Algae Order II: Chlorococcales

Order II: Chlorococcales

• Families: Chlorococcaceae, Protosiphonaceae, and Hydrodictyaceae
• Hydrodictyaceae:
  • Examples: Pediastrum and Hydrodictyon
  • Pediastrum:
    • Unicellular, planktonic, and flat colony
    • Peripheral cells have one or two protrusions
    • Asexual reproduction: formation of autocoenobia
    • Autocoenobia can grow into new coenobia if released from the enclosing mother cell wall
  • Hydrodictyon:
    • Name means "water net"
    • Nets are cylindrical with closed ends
    • Cells are cylindrical with many nuclei in the peripheral cytoplasm
    • Chloroplast is reticulate with pyrenoids
    • Asexual reproduction: mainly through autocoenobia formation
    • Sexual reproduction: isogamous

Order III: Ulothricales

• Family: Ulothricaceae
• Members: found in various habitats, including freshwater, marine, and soil
• Ulothrix:
  • Unbranched, uniseriate, cylindrical filament
  • Mostly grows attached to solid objects
  • Cells have single chloroplast, ring- or girdle-shaped, with one or many pyrenoids
  • Nucleus is enclosed in the chloroplast
  • Reproduction:
    • Vegetative: through fragmentation
    • Asexual: by zoospores, aplanospores, and hypnospores
    • Sexual: isogamous
  • Zoospores:
    • Either biflagellate or quadriflagellate
    • When at rest, they attach themselves to solid objects by flagella and grow into new filaments
  • Hypnospores:
    • Aplanospores that acquire thick walls and dark pigments in unfavorable environmental conditions
  • Isogametes:
    • Biflagellate
    • Facultative gametes that can behave as zoospores if they fail to fuse
  • Life cycle: haplontic (zygote is the only diploid phase)

Order IV: Oedogoniales

• Family: Oedogoniaceae
• Only family in this order
• Comprises only three genera: Oedogonium, Bulbochaete, and Oedocladium
• Oedogonium:
  • Unbranched, uniseriate filament
  • Cells are uninucleate
  • Occurs in freshwater, attached by means of a basal holdfast as an epilithic or epiphytic alga
  • Chloroplasts possess many pyrenoids
  • Nucleus lies in the periphery of the cell
  • Filaments are polar due to the presence of the holdfast
  • Caps are residual portions of cells that divide to bring about intercalary growth
  • Reproduction:
    • Vegetative: through fragmentation
    • Asexual: by zoospores, akinetes, or aplanospores
    • Sexual: by oogamy
  • Zoospores:
    • Flagella are numerous and crown-like (up to 120)
    • After a period of motility, they attach themselves to a substratum and give new filaments
  • Sexual reproduction:
    • Oedogonium may be heterothallic or homothallic
    • Oogonium is formed by enlargement of a cap cell, which becomes spherical and bright orange
    • Antheridium can occur in one of two ways:
      • Macrandrous: small flattened antheridia are formed by successive rapid divisions of cap cells
      • Nannandrous: swarmer released from an antheridial-like cell, which does not work as an antherizoid, termed an androspore
  • Androspores:
    • Swim towards the oogonium, but do not fuse with it
    • Instead, they become attached to the oogonial wall or a cell nearby and germinate into small two- or three-celled filaments
  • The distal end of the filament functions as the antheridium
  • Zygote develops a two- or three-layered wall and undergoes a resting period before germinating by the production of zoospores