Phaeophyceae

The Phaeophyceae (or brown algae) include the largest of all the algae, with some representatives (eg the Californian giant kelp) reaching over 30 m in length. The brown algae are almost exclusively marine. The colour comes from the pigments chlorophyll 'c' and fucoxanthin. The pigment fucoxanthin absorbs light at the yellow part of the spectrum and so brown algae can thrive deeper than the green algae (which utilise red light), although not as deep as red algae (which uses blue light).

There are around 150 species of brown algae around the British Isles ranging from microscopic species such as Vaucheria sp. and relatively giant species such as the northern kelp Laminaria hyperborea (greater than 2 m long). There are two main orders of Phaeophyceae; the Fucophyceae and the Xanthophyceae. Brown seaweeds reproduce in a variety of ways, including vegetatively (eg fragmentation), asexually (from non-sexually produced spores) and sexually with the production of gametes. Male and female gametangia can either be produced on the same thallus (monecious) as in Giffordia or on different male and female thalli (dioecious) as in Laminaria.

Good texts for identifying brown seaweed around the British Isles include: A Handbook of the British Seaweeds by Lily Newton (1931), Seaweeds of the British Isles, Volume 3 Part 1 Fucophyceae (Phaeophyceae) by Robert L. Fletcher, and Field Studies Council AIDGAP publication ‘A Field Key to the British Brown Seaweeds’ by Sue Hiscock.