Introduction to the Marine Algae: Overview 9
Red algae are probably best known for their economic and ecological importance. There are over 300
species of economic importance, either for direct consumption (e.g., nori), or extraction of commercial
colloids for laboratory cell-culture media or industrial additives (Graham et al. 2009).
Chlorophyta
The Chlorophyta are a large group of algae (with about 6,000 species recorded) exhibiting a wide range
of somatic differentiation, from flagellates to complex multicellular thalli and including unicellular,
colonial, filamentous, siphonous, and parenchymatous organisms (Thomas 2002). The different level
of thallus organization is so distinct and characteristic that it has traditionally been used as the basis of
the classification within this phylum. Green algae are dominated by unicellular freshwater species, the
marine species being mostly macroalgae.
Most green algae are uninucleated. Exceptions occur in the orders Siphonocladales (e.g.,
Ernodesmis, Valonia), Cladophorales (Anadyomene, Cladophora), and Bryopsidales (e.g., Caulerpa,
Codium, Halimeda). This later order includes coenocytic forms which are multinucleated and single-cell
organisms in which there are no cell walls or cell membranes separating the nuclei of the entire plant.
The Chlorophyta contain chlorophyll a and b, responsible for the typical green colour, β and γ
carotene, and various xanthophylls. The cell structure is eukaryotic and the main food reserve is starch.
The structure and composition of green algal cell walls range from cellulose microfibrils (Ulvales) to
highly crystalline cellulose (Siphonocladales, Cladophorales), or to polymers of xylan and mannan,
which form microfibrils (Dasycladales). Several tropical green algae of the orders Bryopsidales and
Dasycladales have calcified walls consisting of the aragonite form of calcium carbonate crystals. These
calcified species give a significant contribution to the calcium carbonate sediments in tropical waters
(Hillis-Colinvaux 1980).
The sexual life cycle can be haplontic (e.g., Bryopsidophyceae) or diplohaplontic (e.g.,
Cladophorophyceae) and is predominantly isomorphic (Bold and Wynne 1985).
Like other groups, some green algae are important economically and used as a direct food source,
and additives for several purposes (Graham et al. 2009).
Ochrophyta
With more than 100,000 known species (van den Hoek et al. 1995), this phylum comprises mostly
marine organisms and the whole variety of forms of growth and sizes, including the giant kelps (large
Phaeophyceae responsible for the submerged forests that characterize the rocky sea bottom of cold
and temperate regions; van den Hoek et al. 1995), the single-celled and colonial Chysophyceae, the
unicellular, filamentous, and siphonous Xantophyceae, the coccoids unicells of the Eustigmatophyceae,
and the well-known diatoms (Bacillariophyceae). Important members of this phylum are the brown algae
belonging to the single class Paeophyceae (Silva 1980), which includes about 1,500 to 2,000 species (van
den Hoek et al. 1995). Almost exclusively marine and dominant in temperate to arctic latitudes, these
organisms occur both intertidally and subtidally down to depths of 100 m and more. Most require stable
hard substrata for attachment but some Sargassum species occur only as drift populations (Dawes 1998).
The Phaeophyceae are the most complex algae and encompass filamentous, syntagmatic, or
parenchymatous organisms ranging from small filamentous forms (Ectocarpus) and crustose forms
(Ralfsia), to massive cartilaginous thallus (Fucus, Ascophyllum, the giant kelps). The major taxa are
separated on the basis of life history and gene sequence information, but taxonomic classification is still
in flux. The order Ectocarpales, which encompasses about 30 genera including Ectocarpus, includes
the most primitive species of brown algae in which growth occurs by intercalary cell divisions of the
uniseriately branched filaments. On the opposite side, the Laminariales include the largest and more
complex of all algae (Laminaria, Macrocystis), which can exhibit substantial tissue differentiation
including an epidermis, outer and inner cortex, and a central medulla that contains sieve like cells called
trumpet hypae with a conduction function similar to sieve cells in higher plants (Schmitz and Srivastava
1980).
