomonads. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001976.pub2]