Page 36 - Algae

General Overview 19

have a starch-like reserve polysaccharide. These prokaryotes contribute a large percentage of the
total organic carbon in the global tropical oceans, making up to 25–60% of the total chlorophyll
a biomass in the tropical and subtropical oceans. They are also able to fix nitrogen, though not
in heterocysts. Both blue-green algae and prochlorophytes contain polyhedral bodies (carboxy-
somes) containing RuBisCo (ribulose bisphospate carboxylase/oxygenase, the enzyme that
converts inorganic carbon to reduced organic carbon in all oxygen evolving photosynthetic
organisms), and have similar cell walls characterized by a peptoglycan layer. Blue-green
algae and Prochlorophytes can be classified as obligate photoautotrophic organisms. Reproduction
in both divisions is strictly asexual, by simple cell division of fragmentation of the colony or
filaments.

GLAUCOPHYTA
Glaucophytes (Figure 1.28) are basically unicellular flagellates with a dorsiventral construction;
they bear two unequal flagella, which are inserted in a shallow depression just below the apex of
the cell. Glaucophytes are rare freshwater inhabitants, sometimes collected also from soil
samples. They posses only chlorophyll a and accessory pigments such as phycoerythrocyanin, phy-
cocyanin, and allophycocyanin are organized in phycobilisomes. Carotenoids such as b-carotene
and xanthophylls such as zeaxanthin are also present in their chloroplast. This unusual chloroplast
lies in a special vacuole and presents a thin peptidoglycan wall located between the two
plastid outer membranes. Thylakoids are not stacked. The chloroplast DNA is concentrated in
the center of the chloroplast, where typically carboxysomes are present, which contain the
RuBisCo enzyme. Starch is the reserve polysaccharide, which is accumulated in granular form
inside the cytoplasm, but outside the chloroplast. Glaucophytes live photoautotrophically with
the aid of blue-green plastids often referred to as cyanelles. Cyanelles are presumed to be phylo-
genetically derived from endosymbiotic cyanobacterium. Sexual reproduction is unknown in this
division.

FIGURE 1.28 A group of eight autospore of Glaucocystis nostochinearum still retained within parent cell
wall. (Bar: 10 mm.)