Page 38 - Algae

General Overview 21

FIGURE 1.30 Marine diatom. FIGURE 1.31 Freshwater diatom. (Bar: 20 mm.)
(Bar: 10 mm.)

(Figure 1.33). Phaeophyceae are multicellular, from branched filaments to massive and complex
kelp (Figure 1.34). Other groups of algae have been described as belonging to this division, such
as Pelagophyceans and Sarcinochrysidaleans sensu Graham and Wilkox (2000) and Parmales
sensu Van den Hoek et al. (1995). Heterokontophyta are mostly marine; but they can be found
also in freshwater and terrestrial habitats. They show a preponderance of carotenoids over chlor-
ophylls that result in all groups having golden rather than grass green hue typical of other major
algal divisions. The members of this division possess chlorophylls a, c 1, c 2 , and c 3 with the excep-
tion of the Eustigmatophyceae that have only chlorophyll a. The principal accessory pigments are
b-carotene, fucoxanthin, and vaucheriaxanthin. The thylakoids are grouped into stacks of three,
called lamellae. One lamella usually runs along the whole periphery of the chloroplast, which
is termed girdle lamella, absent only in the Eustigmatophyceae. The chloroplasts are enclosed
in their own double membrane and also by a fold of the endoplasmic reticulum. The chloroplastic
DNA is usually arranged in a ring-shaped nucleoid. Dictyochophyceae species possess several
nucleoids scattered inside the chloroplast. The main reserve polysaccharide is chrysolaminarin,
a b-1,3-glucan, located inside the cytoplasm in special vacuoles. The eyespot consists of a
layer of globules, enclosed within the chloroplast, and together with the photoreceptor, located
in the smooth flagellum, forms the photoreceptive apparatus. The members of this division can
grow photoautotrophically but can also combine different nutritional strategies such as heterotro-
phy. The Heterokontophyta species that reproduce sexually have a haplontic (Chrysophyceae),
diplontic (Bacillariophyceae) or diplohaplontic (Phaeophyceae) life cycle.

HAPTOPHYTA
The great majority of Haptophyta are unicellular, motile, palmelloid, or coccoid (Figure 1.35), but a
few form colonies or short filaments. These algae are generally found in marine habitats, although
there are a number of records from freshwater and terrestrial environments. Flagellate cells bear
two naked flagella, inserted either laterally or apically, which may have different length. A structure
apparently found only in algae of this division is the haptonema, typically a long thin organelle