Page 131 - Algae Anatomy, Biochemistry, and Biotechnology

114 Algae: Anatomy, Biochemistry, and Biotechnology

The thylakoid membranes of Prochloron (Prochlorophyta) are organized into stacked and
unstacked regions reminiscent of the grana and stromal lamellae of higher plant chloroplasts.
They differ from cyanobacteria in that they contain chlorophyll a/b light harvesting systems
rather than phycobiliproteins organized in the phycobilisomes.

Glaucophyta

As already described in Chapter 1 these algae possess inclusion termed cyanelles that are probably
symbiotic cyanobacteria functioning as chloroplasts. Each cyanelle, surrounded by a reduced pep-
toglycan cell wall (except in Glaucosphaera sp.), is enclosed in a vesicle of the host cytoplasm.
Cyanelles do not fix molecular nitrogen in contrast with cyanobacteria; they contain polyphospate
granules and a conspicuous central carboxisome similar to the pyrenoids of other algae. The
thylakoid are not stacked but they are single and equidistant with a concentric arrangement.
Cyanelle pigments are chlorophyll a and b-carotene, which represents the main carotenoid.
Interthylakoidal phycobilisomes contain allophycocyanin and phycocianin. Phycoerytrin is
absent from Glaucophyta but phycoerythrocyanin can be found in some species.

Rhodophyta
Ultrastructurally, red algal chloroplasts are composed of a double-membrane envelope inside of
which are one or more parallel, thylakoidal photosynthetic lamellae. These chloroplasts are not
associated with the endoplasmic reticulum, a feature shared with Glaucophyta and Chlorophyta.
Encircling thylakoids are present in all Florideophyceae and in some taxa of Bangiophyceae
while the thylakoids in the other red algae lie equidistant and are single, that is, not stacked,
unlike any other group of eukaryotic algae (except Glaucophyta), and typically oriented parallel
to each other. All thylakoids have phycobilisomes attached to their stromal surface, which
contain the accessory phycobiliprotein pigments, that is, allophycocianin, phycocyanin, and five
forms of phycoerythrin (Figure 2.77). Chlorophyll a is the only chlorophyll present in the thylakoid
membrane, together with carotenoids such as b-carotene and lutein. Plastid number, shape, and
position (many, discoid, and parietal) is rather uniform throughout the Florideophyceae, and
pyrenoids may or may not be present. A single stellate plastid with a central pyrenoid is commonly
associated with bangiophycidean red algae, such as Phorphyridium. DNA is organized into numer-
ous nucleoids scattered throughout the chloroplast.

Heterokontophyta
Some ultrastructural features of the chloroplast compartments of these algae are common to all the
seven classes of the division, with few exceptions. Four membranes surround the chloroplasts, the
outer two being the chloroplast endoplasmic reticulum and the inner two being the chloroplast

FIGURE 2.77 Transmission electron microscopy image of rhodophyte thylakoid showing with regularly
arranged phycobilisomes. (Bar: 0.05 mm.)