Green Algae Secondary article Mark A Buchheim, University of Tulsa, Tulsa, Oklahoma, USA Article Contents . Introduction The green algae comprise a large and diverse group of organisms that range from the . Major Groups microscopic to the macroscopic. Green algae are found in virtually all aquatic and some . Economic and Ecological Importance terrestrial habitats. Introduction generalization). The taxonomic and phylogenetic status of The green algae comprise a large and diverse group of the green plant group is supported by both molecular and organisms that range from the microscopic (e.g. Chlamy- nonmolecular evidence (Graham, 1993; Graham and domonas) to the macroscopic (e.g. Acetabularia). In Wilcox, 2000). This group of green organisms has been addition to exhibiting a considerable range of structural termed the Viridaeplantae or Chlorobionta. Neither the variability, green algae are characterized by extensive euglenoids nor the chlorarachniophytes, both of which ecological diversity. Green algae are found in virtually all have apparently acquired a green chloroplast by a aquatic (both freshwater and marine) and some terrestrial secondary endosymbiosis, are included in the green plant habitats. Although most are free-living, a number of green lineage (Graham and Wilcox, 2000). Furthermore, the algae are found in symbiotic associations with other Chloroxybacteria (e.g. Prochloron), which possess chlor- organisms (e.g. the lichen association between an alga ophyll a and b organized on thylakoids, are true and a fungus). Some green algae grow epiphytically (e.g. prokaryotes, and are not, therefore, included in the green Characiochloris, which grows on other filamentous algae plant lineage. The green algal division Chlorophyta forms or higher aquatic plants), epizoically (e.g. Basicladia, one branch of the green plant lineage and the green algal which grows on the backs of turtles), or even endophyti- division Charophyta is part of the other branch of the green cally (e.g. Chlorochytrium, which grows inside the thallus plant lineage that also includes the embryophytes. Thus, of the aquatic duckweed plant, Lemna). Ahandful of green the charophytes and embryophytes form a monophyletic algae, all of which have lost the ability to photosynthesize, group that has been termed the Streptophyta (Chapman et can be listed as human pathogens that generally cause al., 1998; Graham and Wilcox, 2000). (see Algal taxonomy: epidermal infections. The major groups of green algae are historical overview.)(see Molecular phylogeny reconstruction.) briefly described in the following sections. (see Algal ecology.) (see Algae: phylogeny and evolution.)(see Algal chloroplasts.) (see Algal symbioses.) (see Protist systematics.) Table 1 illustrates the phylogenetic hierarchy of major green algal groups, as currently assessed by phylogenetic analysis of molecular and morphological evidence. Major Groups The Chlorophyta The green algae have been placed at a variety of formal linnean ranks. Many traditional authors support divi- The Chlorophyta are characterized by motile cells that sional (phylum) status for all organisms generally referred exhibit a cruciate (cross-shaped) arrangement of four sets to as the green algae (i.e. the divisions Chlorophyta and of microtubules that anchor the flagellar apparatus. Charophyta). The divisions Chlorophyta and Charophyta are comprised of five principal groups (i.e. classes) that are characterized by life history, biochemistry and ultrastruc- Table 1 Phylogenetic hierarchy of major green algal groups tural features of the flagellar apparatus and/or the mitotic Viridaeplantae/Chlorobionta and cytokinetic apparatus (Mattox and Stewart, 1984; Streptophyta Mishler et al., 1994; van den Hoek et al., 1995; Graham and Embryophyta Wilcox, 2000). Recent molecular phylogenetic analyses Charophyta have also shed considerable light on diversity and relation- Charophyceae ships among green algae (Mishler et al., 1994; Friedl, 1997; Chlorophyta et al Lewis, 1997; Chapman ., 1998). The Chlorophyta, Chlorophyceae Charophyta and Embyrophyta (i.e. the embryo-producing Trebouxiophyceae land plants) form a monophyletic group of green- Ulvophyceae pigmented plants that bear chlorophyll a and b in Prasinophyceae chloroplasts (see Prototheca for an exception to this ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 1 Green Algae Flagella are inserted apically in motile cells of the Class Trebouxiophyceae Chlorophyta. Two sets of flagellar microtubules always The Trebouxiophyceae have their origins with the class form pairs of microtubules, whereas the remaining two sets Pleurastrophyceae (Mattox et al., 1984), which is no longer of microtubules vary in number. Each of these sets of valid because the type genus, Pleurastrum, has