Introduction to the Heterokontophyta
- A large and diverse group which includes both phototrophs and heterotrophs
- Flagellate stages are typically heterokont -- i.e., have two
- Plastids, when present, are secondary plastids without nucleomorphs
- Pigmentation with chlorophylls a, c1 and c2, with fucoxanthin or vaucheriaxanthin
as accessory pigments
- This gives the cells a golden color
- Heterokonts, along with haptophytes and dinoflagellates, are therefore
referred to as "chromophytes"
- Heterokonts are grouped together on the basis of their ultrastructure,
and this grouping has been supported by biochemical and molecular studies.
- They are extremely diverse in terms of their gross structure, and classical
treatments didn't always put them together.
- Structure & metabolism
- Flagellate cells of many with two different kinds of flagella
- Forward-directed flagellum with mastigonemes
- Pleuronematic (literally "with lateral threads") flagellum,
flimmer flagellum, tinsel flagellum; hence can be said to be "tinsellate"
- Mastigonemes are three-parted tubular bristles with terminal
- Composed of glycoprotein
- Assembled in ER, transported to surface
- Reverse-directed flagellum is smooth, and typically shorter than the
- Base of flagellum often with a transitional helix
- (exceptions are Bacillariophyceae, Raphidophyceae, Phaeophyceae)
- Eyespot is composed of a row of spherical, red or orange pigment
- Swelling at base of the rear (smooth) flagellum, lies near eyespot
- Thought to be involved with light perception
- Phototaxis does occur, although some phototactic organisms do not
have obvious eyespots.
- Phototaxis may be negative or positive, depending upon the light
- Eyespot itself is a cluster of lipid spheres within the chloroplast,
densely packed with carotenoids
- The flagellar swelling is close to the eyespot, and presses into
an invagination in the cell membrane
- It has been proposed that the eyespot shades the flagellar swelling
and that the flagellar swelling is the actual site of photoreception
- If correct, this could also explain why organisms with no detectable
eyespot (including eyespot-lacking mutants) can be capable of phototaxis
- 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
- The eyespot of Euglena (Euglenophyta) seems to be rather similar
to that of heterokonts.
- Chloroplast is secondary
- Chloroplast ER is present, as is a periplastidal network
- No nucleomorph is present
- Thylakoids are stacked in groups of three (lamellae), and a girdle
lamella is present in most
- Pigmentation, Chlorophylls a, c1, and c2, fucoxanthin or vaucheriaxanthin
- Reserve polysaccharide is chrysolaminarin (beta 1,3 glucan, with beta
- Forms in vessicles outside of plastid
- Chrysophyceae - "golden algae" - are typically freshwater,
- Some classifications split addtional groups from the chrysophytes:
- Xanthophyceae - often multicellular
- Eustigmatophyceae - mostly soil algae, some marine picoplankton (0.2-2 micrometers
- Bacillariophyceae - diatoms - have silica frustules, unicellular
or weakly filamentous
- Raphidophyceae - unicellular, with apical flagella (one points backwards
in a groove)
- Dictyophyceae - silicoflagellates, two extant species, but important in
- Phaeophyceae - brown algae, imporant marine macrophytes in colder
- Bicocoecida - not photosynthetic, unicellular flagellates
- Oomycota - not photosynthetic
- "water molds"
- Phytophthora infestans -- causal agent of late blight of potatoes
- Hyphochytridiomycetes - not photosynthetic
- Resemble chytrids, but with anterior, tinsellate flagellum
- Labyrinthulomycetes - not photosynthetic
- The "slime nets"
Tomas, C.R. (ed.) 1997. Identifying marine phytoplankton. Academic Press,
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.