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Chlorophyceae: The „Real“ Green Algae
  • Appearance: non-flagellated unicells, flagellates, filaments, colonies, coenobia, sarcinoid cell aggregations, siphonous forms
  • Cell division always involves closed mitosis and a phycoplast (microtubuli parallel to the division plane)
  • Plasmodesmata are present in species that build cell wall by Golgi vesicle depositions but are lacking in species that divide by furrowing
  • Sexual reproduction by isogamy, anisogamy, or oogamy result in thick-walled, spined hypnozygotes (zygospores); zygotes serve as „resting spores“ and germinate by meiosis

  • Asexual reproduction by 
    • Zoospores: flagellates with eye-spot and contractile vacuole
    • Aplanospores: non-flagellated cells with contractile vacuole
    • Autospores: non-flagellated cells lacking contractile vacuole
  • Terrestrial species predominantly form autospores (no liquid medium)
Sphaeropleales 
  • Monphyletic group of chlorophyceans
  • Mostly non-flagellated species but flagellates spores and gametes
  • Ankistrodesmus produces 1-16 autospores per cell

  • Pediastrum is a very common freshwater colony with species-specific number of cells; exhibits autocolony formation: each cell can divide into the same number of zoospores as the number in the colony; zoospores are retained within a vesicle and arrange in the same planar pattern as the mother colony; young colonies are released from the vesicle


  • Hydrodictyon, the „water net“, 4-6 cm wide; single cells of up to 1 cm, multinucleate; autocolony formation by ca. 20,000 bi-flagellate zoo-spores within mother cell; isogamous sexual reproduction

Chlamydomonas 
  • Chlamydomonas: biflagellate unicell with cuplike chloroplast, very common; 500 species in the genus
  • Major model system for genetic and physiological research
  • Asexual reproduction by 2-16 mitotic cells

  • Sexual reproduction by isogamous or anisogamous biflagellate gametes; some oogamous species; 



The Volvocales Coenobia
 
     
    • Monophyletic group of coordinated colonies of multiple of two Chlamydomonas-like cells
    • Mucilagenous matrix or cells hold loosely by mucilages in one layer, forming a hollow sphere in more evolved species
    • Large colonies seen as predator defense
    • Common forms in summer freshwater systems
    • Increase in complexity from the genus Gonium to Volvox:
    • Gonium: 4, 8, or 32 cells in flat plate
    • Pandorina: globular colony of 16-32 biflagellate cells that are closely adherent at their bases; eye-spots in anterior cells larger than in posterior cells, first degree of colony polarity
    • Eudorina: globular colony of 16-32 firmly connected biflagellate cells; 
    • Volvox: large (500 to several thousand cells) colony, cells in a peripheral mucilage shell; each cell possesses its own mucilage package; protoplasmic strands connect cells; anterior eye-spots larger, reproduction from posterior cells
    • Flagella origin: whereas V-shaped in Chlamydomonas, the flagella originate parallel in the Volvocales 
    • Flagella beat in the same direction (in opposite directions in Chlamydomonas) and flagella beat is coordinated among cells of the coenobium
    • Positive phototaxis is common among Volvocales
    • Eye-spots are mostly larger in apical cells
    • Swimming path: colonies rotate in flat coenobia (Gonium) and „roll“ through the water in spherical coenobia (Pandorina, Eudorina, Volvox)
    • Interconnection of cells is more complex in more complex coenobia (from Gonium to Volvox)


    • Single flagellates in (from left to right): Gonium, Pandorina, Eudorina, Volvox
    • Autocolony formation: gonidia develop by asymmetric cell divison in the posterior part; at 8-cell stage, a pore opens; when final cell number is reached, colony inverts through pore
    • Sexual reproduction starts by some colonies to produce male gametes (flat packets of 16-64 pale flagellates); they release a pheromone that triggers female gamete (egg) production; 
    • Fertilization: sperms lyze whole in female colony and fertilize eggs; unfertilized eggs may produce colony
    • Zygote develops thick, red, spiny cell wall







     
     
     
     
     
     
     
     




    from top: sperm packet, egg cell, zygote
     
     


    Inversion of the Volvox embryo, cross sections
     

Non-Motile Volvocales
  • Formerly assigned to the class Tetrasporales
  • Large gelatinous coenobia predominantly in the epiphyton
  • Pseudocilia: each cell carries two pseudocilia instead of flagella; pseudocilia lack the two central microtubuli of the typical 9+2 structure and developed by reduction of flagella
  • Reproduction by colony fragmentation, zoospores, and isogamy


  •  
  • Flagella origin: whereas V-shaped in Chlamydomonas, the flagella originate parallel in the Volvocales 
  • Flagella beat in the same direction (in opposite directions in Chlamydomonas) and flagella beat is coordinated among cells of the coenobium
  • Positive phototaxis is common among Volvocales
  • Eye-spots are mostly larger in apical cells
  • Swimming path: colonies rotate in flat coenobia (Gonium) and „roll“ through the water in spherical coenobia (Pandorina, Eudorina, Volvox)
  • Interconnection of cells is more complex in more complex coenobia (from Gonium to Volvox)
The Dunaliella Clade and the Chaetophorales
  • Dunaliella clade: named after Dunaliella, group of flagellates, non-motile unicells, filaments; temporarily grouped by genetic sequence results


  • from left to right: Haematococcus, Dictyosphaerium, Selenastrum, Botryococcus, Dunaliella
     
  • Chaetophorales: branched or unbranched filaments with quadriflagellate reproductive cells

    from left to right: Uronema, Microspora, Chaetophora, Draparnaldia, Fritschiella
Oedogoniales
 
    • Small group (3 genera, 600 species) of freshwater, filamentous algae with uninucleate cells
    • Characteristic genus: Oedogonium
    • Chloroplasts are highly dissected, net-like
    • Sexual reproduction by oogamy
    • Ring of flagella on zoospores and male gametes
    • Dwarf male thallus lives on female thallus
    • Cell wall of Oedogonium possesses characteristic „rings“ near the apical end due to intercalary cell division


    Upper: vegetative cells and zygote of Oedogonium; middle: rings on cell wall of Oedogonium; lower: fertilization of egg cell by sperm (oogamy) in Oedogonium

    The Oedogoniales species Bulbochaete carries long, thin spines at the apical end of the branches