Heterokontophyta II Chrysophyceae and kin
- Unicellular or colonial, often flagellate or with flagellate stages.
A few are multicellular.
- Many form silica cysts and/or scales
- Typically freshwater, sometimes in brackish or marine waters
- Moderately diverse (ca. 200 genera & 1000 species)
- Structure & metabolism
- Flagella are apical, unequal in length, and one has mastigonemes.
- Heterokont-type photoreceptor, with swelling at base of smooth flagellum,
and eyespot within chloroplast
- Transitional helix in both flagellar bases
- Can form silica cysts within the cell
- Cyst forms in a special silica deposition vesicle
- The cyst is sealed with an organic plug
- Scales may also be present, and if so are formed in silica deposition
- Mitosis is open (the nuclear envelope breaks down during mitosis)
- The spindle is long, and is persistant in telophase
- Chloroplasts are secondary, with a CER but lacking a nucleomorph
- Pigmentation includes chlorophylls a- and c-, with fucoxanthin
- The fucoxanthin gives the cells a golden-brown color characteristic
- Chloroplast DNA in a ring-shaped nucleoid
- Do not have a carbonate pump; must use CO2 directly
- Often unicells, chrysophytes can also be colonial (Synura), filamentous
(Phaeothamnion), or thalloid (Thallochrysis). They
may be primarily flagellate, amoeboid, palmelloid (clusters of
cells in a mucilaginous sheath; Chrysocapsa), or coccoid (non-motile
spheres with cell walls, organized in loose colonies; Chrysosphaera).,
and some species have stages with two or more of these forms.
- Several chrysophyte morphologies resemble those seen in other groups
of algae, particularly green algae. This provides a powerful example of
parallel evolution, and is important to bear in mind when identifying
algae, particularly in freshwater.
- Sexual reproduction is not well characterized (it is unknown in Ochromonas)
- The life cycle is probably haplontic (i.e., meiosis is zygotic)
- Sex has been documented in Dinobryon, a sessile , stalked chrysophyte.
Isogamous, with male and female strains. Gametes are not morphologically
distinct from vegetative cells.
- Female cells secrete hormone (pheremone), and male cells swim toward
- The putative zygote forms a silica cyst, and serves as a resistant
stage, but meiosis has not yet been documented.
- Sexual reproduction is similar in Synura, where individual cells
functioning as male gametes swim away from their colony and fertilize
cells in female colonies.
- Chrysophyceae - "golden algae" - are typically planktonic
- Ochromonas danica - a well studied chrysophyte
- Freshwater, unicellular flagellate
- Good representative for cell structure in heterokonts
- Naked -- no cell wall -- and capable of amoeboid movement
- Amoeboid movement in drier or more confined areas, flagellate swimming
in open water
- Ochromonas is mixotrophic, and pseudopodia can also capture
- Mucilage bodies are found near the surface in many species
- Mucilage is discharged when cell is irritated
- Discobolocyte is an elaborate mucilage body
- Contractile vaculoles (one or more) are located near flagella, and
are visible in the light microscope.
- Chloroplasts are typical heterokont plastids, but lack pyrenoids
- Two unequal flagella, with typical heterokont features
- Longer, tinsellate flagellum provides propulsion, shorter, smooth
flagellum serves as a rudder
- Eyespot is assocated with the smooth flagellum
- Cell division initially longitudinal, with new flagella forming
next to old ones. The flagella migrate to opposite poles, cytokinesis
finishes at center of a barbell-shaped cell.
- Silaceous cysts (statospores) are formed internally, in a silica
- During cyst development, a narrow channel provides communication
between inner and outer cytoplasm
- This is eventually plugged by polysaccharide
- Cyst formation is a function of population density
- A chrysophyte 'cyst bank' exists in the sediments of suitable
lakes, and can give rise to sporadic population spikes
- Chrysodidymous and Didymochrysis are genera apparently
arrested at different stages in cell division.
- Dinobryon looks like Ochromonas in a tiny wine glass
- Mallomonas - unicell with fancy silica scales
- Synura - like a colonial Mallomonas
- Chrysamoeba - amoeboid vegetative stage
- Freshwater, with a few exceptions
- May be photosynthetic (autotrophic), mixotrophic, or heterotrophic.
- Even photosynthetic species will live heterotrophically if given sucrose.
- Prefer oligotrophic waters and slightly acidic pH
- DYIII is a culture medium well suited to chrysophytes -- no added phosphate,
and typically adjusted to pH 6
- In nature, are often found in colder waters; arctic and alpine lakes,
and temperate lakes in colder seasons
- They do, however, grow well at higher temperatures
- Some species do occur in the ocean, and these may be significant members
of the nannoplankton (2-20 µm) and picoplankton (0.2-2µm).
Some classifications split addtional groups from the chrysophytes, or are of
- Synura is a colonial flagellate reminiscent of the green algae
in the Volvocales
- A cluster of cells joined by posterior ends
- Moderately ornate silica scales
- Cell structure separates them from other chrysophytes, but they are
definitely closely related
- Possibly an outgroup to the Chrysophyceae sensu stricto
- Unicellular flagellates, radially symmetrical along long axis
- A single anterior flagellum with a single row of hairs projects forward,
and there is a basal body for a second, vestigal flagellum
- Capable of photosynthesis, but also feed actively
- Possible outgroup to Bacillariophyceae (diatoms)
- Tiny, coccoid algae with silica plates covering the cell
- Important members of marine nannoplankton
- Pentalamina -- ca 5 µm dia
- Possible outgroup to Phaophyceae (brown algae)
- Mostly filamentous or thalloid marine or estuarine algae, with a few
- Lateral flagella
- Pyrenoid protrudes from plastid
- Cellulosic cell walls, but lacking alginate
- Flagella with transitional helix
- Do not have unilocular or plurilocular sporangia
Required Reading: VdH Chapter 6, 10, 11
Bourrelly, P. 1968. Les Algues D'eau Douce, Tome II: Les Algues Jaunes et
Brunes. Editions N. Boubee & Cie, Paris.