Rhodophyta

  1. Introduction
    1. The Rhodophyta are a moderately diverse, but extremely ancient, group of marine organisms
    2. About 500 genera, with about 5000 species.
    3. Fossil record dates back to the mid-proterozoic, or nearly 2 ba
      1. Thus red algal fossils are among the most ancient of any eukaryote.
    4. Complex life cycle in many
    5. There is a lot of specialized terminology associated with the Rhodophyta
  2. Structure & metabolism
    1. No flagellate stages
    2. Generally complex, multicellular thalli (singular = thallus)
    3. Reproduction, if present, is oogamous, involving non-motile male spermatia
    4. Mitosis is closed (the nuclear envelope does not break down)
      1. Telophase spindle is perisistant
      2. Centrioles are lacking; a ring-shaped microtubular organizing center (MTOC) is present at the spindle poles during spindle formation
    5. Cytokineisis is by furrowing, but is typically incomplete
      1. Sibling cells are separated by a pit connection, which is a proteinaceous plug that fills the junction between the cells.
    6. Cell walls
      1. Cellulose fibrils embedded in an extensive gelatinous matrix
      2. Porphyra and Bangia have xylan fibrils rather than cellulose
      3. The gelatinous matrix is composed of a variety of sulphated galactose polymers
      4. Agar
      5. Carageenan
    7. Polysaccharide storage typically as floridean starch (alpha 1,4 glucan), which accumulates in in the cytoplasm
    8. Chloroplasts
      1. Primary plastids, surrounded by two unit membranes
      2. Chlorophyll a
      3. Phycobilins arranged in phycobilisomes; thylakoids are not stacked
  3. Reproduction
    1. Typically diplohaplontic -- alternation of haploid and diploid stages
    2. Oogamous
      1. Egg is in carpogonium, receptive surface is trichogyne
      2. Spermatia are non-motile, tiny cells that function as male gametes
    3. Meiosis
      1. Tetrasporangia produce tetraspores via meiosis
    4. Bangiophyceae
      1. Isomorphic or heteromorphic alternation of generations
      2. Life cycle is diplohaplontic
      3. Porphyra has a heteromorphic alternation of generations
        1. This was not originally recognized, so the sporphytes were named Conchocelis
        2. Sporophytes are branched filaments; the familiar sheet-like Porphyra is the (monoecious) gametophyte
    5. Florideophyceae
      1. Life cycle typically diplohaplontic, some are haplontic
      2. Many have a complex modification of a diplohaplontic life cycle
      3. Triphasic alternation of generations
      4. Gametophyte (may or may not be dioecous)
      5. Carposporphyte
      6. Tetrasporophyte
      7. The low efficiency of fertilization
  4. Classification
    1. Bangiophyceae
      1. Porphyridiales
        1. Porphyridium purpureum
        2. Cyanidium caldarium (note! "Cyanidium caldarium strain 14-1-1" is actually Galdieria sulphuraria)
        3. The taxonomic position of Cyanidium is uncertain; it may not even be properly classified with Rhodophyta
      2. Compsopogonales
        1. Compsopogon (a freshwater red found in Maryland)
      3. Bangiales
        1. Porphyra japonica (and Conchocelis phase)
    2. Florideophyceae
      1. Acrochaetiales
        1. Audouinella
      2. Palmariales
        1. Palmaria palmata
      3. Nemaliales
      4. Batrachospermales
        1. Batrachospermum
          1. A freshwater red, found in cold, flowing streams
          2. Intercallary meiosis; the apical cell undergoes meiosis, and the gametophyte develops in place on the top of the sporophyte
      5. Corallinales
        1. Lithophyllum
        2. Corallina
      6. Gelidiales
        1. Gelidium
      7. Gigartinales
        1. Chondrus crispus
        2. Used as a source of carageenan
      8. Rhodymeniales
      9. Ceramiales
        1. Polysiphonia
  5. Ecology
    1. Mostly marine, a few freshwater
    2. Typically live attached to surfaces; not present in the phytoplankton
    3. Light harvesting is very efficient, and red algae can live at tremendous depths.
      1. The record is 268 meters -- roughly 0.001% incident light -- collected by the Littlers, who are at the Smithsonian Institution
      2. It is not certain that these algae were surviving by photosynthesis.
    4. Corraline red algae build up calcium carbonate in their cell walls, and can be reef-building organisms
    5. Parasitism
      1. There are a number of parasitic red algae
      2. Some transfer nutrients from their host plant via haustoria
      3. Other parasitic red algae literally tranfer their nuclei into the host plant, and genetically 'hijack' the host thalli.
  6. Economic Importance
    1. Although not generally considered to be economic organsims, there are some important uses for these algae, particularly associated with their production of polysaccharides.
    2. Agar-agar is a delicacy in Japan. It is a clear, jelly-like food, and is virtually flavorless and undigestable.
    3. Agar has been used for years in microbiology, as it provides a polysaccharide gel which is solid, and nontoxic, yet cannot be digested by most microorganisms.
    4. Agarose -- purified from agar -- is a mainstay of molecular biology, and alternate polysaccharides can be sold at great expense.
    5. Porphyra japonica is eaten in Japan and throughout the Far East.
    6. Japanese name is nori -- used for wrapping sushi, rice crackers, topping on rice, etc.
    7. English term for Porpyra is laver.
    8. The cultivation of nori is a substantial industry
    9. Dulse is eaten in regions surrounding the North Atlantic.
    10. Irish Moss is also used as a source of polysaccharides, e.g., to clarify beer.

 

Required Reading: VdH: Chapter 5

Supplementary Reading:

Freshwater, D.W., S. Fredericq, B.S. Butler, M.H. Hommersand, and M.W. Chase. 1994. A gene phylogeny of the red algae (Rhodophyta) based on plastid rbcL. Proc. Natl. Acad. Sci. USA 91:7281-7285.

Ragan, M.A., C.J. Bird, E.L. Rice, R.R. Gutell, C.A. Murphy, and R.K. Singh. 1994. A molecular phylogeny of the marine red algae (Rhodophyta) based on the nuclear small subunit rRNA gene. Proc. Natl. Acad. Sci. USA 91:7276-7280.

Saunders, G.W., and G.T. Kraft. 1997. A molecular perspective on red algal evolution: focus on the Florideophycidae. In D. Bhattacharya, ed., Origins of Algae and Their Plastids. Springer, New York.

Ragan, M.A., and R.R. Gutell. 1995. Are red algae plants? Bot. J. Linn. Soc. 118:81-105.

Goff, L.J., and A.W. Coleman. 1984. Transfer of nuclei from a parasite to its host. Proc. Natl. Acad. Sci. USA 81:5420-5424.

Goff, L.J., D.A. Moon, P. Nyvall, B. Stache, K. Mangin, and G. Zuccarello. 1996. The evolution of parasitisim in the red algae: molecular comparisons of adelphoparasites and their hosts. J. Phycol. 32:297-312.