Kinds of Algae

Classically, algae were classified based on color and distinct morphology (e.g., “red algae lack motile stages”). However, this was found to be insufficient while dealing with the daunting problem of the interrelationships between different groups of eukaryotes. A problem while interpreting phylogeny based on morphology is the difficultly in selecting key character states and assigning phylogenetic values to them, especially in the distantly related groups with substantial morphological differences and of questionable homology. The plasticity of morphological characters resulted in either misclassification or over-classification of recognizable groups that led to a general confusion among phycologists. A significant advancement in the algal systematics took place with the advent of electron microscopy during mid 20th century, when phycologists began to characterize the ultrastructure of algal cells. Since then, the ultrastructural details -especially that of the plastids and the motile cells (flagellar roots and basal bodies) - have been widely taken as a reliable character state in cladistic analysis of various algal taxa because these characteristics were found to be evolutionarily more conserved (Friedl, 1997). By the end of the 20th century, DNA-based molecular systematics had largely superseded ultrastructure–based systematics and it had been shown that the morphological and biochemical diversity of the algae results from their polyphyletic origins within the eukaryotic lineage of the tree of life (Bhattacharya and Medlin, 1995; Stiller and Hall, 1997). The latest among proposed classification schemes for the tree of life, the “six kingdom” model based on molecular, ultrastructural and paleontological evidences (Cavalier-Smith, 2004), groups algal taxa into three kingdoms of the bikonts (ancestrally uniciliate eukaryotes); viz., Protozoa, Chromista and Plantae (Fig. 1.1).

FIG. 1.1 The tree of life based on Cavalier-Smith’s “six kingdom” model (redrawn from Cavalier-Smith, 2004 to emphasize kingdoms relevant to algae). Nine algal lineages are indicated in italics. Although a chromalveolate, lineage Alveolata (indicated by an asterisk) belongs to the kingdom Protozoa. PE and SE indicate hypothetical primary and secondary endosymbiotic (endocytobiotic) events respectively, explaining the origin of plastids.

Algae are tremendously diverse; the existence of nine distinct lineages, which are mentioned below, is good evidence for its diversity (Fig. 1.1). Major lineages (divisions) are the Chlorophyta (green algae), Rhodophyta (red algae), Euglenophyta, Chlorarachniophyta, Glaucophyta, Heterokonta, Cryptophyta, Haptophyta, and the dinoflagellates (within the Alveolata; here is an excellent introductory article on dinoflagellates). The latter four are grouped together as Chromalveolates or chromophyte algae because they contain various xanthophylls -that make them appear yellow or brown- in addition to the light harvesting pigments chlorophyll a and c. The algae not only include the world's largest protist giant kelp (Macrocystis spp in Heterokonta, that can reach up to 30m in length), but also many unicellular, bacteria- sized coccoid algae (e.g., Micromonas spp in Chlorophyta, that have only 1-3 µm cell length).


Cite this information as follows, Bast, F (2010) Ecophysiology and phylogeography of Monostroma in SW Japan. Ph.D. Dissertation, Kochi University, Japan, 167 pp