Brown algae

Class Phaeophyceae

The Phaeophyta, or the brown algae, are a group of macroscopic, photosynthetic algae. They form some of the largest kelps in the ocean, serving as a crucial foundation for cold-water coastal ecosystems, usually on the west coast of continents. They have structures that are convergent with land plants, such as root-like and leaf-like structures, as well as phloem-like cells for moving sugars. The brown algae possess alginic acid which has many uses including as a thickening agent in edible and non-edible products.

Ecology and form

  • Photosynthetic algae, which can be quite large

    • Brown algae use chlorophyll a & c pigments, as well as fucoxanthin, which is an accessory pigment in the chloroplasts

    • These pigments give these algae a brown color; they absorb light in the blue-green to yellow-green spectrum

  • Kelps: brown algae can grow up to 200ft long; many are highly differentiated and plant-like

    • Holdfast: root-like attachment to rocky strata

    • Stipe: short and stem-like

    • Blade: very long and conducts photosynthesis

    • Air bladders: maintain buoyancy

  • Produce algin or alginic acid

    • Forms a gum-like substance

    • Capable of absorbing 200-300 times its own weight

Above: Macrocytis floating on top of the water

Above: the root-like holdfast and stipe of a kelp

Above: Fucus, rockweed, with air bladders

Above: Macrocytis under the water

Gamete-forming phase (=gametophyte)

  • Plurilocular gametangia: produce gametes through mitosis, which fuse during fertilization

  • These fused gametes will create a diploid zygote which will anchor itself to the seafloor and grow into a macroscopic seaweed

Above: plurilocular gametangia of Ectocarpus

Spore-forming phase (=sporophyte)

  • Unilocular sporangia: produce spores through meiosis

  • Brown algae can be either haplo-diplontic or diplontic

  • In haplo-diplontic algae, the spores will germinate into

Above: unilocular sporangia of Ectocarpus

Diversity

  • 1,500 - 2,000 species in 250 genera

  • e.g. Laminaria, Fucus, Macrocytis, Ectocarpus, Sargassum

  • Mostly marine

  • Dominate rocky shores

  • e.g. seaweeds, rockweeds, kelps

Classification

Eukaryotes

├Chromoalveolata

└Heterokonts

└Brown Algae (Phaeophyta)

Geologic Age

Questions for Thought

  • How are the brown algae similar in structure to land plants?

  • What do the terms "unilocular" and "plurilocular" refer to?

  • How is the life cycle similar to land plants?

  • Why do kelp forests exist on the West coast of North America, but not the eastern coast?

Additional Resources