24
Algae
extends the light-harvesting capacity of these algae into the blue and orange/red regions of the visible
light spectrum. Other pigments are β-carotene and several xantophylls (zeaxanthin is the principal
one). Their thylakoids, which lie free in the cytoplasm, are arranged in stacks. Prochlorophytes have
a starch-like reserve polysaccharide. These prokaryotes contribute a large percentage of the total
organic carbon in the global tropical oceans, making up from 25% to 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 (carboxysomes)
containing RuBisCo (ribulose-1,5-bisphosphate carboxylase/oxygenase, the enzyme that converts
inorganic carbon into reduced organic carbon in all oxygen-evolving photosynthetic organisms) and
have similar cell walls characterized by a peptidoglycan layer. Blue-green algae and prochlorophytes
can be classified as obligate photoautotrophic organisms. Reproduction in both divisions is strictly
asexual, by simple cell division, fragmentation of the colony, or filaments.
glaUcophyTa
Glaucophyceae (Figures 1.1b and 1.37) are basically unicellular flagellates with a dorsiventral construction; they bear two unequal flagella, which are inserted into a shallow depression just below
the apex of the cell. Glaucophyceae are rare freshwater inhabitants, sometimes collected from
FIGURE 1.32 Life cycle of Laminaria sp.: 1, sporophyte; 2, male zoospore; 2′, female zoospore; 3, male gametophyte; 3′, female gametophyte; 4, sperm; 4′, egg and fertilization; 5, zygote; 6, young sporophyte. R!: meiosis.
Algae
extends the light-harvesting capacity of these algae into the blue and orange/red regions of the visible
light spectrum. Other pigments are β-carotene and several xantophylls (zeaxanthin is the principal
one). Their thylakoids, which lie free in the cytoplasm, are arranged in stacks. Prochlorophytes have
a starch-like reserve polysaccharide. These prokaryotes contribute a large percentage of the total
organic carbon in the global tropical oceans, making up from 25% to 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 (carboxysomes)
containing RuBisCo (ribulose-1,5-bisphosphate carboxylase/oxygenase, the enzyme that converts
inorganic carbon into reduced organic carbon in all oxygen-evolving photosynthetic organisms) and
have similar cell walls characterized by a peptidoglycan layer. Blue-green algae and prochlorophytes
can be classified as obligate photoautotrophic organisms. Reproduction in both divisions is strictly
asexual, by simple cell division, fragmentation of the colony, or filaments.
glaUcophyTa
Glaucophyceae (Figures 1.1b and 1.37) are basically unicellular flagellates with a dorsiventral construction; they bear two unequal flagella, which are inserted into a shallow depression just below
the apex of the cell. Glaucophyceae are rare freshwater inhabitants, sometimes collected from
FIGURE 1.32 Life cycle of Laminaria sp.: 1, sporophyte; 2, male zoospore; 2′, female zoospore; 3, male gametophyte; 3′, female gametophyte; 4, sperm; 4′, egg and fertilization; 5, zygote; 6, young sporophyte. R!: meiosis.