Red Algae

Kingdom Rhodoplantae

The Red algae, or Rhodophyta, are a diverse group of marine algae with fewer freshwater representatives. They are photosynthetic organisms with chlorophyll that is masked by secondary pigments, giving them a red color. Some have toxic terpenoids to prevent herbivory, and others (coralline red algae) incorporate calcium carbonate into their structure to provide a rigid defense. They are related to land plants, but have one of the most complex life cycles among living organisms, with three different phases. Agar from red algae is used as binding agent and emulsifier, as well as growing medium for culturing bacteria in lab settings. Nori (Porphyra), which is used in Japanese maki rolls, is the single most valuable marine crop grown by aquaculture with a value in excess of US$1 billion.

Above: close-up of a red alga

Above: Coralline red algae

Form and Ecology

  • Extremely diverse in marine environments

  • Non-motile aquatic group; does not produce flagella in any part of its life cycle

    • Mostly macroscopic algae, and structurally complex

    • Lack centrioles in their cells

  • Photosynthetic organisms

    • Chlorophyll a and d

    • Use phycoerythrin and phycocyanin as accessory pigments (giving them their red color)

    • Use "Floridean Starch" as food reserve (unlike plants which use true starch)

  • Cell walls are made of cellulose and covered by agar and carrageenan

  • Some species incorporate calcium carbonate into tissues

    • Calcium carbonate acts as structural deterrent to herbivorous organisms

    • These algae are called the coralline red algae

Life cycle

  • Complex haplo-diplontic lifecycle

    • Carposporophyte (2n): diploid stage that forms from fertilization and produces asexual carpospores

    • Tetrasporophyte (2n): diploid stage that forms from carpospores, and produces haploid tetraspores through meiosis

    • Gametophyte (1n): haploid stage that forms from tetraspores, and produces gametes

      • Spermatia: non-motile sperm

      • Trichogyne: female stalk that catches spermatia

  • Carposporophyte → mitosis → Carpospores → Tetrasporophyte → meiosis → Tetraspores → mitosis → Gametophyte → mitosis → Gametangia → mitosis → Spermatia /eggs → fertilization → Zygote → mitosis → Carposporophyte

Classification

Eukaryotes

└Archaeplastida

└Red algae

Diversity

  • 2,500 - 6,000 species, about 670 genera

  • e.g. Polysiphonia, Chondrus, Porphyra

Geologic Range

Above: Polysiphonia carposporophyte, inside the pericarp cup. The branches are the female gametophyte

Above: Male gametophytes of Polysiphonia

Above: Tetrasporophyte of Polysiphonia

Above: life cycle of the red algae

Ethnobotany

  • Nori, sheets of dried algae used for sushi, comes from the genus Porphyra

    • Nori used in Japanese maki rolls is the single most valuable marine crop grown by aquaculture with a value in excess of US$1 billion

  • Terpenoids, toxic chemicals, used by algae to prevent herbivory

  • Agar, chemical that coats cell walls of algae, used as binding agent and emulsifier

    • Agar is also used as a growing medium for culturing bacteria

Above: red algae, Porphyra, used for nori

Questions for Thought

  • Why do red algae produce toxic terpenoids?

  • How is the red algae life cycle different from other haplo-diplontic life cycles?

  • How is the red algae life cycle similar to plants?

  • What is the function of the tetrasporophyte?

  • Where does the carposporophyte reside?

  • Why are Rhodophyta, red in color?

Additional Resources