Cryptophyta (cryptomonads)

Cryptomonads are small (~5-50 pm) biflagellate protists found in diverse fresh-water, brackish, and marine habitats. They are characterized by a distinct cellular asymmetry and the presence of extrusive organelles called ejectosomes. Many cryptomonads are photosynthetic; their plastids are diverse in pigmentation and coloration. Plastid-bearing cryptomonads are noteworthy in their possession of a "nucleomorph," a residual nucleus of secondary endosymbiotic origin. Members of the cryptomonad genus Goniomonas lack plastids and ingest bacteria for nutrition. Mixotrophic cryptomonads may also exist, and loss of photosynthesis has given rise to colorless, heterotrophic, leucoplast-bearing species on multiple occasions. Cryptomonad taxonomy was traditionally based on morphology and now includes consideration of ultrastructural features such as the cell shape, the periplast structure, the type of cell invagination present (furrow-gullet system), the flagellar apparatus architecture, and the presence-absence of pyrenoids. However, molecular sequence data suggest that morphology is of limited taxo- nomic utility at the level of species identification. Cellular dimorphisms have been found within clonal cultures, supporting the notion that cryptomonads are capable of sexual reproduction. Approximately 20 genera and <100 species of cryptomonads have been described, although their true diversity and abundance in nature is unknown.