Phylum Rhodophyta The Red Algae

Like the brown algae, most of the more than 5,000 species of red algae are seaweeds (Fig. 18.21) that tend, however, to occur in warmer and deeper waters than their brown counterparts. Some grow attached to rocks in intertidal zones, where they may be exposed at low tide. Others grow at depths of up to 200 meters (656 feet) where light barely reaches them, and in 1984, a new species of red algae was discovered at a depth of 269 meters (884 feet), where the light is only 0.0005% of peak surface sunlight. A few are unicellular, but most are filamentous. The filaments frequently are so tightly packed that the plants appear to have flattened blades or to form branching segments. Some develop as beautiful feathery structures that have the appearance of delicate works of art.1 None match the large kelps in size, the largest species seldom exceeding a meter (3 feet) in length.

The red algae have relatively complex life cycles, often involving three different types of thallus structures. Meiosis usually occurs on a thallus called a tetrasporophyte, while gametes are produced on separate male and female thalli. All of the reproductive cells are nonmotile and are carried passively by water currents. Zygotes may migrate from one cell to another through special tubes, which form bizarre loops in some species.

In Polysiphonia (Fig. 18.22), a feathery red alga that is widespread in marine waters, the three types of thalli (male gametophyte, female gametophyte, and tetrasporophyte) all

1. Most seaweeds produce their own "glue" and are easy to mount on paper for display. Fresh specimens can be laid directly on clean, high rag-content paper, covered with a layer or two of cheesecloth, and pressed between sheets of blotting paper for a day or two until they are dry. Feathery types will make better specimens if they are placed in a shallow pan of water so that their delicate structures float out, as they do in the ocean. Paper should be slid under them and then carefully lifted so that the seaweed spreads out naturally on the surface. Once the specimens are dry, the cheesecloth (which keeps them from sticking to blotting paper) is removed, and they remain glued to the paper. They can then be displayed or stored indefinitely. Green and brown seaweeds can be treated in similar fashion.

Certain marine algae, which form crusty growths or jointed-appearing upright structures on rocks, accumulate calcium salts as they grow and often contribute to the development of coral reefs. These coralline algae need no special treatment to be displayed, although some may lose their natural pinkish or purplish color when they die.

zygote fertilization

Life Cycle Fucus

Figure 18.19 Life cycle of the common rockweed Fucus.

branch bearing antheridia in which meiosis has occurred zygote fertilization

Figure 18.19 Life cycle of the common rockweed Fucus.

Phylum Phaeophyta Fucus Microscope


Figure 18.20 A sectioned female conceptacle of Fucus, xlQQ.


Figure 18.20 A sectioned female conceptacle of Fucus, xlQQ.

branch bearing antheridia in which meiosis has occurred outwardly resemble one another. They are about 2 to 15 centimeters (1 to 6 inches) tall and are branched into many fine, threadlike segments. Spermatangia, the male sex structures, slightly resemble dense clusters of tiny grapes on slender branches of the male gametophyte thallus. Each sper-matangium contains a single spermatium that functions as a nonmotile male gamete.

The female sex structures, called carpogonia, are produced on the female gametophyte thallus. Each carpogo-nium consists of a single cell that looks something like a microscopic bottle with a long neck called a trichogyne. A single nucleus at the base of the carpogonium functions as the female gamete, or egg. Since the spermatia have no flagella, they cannot move of their own accord, but currents may carry them considerable distances. If a spermatium should brush against a trichogyne, it may become attached. The walls between the spermatium and the trichogyne then break down, the nucleus of the spermatium migrates to the egg nucleus, and the two nuclei unite, forming a zygote.

Next, toward the base of a pericarp (an urn-shaped body, the outer part of which is formed by the female gametophyte thallus), the zygote begins to divide and eventually develops a

Chapter 18


cluster of clublike carposporangia. The pericarp and carpospo-rangia combined constitute the cystocarp. Diploid asexual spores called carpospores are produced in the carposporangia and released, to be carried away by ocean currents.

When a carpospore lodges in a suitable location (e.g., a rock crevice or the hull of a ship), it usually germinates and grows into a tetrasporophyte, which closely resembles a gametophyte thallus. Tetrasporangia are formed along the branches of the tetrasporophytes. Each tetrasporangium undergoes meiosis, giving rise to four haploid tetraspores. When tetraspores germinate, they develop into male or female gametophytes, thereby completing the life cycle.

The red to purplish colors of most red algae are due to the presence of varying amounts of red and blue accessory tetrasporophyte carposporangia carposporangia

Polysiphonia Cystocarp

zygote fertilization tetrasporophyte tetrasporangia

Polysiphonia Tetrasporophyte

zygote becomes carposporangia within developing cystocarp tetrasporangia zygote becomes carposporangia within developing cystocarp zygote fertilization spermatial nucleus migrates down trichogyne' and unites with egg spermatium trichogyne egg carpogonium spermatium trichogyne egg carpogonium

Polysiphonia Reproduction

female gametophyte

Figure 18.22 Life cycle of the red alga Polysiphonia.

female gametophyte

Figure 18.22 Life cycle of the red alga Polysiphonia.

pigments called phycobilins, similar to those found in the cyanobacteria. The similarity led to the belief of some phy-cologists (those who specialize in the study of algae) that the red algae may have been derived from the cyanobacteria. Several other pigments, including chlorophyll a and sometimes chlorophyll d, are also present in the chloroplasts. The principal reserve food is a carbohydrate called floridean starch. A number of red algae also produce agar and other important gelatinous substances discussed in "Agar" under the section entitled "Human and Ecological Relevance of the Algae," which begins on page 344.

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  • philipp
    Why phylum rhodophyta known as red algae?
    8 years ago
  • ronja hahli
    What is the name of the life cycle of red algae?
    8 years ago
  • brigitte vogel
    Where is the zygote develops in phaeophyta?
    8 years ago
  • Joseph
    Does red algae have flowers?
    8 years ago
    Is red algae a flowering plant ?
    8 years ago
  • Kaiju
    Which of the life cycles is typical for plants and some algae?
    8 years ago
  • Hildigard
    What is the name of fruiting of polysiphonia algae?
    4 years ago
  • henri
    Is seldom a specie of red algae?
    2 years ago

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