Heterokontophyta II — Chrysophyceae and kin


  1. Introduction
    1. Unicellular or colonial, often flagellate or with flagellate stages. A few are multicellular.
    2. Many form silica cysts and/or scales
    3. Typically freshwater, sometimes in brackish or marine waters
    4. Moderately diverse (ca. 200 genera & 1000 species)
  2. Structure & metabolism
    1. Flagella are apical, unequal in length, and one has mastigonemes.
      1. Heterokont-type photoreceptor, with swelling at base of smooth flagellum, and eyespot within chloroplast
      2. Transitional helix in both flagellar bases
    2. Can form silica cysts within the cell
      1. Cyst forms in a special silica deposition vesicle
      2. The cyst is sealed with an organic plug
    3. Scales may also be present, and if so are formed in silica deposition vesicles
    4. Mitosis is open (the nuclear envelope breaks down during mitosis)
      1. The spindle is long, and is persistant in telophase
    5. Chloroplasts are secondary, with a CER but lacking a nucleomorph
      1. Pigmentation includes chlorophylls a- and c-, with fucoxanthin
      2. The fucoxanthin gives the cells a golden-brown color characteristic of heterokonts
      3. Chloroplast DNA in a ring-shaped nucleoid
    6. Do not have a carbonate pump; must use CO2 directly
    7. 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.
    8. 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.
  3. Reproduction
    1. Sexual reproduction is not well characterized (it is unknown in Ochromonas)
    2. The life cycle is probably haplontic (i.e., meiosis is zygotic)
    3. Sex has been documented in Dinobryon, a sessile , stalked chrysophyte.
        Isogamous, with male and female strains. Gametes are not morphologically distinct from vegetative cells.
      1. Female cells secrete hormone (pheremone), and male cells swim toward female
      2. The putative zygote forms a silica cyst, and serves as a resistant stage, but meiosis has not yet been documented.
    4. Sexual reproduction is similar in Synura, where individual cells functioning as male gametes swim away from their colony and fertilize cells in female colonies.
  4. Classification
    1. Chrysophyceae - "golden algae" - are typically planktonic unicells
    2. Ochromonas danica - a well studied chrysophyte
      1. Freshwater, unicellular flagellate
        1. Good representative for cell structure in heterokonts
      2. Naked -- no cell wall -- and capable of amoeboid movement
      3. Amoeboid movement in drier or more confined areas, flagellate swimming in open water
      4. Ochromonas is mixotrophic, and pseudopodia can also capture prey
      5. Mucilage bodies are found near the surface in many species
        1. Mucilage is discharged when cell is irritated
      6. Discobolocyte is an elaborate mucilage body
      7. Contractile vaculoles (one or more) are located near flagella, and are visible in the light microscope.
      8. Chloroplasts are typical heterokont plastids, but lack pyrenoids
      9. Two unequal flagella, with typical heterokont features
        1. Longer, tinsellate flagellum provides propulsion, shorter, smooth flagellum serves as a rudder
      10. Eyespot is assocated with the smooth flagellum
      11. 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.
      12. Silaceous cysts (statospores) are formed internally, in a silica deposition vesicle
        1. During cyst development, a narrow channel provides communication between inner and outer cytoplasm
        2. This is eventually plugged by polysaccharide
        3. Cyst formation is a function of population density
        4. A chrysophyte 'cyst bank' exists in the sediments of suitable lakes, and can give rise to sporadic population spikes
    3. Chrysodidymous and Didymochrysis are genera apparently arrested at different stages in cell division.
    4. Dinobryon looks like Ochromonas in a tiny wine glass
    5. Mallomonas - unicell with fancy silica scales
    6. Synura - like a colonial Mallomonas
    7. Chrysamoeba - amoeboid vegetative stage
  5. Ecology
    1. Freshwater, with a few exceptions
    2. May be photosynthetic (autotrophic), mixotrophic, or heterotrophic.
    3. Even photosynthetic species will live heterotrophically if given sucrose.
    4. Prefer oligotrophic waters and slightly acidic pH
    5. DYIII is a culture medium well suited to chrysophytes -- no added phosphate, and typically adjusted to pH 6
    6. In nature, are often found in colder waters; arctic and alpine lakes, and temperate lakes in colder seasons
    7. They do, however, grow well at higher temperatures
    8. Some species do occur in the ocean, and these may be significant members of the nannoplankton (2-20 µm) and picoplankton (0.2-2µm).

Allied Groups

Some classifications split addtional groups from the chrysophytes, or are of uncertain placement:

  1. Synurophyceae
    1. Synura is a colonial flagellate reminiscent of the green algae in the Volvocales
    2. A cluster of cells joined by posterior ends
    3. Moderately ornate silica scales
    4. Cell structure separates them from other chrysophytes, but they are definitely closely related
    5. Possibly an outgroup to the Chrysophyceae sensu stricto
  2. Pedinellophyceae
    1. Unicellular flagellates, radially symmetrical along long axis
    2. A single anterior flagellum with a single row of hairs projects forward, and there is a basal body for a second, vestigal flagellum
    3. Capable of photosynthesis, but also feed actively
    4. Pedinella
  3. Parmophyceae
    1. Possible outgroup to Bacillariophyceae (diatoms)
    2. Tiny, coccoid algae with silica plates covering the cell
    3. Important members of marine nannoplankton
    4. Pentalamina -- ca 5 µm dia
  4. Sarcinochrysidophyceae
    1. Possible outgroup to Phaophyceae (brown algae)
    2. Mostly filamentous or thalloid marine or estuarine algae, with a few planktonic unicells
    3. Lateral flagella
    4. Pyrenoid protrudes from plastid
    5. Cellulosic cell walls, but lacking alginate
    6. Flagella with transitional helix
    7. Do not have unilocular or plurilocular sporangia
    8. Olisthodiscus
    9. Sarcinochrysis

Required Reading: VdH Chapter 6, 10, 11

Supplementary Reading:

Bourrelly, P. 1968. Les Algues D'eau Douce, Tome II: Les Algues Jaunes et Brunes. Editions N. Boubee & Cie, Paris.