Definition of Algae

Occurrence and Distribution
Structure of Thallus
  Unicells and Unicell Colonial Algae
  Filamentous Algae
  Siphonous Algae
  Parenchymatous and Pseudoparenchymatous Algae
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 O2-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, use similar defense strategies against predators and parasites, and a strong morphological similarity exists between some algae and plants. Then, how to distinguish 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 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 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 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 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 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, 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 microalgae. The number of algal species has been estimated to be one to ten million, and most of them are microalgae.