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Home , Acrosiphonia, Bracteacoccus, Cephaleuros, Chaetomorpha, Chaetopeltidales, Chaetopeltis

Green Secondary article

Mark A Buchheim, University of Tulsa, Tulsa, Oklahoma, USA Article Contents

. Introduction The 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 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. ). 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 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 association between an alga ophyll a and b organized on , are true and a ). Some green algae grow epiphytically (e.g. prokaryotes, and are not, therefore, included in the green , which grows on other filamentous algae plant lineage. The green algal division forms or higher aquatic ), 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 is part of the other branch of the green cally (e.g. , which grows inside the thallus plant lineage that also includes the . 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 (Chapman et can be listed as human pathogens that generally cause al., 1998; Graham and Wilcox, 2000). (see Algal : epidermal infections. The major groups of green algae are historical overview.)(see Molecular phylogeny reconstruction.) briefly described in the following sections. (see Algal .) (see Algae: phylogeny and evolution.)(see Algal .) (see Algal symbioses.) (see 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 () 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- ships among green algae (Mishler et al., 1994; Friedl, 1997; Chlorophyta et al Lewis, 1997; Chapman ., 1998). The Chlorophyta, Charophyta and Embyrophyta (i.e. the embryo-producing land plants) form a monophyletic group of green- pigmented plants that bear a and b in 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 , , 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 . 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 (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. 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 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 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 .)(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 is one although some species (e.g. ) 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 (e.g. the Ulvophyceae possess body and flagellar scales. A ), 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 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. These flagellar Cephaleuros, none of the Ulvophyceae are thought to apparatus features are currently regarded as apomorphic produce a phragmoplast system of microtubules (see for the class Chlorophyceae. Molecular phylogenetic Charophyta). The Ulvophyceae are frequently character- analyses also generally support the monophyly of the ized as comprising two groups, the ulotrichalean ulvo- Chlorophyceae (Mishler et al., 1994; Chapman et al., phytes (e.g. Ulothrix) and the siphonous ulvophytes (e.g. 1998). (see Biogeography of freshwater algae.)(see Cell motility.) Acetabularia). The two groups differ from one another in

2 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Green Algae

Hydrodictyon Chaetopeltis Fritschiella Chlorophyceae Oedocladium Bulbochaete Botryococcus Dunaliella Chlorella vulgaris Chlorella kessleri Chlorella minutissima Trebouxiophyceae Prototheca Dictyochloropsis Chlorella saccharinum Gloeotilopsis Ulothrix Monostroma Acrosiphonia Chaetomorpha Cladophora Ulvophyceae Siphonocladus Chamaedoris Neomeris Acetabularia Batophora Nephroselmis Mantoniella Prasinophyceae Coleochaete Glycine Charophyceae Oryza + Zamia Embryophytes Chara Nitella Cyanophora

Figure 1 Consensus phylogeny of selected exemplars from the green algae (plus embryophytes) based on maximum parsimony analysis of equally weighted 18S rDNA sequence data (Buchheim, unpublished observations). Weakly supported branches (as determined by bootstrap analysis) are identified as dashed lines. life history and in form (van den Hoek et al., 1995; Prasinophyceae et al Chapman ., 1998; Graham and Wilcox, 2000). From a The Prasinophyceae are a heterogeneous, nonmonophy- phylogenetic perspective, the siphonous ulvophytes form a letic assemblage of flagellate and coccoid unicells that distinct group, as do the ulotrichalean ulvophytes (Figure1), include both freshwater and marine representatives. but global monophyly of the two ulvophyte groups is in Flagellated forms vary from uniflagellate to 16-flagellated dispute at both the molecular and morphological levels cells. Most prasinophytes lack true cell walls, but instead et al et al (Mishler ., 1994; Chapman ., 1998). possess scaly surfaces. Both body scales and flagellar scales ( ) see Biogeography of marine algae. are present in many prasinophytes and this feature has

ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 3 Green Algae been used as a taxonomic marker (van den Hoek et al., below represent a sampling of some of the distinctive 1995; Chapman et al., 1998). Anumber of prasinophytes diversity within the green algae. have been shown to possess a unique light-harvesting complex that differs from that in all other organisms (Graham and Wilcox, 2000). Most prasinophytes fall at the Botryococcus base of the chlorophyte branch (Figure 1). ,a Amember of the chlorophycean lineage ( Figure 1), freshwater prasinophyte, is currently thought to be the Botryococcus is a coccoid unicell that secretes lipids, called most basal member of the Streptophyta lineage (Char- botryococcenes, which have been touted as a source of ophyta and Embyrophyta). The current consensus is that renewable, carbon-based energy for the future (Graham the Prasinophyceae will certainly be broken up into several and Wilcox, 2000). Furthermore, it is thought that distinct classes and some of its members transferred to Botryococcus may have been a significant contributing other classes within the green algae (Graham and Wilcox, source in the development of oil shale and coal (Graham 2000). (see Algal cell walls.)(see Algal .)(see Light- and Wilcox, 2000). (see Algal storage products.) harvesting complex.) Chlamydomonas The Charophyta Amember of the chlorophycean lineage ( Figure 1), The Charophyta are characterized by motile cells that Chlamydomonas is a biflagellated unicell that exhibits an exhibit an asymmetric set of flagellar roots. One flagellar ability to reproduce both asexually and via the formation root is associated with a multilayered structure or MLS. of isogametes ( and are evident in some Flagella in motile cells of the Charophyta are inserted species). Chl. reinhardtii remains one of the most useful subapically. Most members of the Charophyta exhibit an organisms for the study of basic physiological processes, open mitotic spindle in which the nuclear membrane including photosynthesis and carbon uptake (van den disappears during mitosis (some prasinophytes also have Hoek et al., 1995; Graham and Wilcox, 2000). (see Algal open mitotic spindles). The mitotic spindle persists metabolism.) throughout mitosis. In addition to the mitotic/cytokinetic features, most motile stages of the charophytes possess body and flagellar scales. All charophytes have peroxi- Volvox somes that produce glycolate oxidase and catalase. The four classes within the Chlorophyta are briefly described Aglobose colony of Chlamydomonas-like cells, Volvox is below. the largest of the colonial flagellates. It is a member of the chlorophycean lineage (Figure 1) and is a close ally of Chlamydomonas (Chapman et al., 1998). Volvox colonies Charophyceae are comprised of hundreds to thousands of cells per The Charophyceae include those green algae traditionally organism, depending on the species (Graham and Wilcox, linked with embryophytes (i.e. the land plants). The 2000). Like Chlamydomonas, Volvox has been used Charophyceae include Chara and Nitella, Coleochaete, extensively by the research community as a model Klebsormidium, Spirogyra and all desmids (e.g. Micraster- organism. Unlike Chlamydomonas (a unicell), Volvox can ias). Charophycean algae that exhibit an -like be and has been used to study the control of multicellular cytokinetic apparatus (i.e. a phragmoplast) are thought to development. The discovery of transposable elements in be the closest, extant relatives of the embryophytes. Both Volvox has dramatically enhanced its usefulness for the Chara and Coleochaete exhibit a phragmoplast system of study of development at the molecular level and it is likely microtubules that forms perpendicular to the plane of cell that this distinctive alga will play a more prominent role in division and is thought to be the template for plasmodes- developmental research. (see Adhesive specificity and the mata development. The monophyly of the Charophyceae evolution of multicellularity.) has been challenged on the basis of both molecular and morphological evidence, but the issue remains a matter of debate. (see Embryophyta (land plants).)(see Plant mitosis, cytokinesis and cell plate formation.) Amember of the chlorophycean lineage ( Figure 1), Chloromonas, is a biflagellated unicell that is known to be a close ally of both Chlamydomonas and Volvox (Buchheim et al., 1997). Many species of Chloromonas (and some Economic and Ecological Importance species of Chlamydomonas) are distinctive in their ability to grow in snow. Snow that is coloured bright green or red The green algae include a number of economically and often harbour these ‘snow algae’. Moreover, these photo- ecologically important plants. The organisms presented synthetic organisms may be a key component of an

4 ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net Green Algae extraordinary microcosm of organisms that are capable of (Figure 1). Although Prototheca possesses plastids with exploiting a habitat too extreme for the vast majority of (and thus is a plant-like organism), it is distinctive in life. (see Arctic ecosystems.) that it lacks photosynthetic pigments and is thus hetero- trophic. Given its phylogenetic neighbours (e.g. Chlorella), it is clear that Prototheca has lost the ability to produce Bracteacoccus photosynthetic pigment. It has been described as an This is a coccoid member of the chlorophycean lineage opportunistic pathogen of humans (Graham and Wilcox, (Lewis, 1997). Although Bracteacoccus grows well in liquid 2000). culture, it is noted for its typical habitat – the so-called algal crusts found in arid regions of the world (Lewis, 1997). Dunaliella Dunaliella is a unicellular, flagellated ally of Chlamydomo- Fritschiella nas in the class Chlorophyceae (Figure 1), despite the fact that Dunaliella normally grows in saline to hypersaline This is a branched, filamentous organism associated with environments and lacks a cell wall. It has been used the green algal order, Chaetophorales, in the chlorophy- extensively by scientists studying adaptations to saline cean lineage (Booton et al., 1998). Fritschiella exhibits a environments and scientists interested in the production of heterotrichous form of filamentous development, in that it carotenoid pigments. In fact, Dunaliella is currently grown possesses a distinct prostrate system of filaments and an in aquaculture for industrial-scale production of b- erect system of filaments. The habit of Fritschiella, which carotene, an essential component of the human diet grows on wet or moist surfaces, is reminiscent of the (Graham and Wilcox, 2000). (see Halophiles.) . As such, it has been touted as a possible sister group to the embryophytes (which include the mosses). However, ultrastructural evidence from the mitotic and Ulothrix cytokinetic apparatus indicates that Fritschiella is not part of the immediate sister group to the embryophytes This is an unbranched, filamentous member of the class (Figure 1). Ulvophyceae (Figure 1). Moreover, Ulothrix lends its name to a group within the Ulvophyceae, the ulotrichalean ulvophytes. The ulotrichalean ulvophytes exhibit complex life histories in which haploid and diploid phases, called Acrescent-shaped, coccoid green alga, Selenastrum is gametophytes and sporophytes, are produced. This type of frequently encountered in samples from ponds life history is called alternation of generations. Ulva is one and lakes. It is readily grown in culture and has been shown of several macroscopic relatives of Ulothrix that produces to exhibit demonstrable responses to environmental gametophytes and sporophytes that are morphologically toxins. Consequently, Selenastrum has been adopted by indistinguishable. Ulothrix, a freshwater representative, is many governmental agencies, including the US Environ- an exception to the general rule that most ulvophytes are mental Protection Agency, as an organism of choice in the marine organisms. Cytologically, Ulothrix and other testing protocols for environmental toxins (Graham and ulotrichalean organisms are uninucleate. (see Gametophyte Wilcox, 2000). (see Environmental impact assessment.) and sporophyte.)

