The phylum Cryptophyta describes tiny, motile, unicellular organisms with two slightly unequal flagella bearing lateral hairs. Cryptomonads live mainly in marine and freshwater environments.
Some cryptomonads are alga-like, with bluegreen, red, and olive-brown photosynthetic pigments including chlorophylls a, c2, alpha-carotene, xanthophylls (alloxanthin, crocoxanthin, zeaxanthin, and monadoxanthin), and phycobiliproteins (phycoerythrin and phycocyanin).
Cryptomonads are found in a variety of moist places, such as algal blooms in the ocean or in fresh water, and on beaches. Some members are intestinal parasites in animals.
Historically, botanists and zoologists alike have adopted cryptomonads. Botanically, cryptomonads would be included in kingdom Plantae, phylum Cryptophyta, class Cryptophyceae, order Cryptomonadales, and family Cryptomonadaceae.
Zoologists would place the cryptomonads in kingdom Animalia, phylum Sarcomastigophora, class Phytomastigophora, order Cryptomonadida, and family Cryptomonadidae.
Also, the cryptomonads are currently included in a separate kingdom, Protoctista (also known as Protista), with phylum, class, order, and family taxa being the same as that for the botanical taxa above.
Synonyms for Cryptophyta are Cryptomonadales, Cryptophyceae, and Chromophyta, and they were once placed with the algae. Genera for the cryptomonads include the following: Chilomonas, Chroomonas, Cryptomonas, Cyathomonas, Falcomonas, Geminigera, Goniomonas, Guillardia, Hemiselmis, Komma, Plagiomonas, Pyrenomonas, Rhodomonas, Storeatula, and Teleaulax.
Guillardia theta, formerly known as Cryptomonas phi, has been studied most extensively, and the complete chloroplast genome is known (Genbank accession number AF041468).
The cryptomonads are part of the nanoplankton (typically phytoplankton between 2 to 20 micrometers in diameter) and are a relatively small but ecologically and evolutionarily important taxon. Both freshwater and marine representatives are known.
Many photosynthetic species likely retain their capacity to eat prey (mixotrophy). Fluctuations in their numbers are correlated with increases in levels of nitrogen in the water in which they live. Some species of cryptomonads form gelatinous colonies.
A few cryptomonads have reached the palmelloid or sessile stage of organization, but most are free-living flagellates common in nutrient-rich water. A weakly filamentous member of the cryptomonads is Bjornbergiella. Well-known examples of cryptomonads are Cryptomonas ovata, C. similis, and Chilomonas paramecium.
The cryptomonads can form major blooms in Arctic and Antarctic waters as well as in North America’s Chesapeake Bay. They can be very important food sources for smaller heterotrophic or mixotrophic plankton, including ciliates and dinoflagellates. Cryptomonads are found in freshwater lakes, ponds, and ditches—especially in colderwaters.
They are dominant species in many Antarctic lakes, and they are also found in interstitial water on sandy beaches. Reproduction in the cryptomonads is generally asexual in culture, but sexual reproduction has been documented in the Cryptophyta.
Cryptomonads are tiny flagellates, 5 to 30 microns (most around 10 to 20microns). They are flattened dorsiventrally in shape and are asymmetrical, with a periplast (a proteinaceous structure that lies inside the plasmalemma and is attached to it). Cryptomonads are mostly algal forms, with an anterior (ventral) groove or pocket and a gullet, which has refractile ejectosomes or trichocysts.
The unequal flagella are inserted nearly parallel to the pocket, not inside the gullet as in the euglenophytes. Mitochondria have flat cristae, while the plastids are secondary with a highly reduced eukaryotic nucleus, the nucleomorph.
Although there are a few colorless forms, such as Chilomonas, most cryptomonads have a chloroplast. The chloroplast is not contained directly, however, because there is a reduced eukaryote symbiotic within the cell containing a normal prokaryote chloroplast. Usually, there are two chloroplasts, which are secondary plastids.
The chloroplast is bound by four membranes (two being chloroplast endoplasmic reticulum, or CER, continuous with the nuclear envelope and homologous to a food vacuole) with a tiny nucleus (nucleomorph) between the middle two membranes.
Much like the dinoflagellates (with which the cryptomonads were formerly grouped), it has chlorophylls a and c; chlorophyll b is never present. Thylakoids are paired, and phycobilin pigments are present in the spaces between the thylakoids but not in phycobilisomes, such as in the cyanobacteria and Rhodophyta.
Food reserves are starch like, accumulating in the periplastidal space stored between the starch envelope and the chloroplast reticulum. If there is an eyespot, it is inside a plastid not associated with the flagella.
There is a large nucleus at the posterior end. Mitosis is open, and centrioles are not associated with mitosis. Cell division is by furrowing. No histones are associated with the chromosomes.
The unique nucleomorph has deoxyribonucleic acid (DNA), is contained within a double membrane, and also has a nucleolus-like region.Molecular data suggest and strongly support the idea that the nucleomorph is a vestigial nucleus from the original endosymbiont, which became the cryptophyte chloroplast.
Three chromosomes are associated with the nucleomorph: 240 kilobase pairs (kb), 225 kb, and 195 kb. The bulk of chromosome II (175 kb) is now sequenced with a preponderance of “housekeeping genes” apparently existing for the service of just a few genes encoding plastid proteins.
There are parallels between the nucleomorphs of cryptomonads and chlorarachniophytes. The cryptomonad nucleomorph is depauperate in introns, with ribosomal ribonucleic acid (rRNA) genes at the chromosome ends just within the telomeres.
The longer flagellum has two rows of mastigonemes (lateral hairs), while the shorter flagellum has a single row. Mastigonemes are two-parted bristles or hairs made up of a rigid, tubular base and, usually, two terminal hairs.
These bristles are formed within the endoplasmic reticulum (or nuclear envelope) and are thus transported to the exterior of the cell. The flagella are covered with scales, too.
Trichocysts or ejectosomes are in the oral groove, and they are scattered around the cell surface. There is a tightly spooled protein in the trichocysts, which can undergo a very rapid, irreversible conformational change in which it pops out of the cell, pushing the cell backward as a result. The trichocysts are considered a defense mechanism or are perhaps involved in predation.
Evolution and phylogeny of cryptomonads have not been well documented using molecular techniques. An 18S ribosomal RNA phylogeny of cryptomonads has been made, however.
The nucleus of the endosymbiont (the nucleomorph) does not seem to have a complete complement of genes for photosynthesis, these being relocated to the host nucleus now. Plastid targeting mechanisms of cryptomonads for light-harvesting complex proteins have been studied, though.
The primary plastid is of unknown origin and remains under scrutiny but is probably from a red algal lineage, as the presence of phycobilins is suggestive of the Rhodophyta (red algae). Chlorophyllc is unknown among the red algae, however.
Molecular phylogenetic studies place the nucleomorph close to red algae, and the chloroplast genomemap has characteristics suggesting a reduction series from red algae to cryptomonads to heterokonts. This does not imply any descendant relationship among extant groups but rather retention of ancestral character states from common ancestors.
Lateral gene transfer from an ancestral cryptomonad to a dinoflagellate is postulated from sequence analysis of two nuclear-encoded glyceraldehyde-3-phosphate dehydrogenase (GAPDH) genes isolated from the dinoflagellate Gonyaulax polyhedra.
The plastid sequence forms amonophyletic group with the plastid isoforms of cryptomonads, distinct from all other plastid GAPDHs. This provides the first example of genetic exchange accompanying symbiotic associations between cryptomonads and dinoflagellates, which are common in present-day cells.