4 Marine Macro- and Microalgae: An Overview
The more complex types and forms (cartilaginous) have a parenchymatous or pseudoparenchymatous
construction, which originates from a meristem with cell divisions in three dimensions. In the
parenchymatous thallus, cells of the primary filament divide in all directions and the essential filamentous
structure is lost (e.g., Ulva, Fig. 2H). The pseudoparenchymatous thallus is made by the aggregation,
in a loose or compact way, of numerous, intertwined, branched filaments held together by mucilages to
collectively form the thallus, which therefore has little internal differentiation. A good example is found
in the red alga Schizymenia (Fig. 2I).
Reproduction
The reproduction in algae is highly variable. Asexual reproduction, arguably the most widespread, can
be vegetative (by the division of a single cell or by the fragmentation of a colony or filament) or by the
production of motile spores. This mode of reproduction allows stability of an adapted genotype from one
generation to the next, but restricts genetic variability. Sexual reproduction, involving the union of gametes,
results in the genetic recombination needed for adaptation to changing environments (Lee 2008).
The binary fission of unicellular algae is the simplest form of asexual reproduction, in which the
parent organism divides into two equal parts, each containing the same hereditary information as the
parent. The autocolony formation is a specific mode in which each cell of the colony can produce a new
colony similar to the one to which it belongs. Cell divisions produce a sort of embryonic colony smaller
than the parent colony but equal in cell number (Fig. 2A).
In the fragmentation mode, the filaments and/or non-coenobic colonies break into two or several
parts, each having the capacity of developing into new individuals.
Asexual spores can be flagellate motile (zoospores) and produced by a parental vegetative cell,
aflagellate (aplanospores) that begin their development within the parent cell before being released, and
autospores that are aflagellate daughter cells that will be released by the rupture of the parent cell wall.
These later are almost perfect replicas of the mother cell as in the green alga Chlorella (Fig. 3). Spores
may be produced within ordinary vegetative cells or within specialized structures called sporangia.
Under unfavorable conditions many algae can produce thick-walled resting cells, such as hypnospores,
hypnozygotes, statospores, and akinetes, and enter a dormancy period. The former two are produced by
protoplasts that were previously separated from the walls of the parental cells. They enable the species
to survive temporary drying out of water bodies. Statospores are endogenous cysts formed within the
vegetative cell. Their silica enriched walls, often ornamented with spines and other projections, make
them good fossil records. Akinetes are of widespread abundance in the blue-green algae and used in Alfa
taxonomy. They are essentially enlarged vegetative cells that develop a thickened wall in response to
limiting environmental nutrients or limiting light.
Fig. 3. Chlorella spores resembling mother cells. Photos from Island Aquatic Research, Azorean Biodiversity Group/Center
for Ecology, Evolution and Environmental Changes (IAE/GBA/cE3c).
The more complex types and forms (cartilaginous) have a parenchymatous or pseudoparenchymatous
construction, which originates from a meristem with cell divisions in three dimensions. In the
parenchymatous thallus, cells of the primary filament divide in all directions and the essential filamentous
structure is lost (e.g., Ulva, Fig. 2H). The pseudoparenchymatous thallus is made by the aggregation,
in a loose or compact way, of numerous, intertwined, branched filaments held together by mucilages to
collectively form the thallus, which therefore has little internal differentiation. A good example is found
in the red alga Schizymenia (Fig. 2I).
Reproduction
The reproduction in algae is highly variable. Asexual reproduction, arguably the most widespread, can
be vegetative (by the division of a single cell or by the fragmentation of a colony or filament) or by the
production of motile spores. This mode of reproduction allows stability of an adapted genotype from one
generation to the next, but restricts genetic variability. Sexual reproduction, involving the union of gametes,
results in the genetic recombination needed for adaptation to changing environments (Lee 2008).
The binary fission of unicellular algae is the simplest form of asexual reproduction, in which the
parent organism divides into two equal parts, each containing the same hereditary information as the
parent. The autocolony formation is a specific mode in which each cell of the colony can produce a new
colony similar to the one to which it belongs. Cell divisions produce a sort of embryonic colony smaller
than the parent colony but equal in cell number (Fig. 2A).
In the fragmentation mode, the filaments and/or non-coenobic colonies break into two or several
parts, each having the capacity of developing into new individuals.
Asexual spores can be flagellate motile (zoospores) and produced by a parental vegetative cell,
aflagellate (aplanospores) that begin their development within the parent cell before being released, and
autospores that are aflagellate daughter cells that will be released by the rupture of the parent cell wall.
These later are almost perfect replicas of the mother cell as in the green alga Chlorella (Fig. 3). Spores
may be produced within ordinary vegetative cells or within specialized structures called sporangia.
Under unfavorable conditions many algae can produce thick-walled resting cells, such as hypnospores,
hypnozygotes, statospores, and akinetes, and enter a dormancy period. The former two are produced by
protoplasts that were previously separated from the walls of the parental cells. They enable the species
to survive temporary drying out of water bodies. Statospores are endogenous cysts formed within the
vegetative cell. Their silica enriched walls, often ornamented with spines and other projections, make
them good fossil records. Akinetes are of widespread abundance in the blue-green algae and used in Alfa
taxonomy. They are essentially enlarged vegetative cells that develop a thickened wall in response to
limiting environmental nutrients or limiting light.
Fig. 3. Chlorella spores resembling mother cells. Photos from Island Aquatic Research, Azorean Biodiversity Group/Center
for Ecology, Evolution and Environmental Changes (IAE/GBA/cE3c).