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molecular systematics had largely superseded ultrastructure–based systematics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 result
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).