Page 18 - Algae

1 General Overview

DEFINITION

The term algae has no formal taxonomic standing. 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), noncohesive, and artificial assemblage of O 2 -evolving, photosynthetic organ-
isms (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, use similar defense strategies against predators and parasites,
and a strong morphological similarity exists between some algae and plants. Then, how to dis-
tinguish algae from plants? The answer is quite easy because the similarities 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 sur-
rounded 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 hence the name embryophytes is 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 game-
tophyte 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 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 cell, 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 for and the other 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 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 mm 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 poly-
saccharides, and 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 both macroalgae and a highly diversified group of microorganisms known as micro-
algae. The number of algal species has been estimated to be one to ten million, and most of
them are microalgae.

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