Introduction to the Heterokontophyta

  1. Introduction
    1. A large and diverse group which includes both phototrophs and heterotrophs
    2. Flagellate stages are typically heterokont -- i.e., have two dissimilar flagella
    3. Plastids, when present, are secondary plastids without nucleomorphs
    4. Pigmentation with chlorophylls a, c1 and c2, with fucoxanthin or vaucheriaxanthin as accessory pigments
    5. This gives the cells a golden color
    6. Heterokonts, along with haptophytes and dinoflagellates, are therefore referred to as "chromophytes"
    7. Heterokonts are grouped together on the basis of their ultrastructure, and this grouping has been supported by biochemical and molecular studies.
    8. They are extremely diverse in terms of their gross structure, and classical treatments didn't always put them together.
  2. Structure & metabolism
    1. Flagellate cells of many with two different kinds of flagella
    2. Forward-directed flagellum with mastigonemes
    3. Pleuronematic (literally "with lateral threads") flagellum, flimmer flagellum, tinsel flagellum; hence can be said to be "tinsellate"
    4. Mastigonemes are three-parted tubular bristles with terminal hairs.
      1. Composed of glycoprotein
      2. Assembled in ER, transported to surface
    5. Reverse-directed flagellum is smooth, and typically shorter than the forward flagellum
    6. Base of flagellum often with a transitional helix
      1. (exceptions are Bacillariophyceae, Raphidophyceae, Phaeophyceae)
    7. Eyespot is composed of a row of spherical, red or orange pigment bodies
      1. Swelling at base of the rear (smooth) flagellum, lies near eyespot in chloroplast
      2. Thought to be involved with light perception
      3. Phototaxis does occur, although some phototactic organisms do not have obvious eyespots.
        1. Phototaxis may be negative or positive, depending upon the light environment
      4. Eyespot itself is a cluster of lipid spheres within the chloroplast, densely packed with carotenoids
      5. The flagellar swelling is close to the eyespot, and presses into an invagination in the cell membrane
      6. It has been proposed that the eyespot shades the flagellar swelling and that the flagellar swelling is the actual site of photoreception
      7. If correct, this could also explain why organisms with no detectable eyespot (including eyespot-lacking mutants) can be capable of phototaxis
      8. Precise mechanism for photoreception is unknown, but the action spectrum has been determined (maximum at 420-490 nm), and a flavoprotein with a similar absorbtion range has been found in some heterokonts
      9. The eyespot of Euglena (Euglenophyta) seems to be rather similar to that of heterokonts.
    8. Chloroplast is secondary
      1. Chloroplast ER is present, as is a periplastidal network
      2. No nucleomorph is present
      3. Thylakoids are stacked in groups of three (lamellae), and a girdle lamella is present in most
      4. Pigmentation, Chlorophylls a, c1, and c2, fucoxanthin or vaucheriaxanthin
    9. Reserve polysaccharide is chrysolaminarin (beta 1,3 glucan, with beta 1,6 branches)
      1. Forms in vessicles outside of plastid


  1. Chrysophyceae - "golden algae" - are typically freshwater, planktonic unicells
    1. Some classifications split addtional groups from the chrysophytes:
    2. • Synurophyceae
    3. • Pedinellophyceae
    4. • Parmophyceae
    5. • Sarcinochrysidophyceae
  2. Xanthophyceae - often multicellular
  3. Eustigmatophyceae - mostly soil algae, some marine picoplankton (0.2-2 micrometers diameter)
  4. Bacillariophyceae - diatoms - have silica frustules, unicellular or weakly filamentous
  5. Raphidophyceae - unicellular, with apical flagella (one points backwards in a groove)
  6. Dictyophyceae - silicoflagellates, two extant species, but important in fossil record
  7. Phaeophyceae - brown algae, imporant marine macrophytes in colder waters
  8. Bicocoecida - not photosynthetic, unicellular flagellates
    1. Cafeteria
  9. Oomycota - not photosynthetic
    1. "water molds"
    2. Phytophthora infestans -- causal agent of late blight of potatoes
  10. Hyphochytridiomycetes - not photosynthetic
  11. Resemble chytrids, but with anterior, tinsellate flagellum
    1. Hyphochytrium
  12. Labyrinthulomycetes - not photosynthetic
    1. The "slime nets"
    2. Labyrinthuloides

Supplementary Reading:

Tomas, C.R. (ed.) 1997. Identifying marine phytoplankton. Academic Press, San Diego.

Williams, D.M. 1991. Phylogenetic relationships among the chromista: a review and preliminary analysis. Cladistics 7:141-156. [a cladistic analysis primarily of morphological data]

Medlin, L.K., A. Cooper, C. Hill, S. Wrieden, and U. Wellbrock. 1995. Phylogenetic position of the Chromista plastids based on small subunit rRNA coding regions. Curr. Genet. 28:560-565.

Reith, M. 1995. Molecular biology of rhodophyte and chromophyte plastids. Annu. Rev. Plant Physiol. Plant Mol. Biol. 46:549-575.