Red algae are probably best known for their economic and ecological importance. There are over 300
species of economic importance, either for direct consumption (e.g., nori), or extraction of commercial
colloids for laboratory cell-culture media or industrial additives (Graham et al. 2009).
Chlorophyta
The Chlorophyta are a large group of algae (with about 6,000 species recorded) exhibiting a wide range
of somatic differentiation, from flagellates to complex multicellular thalli and including unicellular,
colonial, filamentous, siphonous, and parenchymatous organisms (Thomas 2002). The different level
of thallus organization is so distinct and characteristic that it has traditionally been used as the basis of
the classification within this phylum. Green algae are dominated by unicellular freshwater species, the
marine species being mostly macroalgae.
Most green algae are uninucleated. Exceptions occur in the orders Siphonocladales (e.g.,
Ernodesmis, Valonia), Cladophorales (Anadyomene, Cladophora), and Bryopsidales (e.g., Caulerpa,
Codium, Halimeda). This later order includes coenocytic forms which are multinucleated and single-cell
organisms in which there are no cell walls or cell membranes separating the nuclei of the entire plant.
The Chlorophyta contain chlorophyll a and b, responsible for the typical green colour, β and γ
carotene, and various xanthophylls. The cell structure is eukaryotic and the main food reserve is starch.
The structure and composition of green algal cell walls range from cellulose microfibrils (Ulvales) to
highly crystalline cellulose (Siphonocladales, Cladophorales), or to polymers of xylan and mannan,
which form microfibrils (Dasycladales). Several tropical green algae of the orders Bryopsidales and
Dasycladales have calcified walls consisting of the aragonite form of calcium carbonate crystals. These
calcified species give a significant contribution to the calcium carbonate sediments in tropical waters
(Hillis-Colinvaux 1980).
The sexual life cycle can be haplontic (e.g., Bryopsidophyceae) or diplohaplontic (e.g.,
Cladophorophyceae) and is predominantly isomorphic (Bold and Wynne 1985).
Like other groups, some green algae are important economically and used as a direct food source,
and additives for several purposes (Graham et al. 2009).
Ochrophyta
With more than 100,000 known species (van den Hoek et al. 1995), this phylum comprises mostly
marine organisms and the whole variety of forms of growth and sizes, including the giant kelps (large
Phaeophyceae responsible for the submerged forests that characterize the rocky sea bottom of cold
and temperate regions; van den Hoek et al. 1995), the single-celled and colonial Chysophyceae, the
unicellular, filamentous, and siphonous Xantophyceae, the coccoids unicells of the Eustigmatophyceae,
and the well-known diatoms (Bacillariophyceae). Important members of this phylum are the brown algae
belonging to the single class Paeophyceae (Silva 1980), which includes about 1,500 to 2,000 species (van
den Hoek et al. 1995). Almost exclusively marine and dominant in temperate to arctic latitudes, these
organisms occur both intertidally and subtidally down to depths of 100 m and more. Most require stable
hard substrata for attachment but some Sargassum species occur only as drift populations (Dawes 1998).
The Phaeophyceae are the most complex algae and encompass filamentous, syntagmatic, or
parenchymatous organisms ranging from small filamentous forms (Ectocarpus) and crustose forms
(Ralfsia), to massive cartilaginous thallus (Fucus, Ascophyllum, the giant kelps). The major taxa are
separated on the basis of life history and gene sequence information, but taxonomic classification is still
in flux. The order Ectocarpales, which encompasses about 30 genera including Ectocarpus, includes
the most primitive species of brown algae in which growth occurs by intercalary cell divisions of the
uniseriately branched filaments. On the opposite side, the Laminariales include the largest and more
complex of all algae (Laminaria, Macrocystis), which can exhibit substantial tissue differentiation
including an epidermis, outer and inner cortex, and a central medulla that contains sieve like cells called
trumpet hypae with a conduction function similar to sieve cells in higher plants (Schmitz and Srivastava
1980).