Chlorella Acetabularia Chlorella is distinguished by a general lack of superficially Amacroscopic green algal member of the ulvophycean distinguishing features! It lacks any known motile stages lineage (Figure 1), Acetabularia is commonly called and reproduces by autosporogenesis – the formation of ‘Neptune’s wineglass’ because of its shape – a central stalk nonmotile within a parental . The capped by radiating arms that form a bowl in most species. simplicity of the organism and its ease of growth have Acetabularia is an ally of a group of ulvophycean called made Chlorella a favourite of plant physiologists interested ‘siphonous’ green algae, most of which have a multi- in studying plant processes such as photosynthesis. nucleate phase. These siphonous algae are generally characterized by complex, macroscopic morphologies, Prototheca yet the thalli are not organized into distinct cells (i.e. the thalli lack crosswalls). Thus, the thalli of Acetabularia are In common with Chlorella, Prototheca is a unicellular, essentially large, unicellular organisms. As a consequence coccoid organism that does not produce any known motile of this distinctive feature, geneticists have exploited stage. It is allied within the class Trebouxiophyceae Acetabularia in studies of algal development. In contrast

ENCYCLOPEDIA OF LIFE SCIENCES / & 2001 Nature Publishing Group / www.els.net 5 Green Algae to the , many members of the siphonous (eds) Molecular Systematics of Plants II. Boston: Kluwer Academic lineage are effectively multinucleate. Publishers. Friedl T (1997) The evolution of the green algae. Plant Systematics and Evolution (supplement) 11: 87–101. Nitella Graham LE (1993) Origin of Land Plants. New York: Wiley. Graham LE and Wilcox LW (2000) Algae. Upper Saddle River, NJ: Nitella is a close ally of Chara in the charophycean lineage Prentice Hall. (Figure 1). Both Nitella and Chara are macroscopic green Lewis LA(1997) Diversity and phylogenetic placement of Bracteacoccus algae. The cells that comprise Nitella thalli are notably Tereg (Chlorophyceae, Chlorophyta) based on 18S rRNAgene large (up to 15 mm in length) and the cytoplasm exhibits sequence data. Journal of Phycology 33: 279–285. pronounced cyclosis, thus Nitella has frequently been used Mattox KR and Stewart KD (1984) Classification of the green algae: a in teaching laboratories to illustrate this remarkable concept based on comparative cytology. In: Irvine DEG and John D (eds) Systematics of the Green Algae. London: Academic Press. phenomenon. Moreover, it has been the organism of Mishler BD, Lewis LA, Buchheim MA et al. (1994) Phylogenetic choice for the study of intracellular movement at the relationships of the ‘green algae’ and ‘’. Annals of the molecular level. Similarly, the large vacuoles of Nitella Missouri Botanical Garden 81: 451–483. have been exploited in the study of basic vacuolar function. van den Hoek C, Mann DG and Jahns HM (1995) Algae. An Introduction to Phycology. Cambridge: Cambridge University Press. References Further Reading Booton GC, Floyd GL and Fuerst PA(1998) Origins and affinities of the filamentous green algal orders Chaetophorales and Oedogoniales Chapman RL, Buchheim MA, Delwiche CF et al. (1998) Molecular based on 18S rRNAgene sequences. Journal of Phycology 34: 312– systematics of the green algae. In: Soltis DE et al. (eds) Molecular 318. Systematics of Plants II. Boston: Kluwer Academic Publishers. Buchheim MA, Buchheim JA and Chapman RL (1997) Phylogeny of Graham LE (1993) Origin of Land Plants. New York: Wiley. Chloromonas: Astudy of 18S rRNAgene sequences. Journal of Graham LE and Wilcox LW (2000) Algae. Upper Saddle River, NJ: Phycology 33: 286–293. Prentice Hall. Chapman RL, Buchheim MAand Delwiche CF (1998) Molecular van den Hoek C, Mann DG and Jahns HM (1995) Algae. An Introduction systematics of the green algae. In: Soltis DE, Soltis PS and Doyle JJ to Phycology. Cambridge: Cambridge University Press.

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