cryptomonads

Cryptomonads, also known as cryptophytes, are a group of unicellular, flagellated protists in the phylum Cryptophyta. They occur in both marine and freshwater environments and are often mixotrophic, combining photosynthesis with phagotrophy of bacteria and other small particles.

Most cells are small, typically a few to a few dozen micrometers, and they usually bear two unequal flagella that emerge from an apical groove. The cell is enveloped by a flexible outer layer called the periplast. A cytostome and cytopharynx enable ingestion of prey, allowing the organisms to take up food particles when light is limited.

Cryptomonads carry chloroplasts derived from a secondary endosymbiotic red alga. The plastid is surrounded by four membranes and contains chlorophylls a and c, the pigments phycoerythrin and phycocyanin, and a small vestigial nucleus called a nucleomorph, housed between two outer membranes. This plastid is the site of photosynthesis in those cells that rely on light.

They are mixotrophs, functioning as primary producers when photosynthesizing and as consumers when feeding on bacteria; many taxa can switch depending on environmental conditions. They inhabit a wide range of aquatic environments and can contribute to microbial food webs and carbon cycling in both oligotrophic and nutrient-rich waters.

Reproduction is primarily asexual, occurring by binary fission. Sexual reproduction has been reported for some species but is not widely documented. Notable genera include Guillardia, Cryptomonas, and Rhodomonas; the model species Guillardia theta has been studied for its nucleomorph and plastid organization. The name derives from Greek kryptos, hidden, reflecting their complex intracellular remnants.