Archaeplastida are a supergroup of protists that comprise red and
green algae, which include unicellular, multicellular, and colonial forms.
LEARNING OBJECTIVE [ edit ]
Describe the relationship between red algae, green algae, and land plants
KEY POINTS [ edit ]
Archaeplastida are typically associated with their relationship to land plants; in addition,
molecular evidence shows that Archaeplastida evolved from an endosymbiotic relationship
between a heterotrophic protist and a cyanobacterium.
Red algae (rhodophytes), are classified as Archaeplastida and are most often characterized by the
presence of the red pigment phycoerythrin; however, there are red algae that lack phycoerythrins
and can be classified as parasites.
Red algae typically exist as multicellular protists that lack flagella; however, they can also exist as
unicellular organisms.
Green algae are the most abundant group of algae and can be further classified as chlorophytes
and charophytes.
Charophytes are the green algae which resemble land plants and are their closest living relative.
Chlorophytes are the green algae which exhibit a wide range of forms; they can be unicellular,
multicellular, or colonial.
TERMS [ edit ]
plankton
a generic term for all the organisms that float in the sea
endosymbiotic
that lives within a body or cells of another organism
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Archaeplastida
Red algae and green algae are included in
the supergroup Archaeplastida. It is well
documented that land plants evolved from
a common ancestor of these protists; their
closest relatives are found within this
group. Molecular evidence supports that
all Archaeplastida are descendants of an
endosymbiotic relationship between a
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heterotrophic protist and a
cyanobacterium. The red and green algae include unicellular, multicellular, and colonial
forms
Red Algae
Red algae, or rhodophytes, are primarily multicellular, lack flagella, and range in size from
microscopic, unicellular protists to large, multicellular forms grouped into the informal
seaweed category. The red algae life cycle is an alternation of generations. Some species of
red algae contain phycoerythrins, photosynthetic accessory pigments that are red in color and
outcompete the green tint of chlorophyll, making these species appear as varying shades of
red. Other protists classified as red algae lack phycoerythrins and are parasites. Red algae are
common in tropical waters where they have been detected at depths of 260 meters. Other red
algae exist in terrestrial or freshwater environments.
Green Algae: Chlorophytes and Charophytes
The most abundant group of algae is the green algae. The green algae exhibit similar features
to the land plants, particularly in terms of chloroplast structure. It is well supported that this
group of protists share a relatively-recent common ancestors with land plants. The green
algae are subdivided into the chlorophytes and the charophytes. The charophytes are the
closest-living relatives of land plants, resembling them inmorphology and reproductive
strategies. Charophytes are common in wet habitats where their presence often signals a
healthy ecosystem.
The chlorophytes exhibit great diversity of form and function. Chlorophytes primarily inhabit
freshwater and damp soil; they are a common component of plankton. Chlamydomonas is a
simple, unicellular chlorophyte with a pear-shaped morphology and two opposing, anterior
flagella that guide this protist toward light sensed by its eyespot. More complex chlorophyte
species exhibit haploid gametes and spores that resembleChlamydomonas.
The chlorophyte Volvox is one of only a few examples of a colonial organism, which behaves
in some ways like a collection of individual cells, but in other ways like the specialized cells of
a multicellular organism . Volvox colonies contain 500 to 60,000 cells, each with two flagella,
contained within a hollow, spherical matrix composed of a gelatinous glycoproteinsecretion.
Individual Volvox cells move in a coordinated fashion and are interconnected by cytoplasmic
bridges. Only a few of the cells reproduce to create daughter colonies, an example of basic cell
specialization in this organism.
Volvox aureus
Volvox aureus is a green alga in the supergroup Archaeplastida. This species exists as a colony,
consisting of cells immersed in a gel­like matrix and intertwined with each other via hair­like
cytoplasmic extensions.
True multicellular organisms, such as the sea lettuce, Ulva, are represented among the
chlorophytes. In addition, some chlorophytes exist as large, multinucleate, single cells.
Species in the genus Caulerpa exhibit flattened, fern-like foliage and can reach lengths of 3
meters . Caulerpa species undergo nuclear division, but their cells do not
complete cytokinesis, remaining instead as massive and elaborate single cells.
Caulerpa taxifolia
Caulerpa taxifolia is a chlorophyte consisting of a single cell containing potentially thousands of
nuclei.