been shown microtubules have been termed flagellar roots. The two- to be a member of the class Chlorophyceae (Friedl, 1997). stranded flagellar roots are always inserted opposite one Like the Chlorophyceae, the Trebouxiophyceae are pre- another, as are the multistranded flagellar roots. This dominantly freshwater organisms, but include a few cruciate arrangement of flagellar roots has also been marine species. Of particular interest is the observation et al referred to as 1808 rotational symmetry (Chapman ., that a number of trebouxiophycean organisms (including 1998). Most members of the Chlorophyta exhibit a closed Trebouxia) are found in lichen associations. The class is mitotic spindle in which the nuclear membrane remains currently diagnosed exclusively on the basis of 18S largely intact during mitosis (exceptions are found among ribosomal deoxyribonucleic acid (rDNA) sequence analy- the Prasinophyceae) (Graham and Wilcox, 2000). Plasmo- sis (Friedl, 1997; Chapman et al., 1998). Cytologically, the desmata have been found in several chlorophycean Trebouxiophyceae exhibit features that are thought to be lineages (i.e. Chaetophorales and Oedogoniales) and in plesiomorphic. These plesiomorphic features include a the Charophyceae (Graham and Wilcox, 2000). Peroxi- metacentric spindle that collapses during telophase, a somes in all but a few prasinophyte species have been found phycoplast system of cytokinetic microtubules, and basal to contain the photorespiratory enzyme, glycolate dehy- bodies offset in a counterclockwise arrangement (Friedl, drogenase (Graham and Wilcox, 2000). In those organisms 1997). No body or flagellar scales have been detected on that produce a sexual stage, the bulk of the Chlorophyta motile cells of the Trebouxiophyceae. Molecular phyloge- exhibit zygotic meiosis where the zygote is the only diploid netic analyses generally support the monophyly of the phase. However, the class Ulvophyceae includes many Trebouxiophyceae (Lewis, 1997; Friedl, 1997; Chapman representatives in which alternation of generations or et al., 1998), but the degree of support varies from analysis gametic meiosis characterizes the life history. In addition, to analysis. On the basis of a shared phycoplast system of the Chaetophorales in the class Chlorophyceae have been cytokinetic microtubules, the Chlorophyceae and Tre- reported to exhibit an isomorphic alternation of genera- bouxiophyceae are thought to be sister taxa (Figure 1) tions, but this observation remains to be fully tested (van within the Chlorophyta (Friedl, 1997; Lewis, 1997; Chap- den Hoek et al., 1995). The four classes within the man et al., 1998). (see Phylogeny based on 16S rRNA/DNA.) Chlorophyta are briefly described below. (see Chlorophyta (see Classification.) (green plants).)(see Algal reproduction.)(see Plant peroxisomes and glyoxysomes.)(see Algal flagella.) Class Ulvophyceae The Ulvophyceae include green algae that range from simple unicells to the largest macrophytes. With a few Class Chlorophyceae notable exceptions, the majority of ulvophytes are marine. The Chlorophyceae are predominantly freshwater forms, The branched, filamentous genus Cladophora is one although some species (e.g. Dunaliella) are found in ulvophyte that has both freshwater and marine species. brackish to marine environments. The class is currently Cytologically, the Ulvophyceae exhibit a centric spindle diagnosed principally on the basis of ultrastructural that persists through telophase and the motile cells possess features of the mitotic, cytokinetic and flagellar systems flagellar apparatus components that are arranged in a (Mattox and Stewart, 1984; Mishler et al., 1994; Graham counterclockwise orientation. Many of the motile cells of and Wilcox, 2000). Except for the Chaetopeltidales (e.g. the Ulvophyceae possess body and flagellar scales. A Chaetopeltis), the Chlorophyceae are generally thought to phycoplast system of microtubules has been observed in lack body and flagellar scales on motile stages. The the ulotrichalean ulvophyte, Gloeotilopsis, but the issue of Chlorophyceae are characterized by a centric mitotic the taxonomic range of this character within the Ulvophy- spindle that collapses during telophase. Asystem of ceae remains a matter of contention (Chapman et al., microtubules, called a phycoplast, develops parallel to 1998). Most green algal taxonomists do not yet list the the plane of cell division. Basal bodies in the flagellar phycoplast as a characteristic of the Ulvophyceae, as it has apparatus of motile cells either exhibit no relative not been demonstrated in most species that have been offset (i.e. are directly opposed) or are offset, relative to studied to date. Except for the ulvophycean genus, one another, in a clockwise fashion.