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General Overview
DEFINITION
The term algae has no formal taxonomic standing; however, it is routinely used to indicate a polyphyletic (i.e., including organisms that do not share a common origin, but follow multiple and independent evolutionary lines), non-cohesive, and artificial assemblage of O 2 -evolving, photosynthetic
organisms (with several exceptions of colorless members undoubtedly related to pigmented forms).
According to this definition, plants could be considered an algal division. Algae and plants produce
the same storage compounds as well as use similar defense strategies against predators and parasites. A strong morphological similarity exists between some algae and plants; however, distinguishing algae from plants is quite easy since the similarities we have listed between algae and plants
are much fewer than their differences. Plants show a very high degree of differentiation, with roots,
leaves, stems, and xylem/phloem vascular network, their reproductive organs are surrounded by a
jacket of sterile cells, they have a multicellular diploid embryo stage that remains developmentally
and nutritionally dependent on the parental gametophyte for a significant period (and this feature is
the source of the name embryophytes given to plants), and tissue-generating parenchymatous meristems at the shoot and root apices producing tissues that differentiate in a wide variety of shapes.
Moreover, all plants have a digenetic life cycle, with an alternation between a haploid gametophyte
and a diploid sporophyte. Algae do not have any of these features, they do not have roots, stems,
leaves, nor well-defined vascular tissues, even though many seaweeds are plant-like in appearance
and some of them show specialization and differentiation of their vegetative cells, they do not form
embryos, their reproductive structures consist of cells that are all potentially fertile and lack sterile
cells covering or protecting them, parenchymatous development is present only in some groups, and
have both monogenetic and digenetic life cycles. Moreover, algae occur in dissimilar forms such
as microscopic single cells, macroscopic multicellular loose or filmy conglomerations, matted or
branched colonies, or more complex leafy or blade forms, which contrast strongly with uniformity
in vascular plants. Evolution may have worked in two ways: one for shaping similarities and one for
shaping differences. The same environmental pressure led to the parallel, independent evolution of
similar traits in both plants and algae, while the transition from relatively stable aquatic environment to a gaseous medium exposed plants to new physical conditions that resulted in key physiological and structural changes necessary to be able to invade upland habitats and fully exploit them.
The bottom line is that plants are a separate group with no overlapping with the algal assemblage.
The profound diversity of size ranging from picoplankton only 0.2–2.0 μm in diameter to giant
kelps with fronds up to 60 m in length, ecology and colonized habitats, cellular structure, levels
of organization and morphology, pigments for photosynthesis, reserve and structural polysaccharides, type of life history reflect the varied evolutionary origins of this heterogeneous assemblage of
organisms, including both prokaryote and eukaryote species. The term algae refers to macroalgae
and a highly diversified group of microorganisms known as microalgae. Estimates of the number
of living algae varies from 30,000 to more than 1 million species, but most of the reliable estimates
refer to the numbers given in AlgaeBase, which currently documents 32,260 species of organisms
generally regarded as algae of an estimated 43,918 described species of algae, corresponding to
about 73%. According to the AlgaeBase estimate of 28,500 species waiting for description, the total
number of algal species is likely to be about 72,500, of which more than 20,000 will be diatomic.
1
General Overview
DEFINITION
The term algae has no formal taxonomic standing; however, it is routinely used to indicate a polyphyletic (i.e., including organisms that do not share a common origin, but follow multiple and independent evolutionary lines), non-cohesive, and artificial assemblage of O 2 -evolving, photosynthetic
organisms (with several exceptions of colorless members undoubtedly related to pigmented forms).
According to this definition, plants could be considered an algal division. Algae and plants produce
the same storage compounds as well as use similar defense strategies against predators and parasites. A strong morphological similarity exists between some algae and plants; however, distinguishing algae from plants is quite easy since the similarities we have listed between algae and plants
are much fewer than their differences. Plants show a very high degree of differentiation, with roots,
leaves, stems, and xylem/phloem vascular network, their reproductive organs are surrounded by a
jacket of sterile cells, they have a multicellular diploid embryo stage that remains developmentally
and nutritionally dependent on the parental gametophyte for a significant period (and this feature is
the source of the name embryophytes given to plants), and tissue-generating parenchymatous meristems at the shoot and root apices producing tissues that differentiate in a wide variety of shapes.
Moreover, all plants have a digenetic life cycle, with an alternation between a haploid gametophyte
and a diploid sporophyte. Algae do not have any of these features, they do not have roots, stems,
leaves, nor well-defined vascular tissues, even though many seaweeds are plant-like in appearance
and some of them show specialization and differentiation of their vegetative cells, they do not form
embryos, their reproductive structures consist of cells that are all potentially fertile and lack sterile
cells covering or protecting them, parenchymatous development is present only in some groups, and
have both monogenetic and digenetic life cycles. Moreover, algae occur in dissimilar forms such
as microscopic single cells, macroscopic multicellular loose or filmy conglomerations, matted or
branched colonies, or more complex leafy or blade forms, which contrast strongly with uniformity
in vascular plants. Evolution may have worked in two ways: one for shaping similarities and one for
shaping differences. The same environmental pressure led to the parallel, independent evolution of
similar traits in both plants and algae, while the transition from relatively stable aquatic environment to a gaseous medium exposed plants to new physical conditions that resulted in key physiological and structural changes necessary to be able to invade upland habitats and fully exploit them.
The bottom line is that plants are a separate group with no overlapping with the algal assemblage.
The profound diversity of size ranging from picoplankton only 0.2–2.0 μm in diameter to giant
kelps with fronds up to 60 m in length, ecology and colonized habitats, cellular structure, levels
of organization and morphology, pigments for photosynthesis, reserve and structural polysaccharides, type of life history reflect the varied evolutionary origins of this heterogeneous assemblage of
organisms, including both prokaryote and eukaryote species. The term algae refers to macroalgae
and a highly diversified group of microorganisms known as microalgae. Estimates of the number
of living algae varies from 30,000 to more than 1 million species, but most of the reliable estimates
refer to the numbers given in AlgaeBase, which currently documents 32,260 species of organisms
generally regarded as algae of an estimated 43,918 described species of algae, corresponding to
about 73%. According to the AlgaeBase estimate of 28,500 species waiting for description, the total
number of algal species is likely to be about 72,500, of which more than 20,000 will be diatomic.
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