Heterokontophyta V — Phaeophyceae

Phaeophyceae - the brown algae

  1. Introduction
    1. Multicellular, from branched filaments to massive and complex kelp
    2. Because of their large size, the brown algae were long given their own division, the Phaeophyta, but they are clearly members of the heterokonts, and are clearly a comparable group to the other major heterokont groups.
    3. Major elements in the temperate marine coastal flora, but less conspicuous in the tropics.
    4. Relative newcomers in the geologic record
      1. Unlike diatoms, brown algae do not have hard parts that fossilize well.
      2. Some ancient fossils have been interpreted as brown algae, but these identifications are very tenuous
    5. About 265 genera, and 1500-2000 species.
    6. Amost exclusively marine
  2. Structure & metabolism
    1. Multicellular
    2. Filamentous
    3. Parenchymatous
      1. Intercallary meristem located at the juncture between blade and stipe
      2. Meristoderm -- cortical tissue responsible for photosynthesis, but also capable of meristematic activity
    4. Flagellate cells are either gametes or zoospores; the vegetative thallus is never motile
      1. Flagellate stages are more or less typical heterokont cells, but with the flagella inserted laterally
      2. A typical heterokont eyespot is present in most, but a few species lack the eyespot
    5. Chloroplasts are generally discoid, and are secondary, with typical heterokont membrane arrangements.
      1. A CER is present
      2. Thylakoids are in groups of three, with girdle lamellae
      3. Pigmentation is Chlorphylls a and c, with fucoxanthin giving them a golden-brown color
    6. No silaceous cell walls or scales are present
    7. Cell walls are cellulosic, with the cellulose stiffened by calcium alginate. Mucilaginous substances, including fucoidan and other alginates form the amorphous portion of the wall.
    8. During mitosis, the nuclear envelope does not break down until late anaphase, and is thus considered to be semi-closed. The spindle is not persistant in telophase, and the daughter nuclei lie relatively close to each other during cytokinesis.
    9. Distinctive sporangia are present, and are diagnostic for the Phaeophyceae
      1. Plurilocular sporangia are involved in gamete formation
      2. Unilocular sporangia release zoospores (also called zoids)
  3. Reproduction
    1. Isogamous, anisogamous, or oogamous.
    2. Most species are diplohaplontic, with an alternation of free-living generations. These may be isomorphic or heteromorphic.
    3. In the Laminariales -- the kelps -- the life cycle is a heteromorphic alternation of generations
      1. The dioecious gametophytes are microscopic and filamentous, living at the same depth as the sporophyte holdfast
      2. The sporophyte is the familar macroscopic kelp
      3. Oogamous
    4. In the Fucales and Durvillaeales -- called rockweeds or "wracks" -- the life cycle is diplontic and oogamous. Meiosis is gametic, and the only haploid stages are the gametes.
  4. Classification
    1. Organisms to know
      1. Ectocarpales
        1. Ectocarpus
        2. Pilayella littoralis (=Pylaiella) widely used in laboratory studies of brown algae
      2. Sphacelariales
        1. Sphacelaria
      3. Syringodermatales
      4. Dictyotales
        1. Dictyota
      5. Scytosiphonales
        1. Scytosiphon
      6. Cutleriales
      7. Dictyosiphonales
      8. Chordariales
      9. Sporochnales
      10. Desmarestiales
      11. Laminariales -- Kelps
        1. Laminaria
        2. Nereocystis
        3. Macrocystis
        4. Postelsia
      12. Fucales Rockweeds
        1. Fucus
        2. Sargassum
      13. Durvillaeales
      14. Ascoseirales
  5. Ecology
    1. Almost all are marine, although there are four or five small genera that occur in fresh waters.
    2. Most abundant and diverse in temperate and polar waters, but a few do occur in tropical waters.
    3. Most live attached to a solid substrate, either rock or other hard surfaces such as barnacles and other molluscs, or sometimes as epiphytes on other algae.
    4. The major exception are some species of Sargassum, which form a huge floating mass in the mid-atlantic, the "Sargasso Sea". This is a very unusual habitat, and there are many distinctive organisms that depend upon the Sargassum for food and shelter. Nutrient cycling in the Sargasso Sea needs further investigation; it is a region of unusually high productivity in relatively nutrient-poor waters.
    5. Laminaria's distribution is limited in part by the opacity of the water, the lower limit being the depth at which about 0.6% of the incident sunlight reaches the sea floor.
    6. Somtimes live at great depth. According to Bold and Wynne (1985), living Lobophora variegata was collected at 220 m near the Bahamas.
    7. Balance between kelp, sea urchins, sea otters
  6. Economic importance
    1. Food
    2. Polysaccharides
    3. Iodine production


Required Reading: VdH Chapter 12, 13

Supplementary Reading:

Ricketts, E.F., J. Calvin, and J.W. Hedgpeth; revised by D.W. Phillips. Between Pacific Tides, 5th edition.Stanford University Press, Stanford, CA. [First published in 1939, the second edition of Between Pacific Tides had a forward by John Steinbeck. The book emphasizes animals, but is fun to read and has some nice information on intertidal zonation and kelps]