Classification

F.E. Fritsch classified the algae into eleven classes based on their photosynthetic pigments, the number and type of flagella, the chemical nature of the stored food reserve, thallus organisation and reproductive mode, publishing the scheme as The Structure and Reproduction of the Algae (1935). The eleven classes are Chlorophyceae, Xanthophyceae, Chrysophyceae, Bacillariophyceae, Cryptophyceae, Dinophyceae, Chloromonadineae, Euglenophyceae, Phaeophyceae, Rhodophyceae and Cyanophyceae (see Table 2.2 for the diagnostic pigment/flagella/food combination of each).

Three of these classes are highlighted for their ecological and structural importance. Chlorophyceae ('green algae') are mostly aquatic — freshwater (Spirogyra) or marine (Ulva) — with a few terrestrial forms (Trentepohlia). Their chloroplast shape varies strikingly by genus: cup-shaped in Chlamydomonas, discoid in Chara, girdle-shaped in Ulothrix, reticulate (net-like) in Oedogonium, spiral in Spirogyra, stellate (star-shaped) in Zygnema and plate-like in Mougeotia. Chlorophyll a and b are the dominant pigments, starch-storing pyrenoids are present, and the cell wall has an inner cellulose layer and an outer pectin layer. All three reproductive modes occur, and examples include Chlorella, Chlamydomonas, Volvox, Spirogyra, Ulothrix, Chara and Ulva.

Phaeophyceae ('brown algae') are almost entirely marine (Pleurocladia is the rare freshwater exception), with a thallus that can be filamentous (Ectocarpus), frond-like (Dictyota) or a giant kelp (Laminaria, Macrocystis); larger forms differentiate into a photosynthetic frond, a stalk-like stipe and an anchoring holdfast. Their pigments — chlorophyll a and c, carotenoids, xanthophylls, and especially the golden-brown pigment fucoxanthin — give the group its olive-green-to-brown colouring, and mannitol plus laminarin serve as reserve food. Motile stages carry two laterally inserted, unequal flagella (one whiplash, one tinsel); sexual reproduction ranges from isogamy to (most commonly) oogamy, and alternation of generation may be isomorphic, heteromorphic or diplontic. Examples include Sargassum, Laminaria, Fucus and Dictyota. …