Cryptophyta

1. Introduction
   1. A relatively small, but ecologically and evolutionarily important group
      1. Typically phytoplankton
      2. In both freshwater and marine environments
   2. Small and delicate cells, consequently often difficult to observe and poorly studied
      1. Distinctive structure; fairly easy to identify except for their small size
   3. Some are not photosynthetic, and are heterotrophic
   4. Plastids are secondary, with a highly reduced eukaryotic nucleus, the nucleomorph
   5. Many photosynthetic species probably retain ability to eat prey (mixotrophy)
2. Structure & metabolism
   1. Flagellate unicells
      1. Dorsiventral, with groove and gullet on ventral side
      2. Some form sessile ("palmelloid") stages
      3. Bjornbergiella is weakly filamentous
   2. Two unequal flagella
      1. Typically the longer flagellum has two rows of mastigonemes, the shorter one a single row
         1. Mastigonemes are two-parted bristles on flagella, which are composed of a rigid, tubular base and (typically) two terminal hairs
         2. Mastigonemes form within the endoplasmic reticulum (or the nuclear envelope) and are transported to the exterior of the cell.
      2. Flagella are also covered with scales
      3. Flagellar root with rhizostile, transverse striated root, and three microtubular roots
   3. Trichocysts in oral groove, and scattered around surface of cell
      1. Trichocysts are composed of a tightly coiled spool of protein
      2. Can undergo a very rapid, irreversible conformational shift
      3. The trichocyst pops suddenly out of the cell, causing the cell to jump backwards as a result
      4. Defense mechanism, may also be involved in predation
   4. Contractile vaculole at anterior end
   5. Large nucleus at posterior end
      1. Mitosis is open
      2. Centrioles are not associated with mitosis
   6. Cell division by furrowing
   7. Cell is stiffened by proteinaceous plates that lie inside of the plasmalemma, and are attached to it
   8. Chloroplast is a secondary plastid, with a chloroplast endoplasmic reticulum (CER) and nucleomorph
      1. CER is continuous with nuclear envelope, and is probably homologous to a food vacuole.
   9. Origin of primary plastid remains under study
      1. Probably from red lineage
      2. Presence of phycobilins is suggestive of red algae, but is the plesiomorphic state for plastids, and chlorophyll c is unknown among reds.
      3. Molecular phylogenetic studies place nucleomorph close to red algae
      4. Chloroplast genome map has features that suggest a reduction series from red algae to cryptomonads to heterokonts
      5. Note, however, that this does not imply ancestor/descendant relationships among the extant groups, but rather retention of ancestral character states from common ancestors.
   10. Nucleomorph with three chromosomes, 240 kb, 225 kb, and 195 kb
       1. Does not form a spindle during mitotsis
   11. Plastid is a secondary plastid
   12. Primary plastid typically with thylakoids in pairs, with no girdle lamellae
       1. Chlorophylls a and c2
       2. Phycobilins (phycoerythrin and phycocyanin), but inside thylakoid lumen, rather than in phycobilisomes
       3. Pyrenoid projects from plastid
       4. Carotenoids: alpha-carotene
       5. Xanthophylls: alloxanthin, crocoxanthin, zeaxanthin, monadoxanthin
       6. Eyespot, if present, is inside plastid, but not associated with the flagella
   13. Food storage is primarily starch, which accumulates in periplastidal space
3. Reproduction
   1. Reproduction is generally asexual in culture.
   2. Sexual reproduction has recently been documented in the Cryptophyta
4. Classification
   1. Only about 12 genera are known
   2. Guillardia theta (formerly Cryptomonas phi) has been studied most extensively
      1. The complete chloroplast genome is known (Genbank accession # AF041468)
   3. Cryptomonas similis
   4. Chilomonas paramecium - nonphotosynthetic but with unpigmented plastids ("leucoplasts")
   5. Cyathomonas - nonphotosynthetic and lacking plastids
5. Ecology
   1. Both freshwater and marine
   2. Important members of the nannoplankton (2-20 micrometers diameter)
   3. Can form major blooms in arctic and antarctic waters as well as in the Chesapeake Bay
      • Sometimes an important food source for smaller heterotropic or mixotrophic plankton, including ciliates and dinoflagellates
      • Myrionecta rubra (=Mesodinium rubrum) is a ciliate that seems to acquire cryptomonad kleptochloroplasts.
   4. Freshwater lakes, ponds, and ditches, particularly in colder waters
   5. Dominant species in some antarctic lakes
   6. Also found in interstitial water on sandy beaches

Supplementary Reading:

Gibbs, S.P. 1981. The chloroplast endoplasmic reticulum: structure, function, and evolutionary significance. Int. Rev. Cytol. 72:49-99.

Gray, M.W. 1994. One plus one equals one: the making of a cryptomonad alga. A.S.M. News 60:423-427.

Douglas,S.E. and Penny,S.L. 1998. The plastid genome from the cryptomonad alga, Guillardia theta: complete sequence and conserved synteny groups confirm its common ancestry with red algae J. Mol. Evol. (1998)

Cavalier-Smith, T., et al. 1996. Cryptomonad nuclear and nucleomorph 18S rRNA phylogeny. Eur. J. Phycol. 31:XXX-XXX.

Krugens, P. 1988. Ultrastructrure of fertilization in a cryptomonad. J. Phycol. 24:385-393.

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