Heterokontophyta III Xanthophyceae & Eustigmatophyceae
- A moderately small group, there are about 100 genera and 600 species
- Primarily freshwater or soil algae, a few are marine
- Typically rare algae found in low abundance, although a few genera,
including Tribonema and Vaucheria, are reasonably common
- Structure & metabolism
- Most are coccoid, or filamentous, but the most distinctive species are
siphonous. A few are flagellate unicells.
- Siphonous algae are composed of multiple tubular cells each
of which has several to many nuclei
- Coenocytic algae are multinucleate, with the entire thallus
composed of a single (often complex) cell. Septa, if present at all,
are only found in specialized structures such as gametangia.
- Zoids with apically inserted heterokont flagella. A transitional helix
is present, and there is a typical heterokont photoreceptor.
- Chloroplasts are secondary, without a nucleomorph
- There are multiple discoid chloroplasts in each cell, but each is
contained within a fold of CER, so that each plastid is surrounded by
- Thylakoids are typically in groups of three, with a girdle lamella,
but there is some variation within the group.
- Pigmentation is chlorophyll a, with small amounts of chl. c,
beta carotene, vaucheriaxanthin, diatoxanthin, diadinoxanthin, and heteroxanthin.
Fucoxanthin is not present.
- Because the dominant pigment is chlorophyll a, the xanthophytes can
easily be mistaken for green algae.
- The reserve polysaccharide is chrysolaminarin
- The cell wall is cellulose, impregnated with silica
- Cysts, if present, are formed from silica, but are in two overlapping
- Reproduction is oogamous in Vaucheria, but has not been described
in any other xanthophyte.
- Tribonema is composed of unbranched filaments
- The walls are formed from distinctive H-shaped wall pieces
- When the filaments break, halves of the H persist, and form hollow
cylinders at the end of the filament
- Look for these to distinguish Tribonema from unbranched filamentous
- Botrydium is relatively common on damp soil. The thalli are composed
of a large, aerial vessicle with colorless rhizoids.
- Ophiocytium is composed of elongate, cylindrical cells with walls
in two unequal parts
- Mature cells are multinucleate and have multiple chloroplasts. When
mature, the cytoplasm cleaves into multiple zoospores (or aplanospores
-- non-motile cells for dispersal), which are released by the separation
of the two parts of the cell wall.
- It is common for some of the zoospores to settle right next to the
opening from which they were released. This produces a distinctive cluster
of cells which at casual inspection will make the thallus look like
a branched filament.
- Vaucheria is a relatively common, siphonous xanthophyte.
- Long presumed to be a green alga, the correct placement of the Vaucheria
with the Xanthophyceae was recognized only on the basis of careful pigment
analysis and ultrastructural study.
- Composed of branched fiaments with long coenocytic tubes, with septa
forming only when gametangia differentiate.
- Reproduction is oogamous, and gametangia have a distinctive structure.
- Mitosis in vaucheria involves a long spindle, which is persistant
far into telophase. This form of mitosis seems to be an adaptation to
siphonous cell structure, and may not be typical of the Xanthophyceae.
- Most Xathtophyceae are freshwater or soil algae, but a few are marine
- Relatively rare and generally not in high abundance, xanthophytes are
- Lichen with Xathophyceae (or sometimes unbranched filamentous
Phaeophyceae) as phycobiont
- Occurs high in intertidal
- A second lichen found in the same zone, Lichina, has a cyanobacterium,
Calothrix, as the phycobiont
- Below the lichen zone but above the barnacle zone, may be a narrow band
of cyanobacteria, including Calothrix, Phormidium, Nodularia,
Gloeothece, and Rivularia. This zone is very narrow in protected
areas, but may be rather wide on exposed shorelines.
- Coccoid unicells with polysaccharide cell walls.
- Only a few species are known, from both fresh and marine waters.
- The Eustigmatophyceae have a peculiar set of characteristics, and may be
incorrectly classified with heterokonts.
- Flagellate stages with one, or sometimes two apically inserted flagella.
- Do not have a typical heterokont eyespot, but have an eyespot
located outside of the chloroplast. This eyespot is composed of carotenoids,
and is membrane bound. There is an associated exension of the pleuronematic
flagellum (in other heterokonts the eyespot is associated with the
- There is a transitional helix.
- Chloroplasts are secondary, and lack a nucleomorph.
- Thylakoids are in groups of three, but there are no girdle lamellae.
A stalked pyrenoid is often present.
- Pigmentation is chlorophyll a (only), with large quantities of violaxanthin,
which serves as a light-harvesting pigment. Beta carotene, vaucheriaxanthin,
and other xanthophylls are also present.
Required Reading: VdH Chapter 7,8
Bourrelly, P. 1968. Les Algues D'eau Douce, Tome II: Les Algues Jaunes et
Brunes. Editions N. Boubee & Cie, Paris.