The algae in the Heterokontophyta usually have cells with an anterior tinsel and posterior whiplash flagellum (Fig. 10.1). The plastids contain chlorophylls a and c along with fucoxanthin. The storage product is usually chrysolaminarin in cytoplasmic vesicles.
Based on molecular evidence (Horn et al., 2007 ; Kai et al., 2008 ; Schmidt et al., 2012 ; Yang et al., 2012), the algae in the Heterokontophyta can be divided into three clades.
Clade 1. Chrysophyceae, Synurophyceae, Eustigmatophyceae, Pinguiophyceae, Synchromophyceae.
Clade 2. Bacillariophyceae, Bolidophyceae, Dictyochophyceae, Pelagophyceae.
Clade 3. Phaeophyceae, Raphidophyceae, Xanthophyceae, Phaeothamniophyceae, Chrysomerophyceae, Schizocladiophyceae, Aurearenophyceae.
Since some of these classes have only a few representatives, and because this is a basic textbook, I have chosen not dealt with the algae in the Synchromophyceae, Chrysomerophyceae, Schizocladiophyceae, and Aurearenophyceae. The following classes are covered here (Andersen, 2004):
Chrysophyceae (golden-brown algae) (Chapter 10)
Synurophyceae (Chapter 11)
Eustigmatophyceae (Chapter 12)
Pinguiophyceae (Chapter 13)
Dictyochophyceae (silicoflagellates) (Chapter 14)
Pelagophyceae (Chapter 15)
Bolidophyceae (Chapter 16)
Bacillariophyceae (diatoms) (Chapter 17)
Raphidophyceae (chloromonads) (Chapter 18)
Xanthophyceae (yellow-green algae) (Chapter 19)
Phaeothamniophyceae (Chapter 20)
Phaeophyceae (brown algae) (Chapter 21).
The Chrysophyceae are distinguished chemically by having chlorophylls a, c 1, and c 2 (Andersen and Mulkey, 1983) and structurally by two flagella inserted into the cell perpendicular to each other, one photoreceptor on the short flagellum that is usually shaded by an eyespot in the anterior portion of the chloroplast, contractile vacuoles in the anterior portion of the cell, chloroplast endoplasmic reticulum, and radially or biradially symmetrical silica scales (if they are present). The storage product is chrysolaminarin. Many members of the class produce statospores enclosed in a silicified wall with a terminal pore. The Chrysophyceae/Synurophyceae probably originated about 268 Ma (million years ago) (Brown, 2010).
Most of the species in the Chrysophyceae are freshwater species, and occur in soft waters (low in calcium). Many of the freshwater species are in the plankton of lakes where they are present in abundance. The coccoid and filamentous genera are found mostly in cold springs and brooks, where they occur as gelatinous or crustous growths on stones and woodwork.