CHLOROPHYTA 217

Fig. 5.86 Trebouxia sp.: (a) vegetative cell; (b) zoospores being released; (c) aplanospores being released. The Trebouxia-containing lichen Xanthoria parietina; (d) whole thallus; (e) section of thallus showing spherical Trebouxia cells. ((a)–(c) after Ahmadjian, 1960; (d),(e) after Fünfstück, 1907.)

area has been to provide food and oxygen for space and submarine travel. In such a setup, carbon dioxide from human respiration is taken up for use in photosynthesis, with the oxygen produced being respired by humans. A man uses about 600 liters of oxygen daily, and a gas exchange unit that would produce this amount of oxygen would also produce almost enough biomass to provide sufficient food for one man as well (Fogg, 1971).

Trebouxiales

This order represents the “lichen algae group” (Lewis and McCourt, 2004). Note that not all lichenized algae are in this group: other phycobionts (algal partner in a lichen symbiosis, the fungus is the mycobiont) include cyanobacteria, Trentepholia (Ulvophyceae), and Heterococcus (Xanthophyceae).

Trebouxia (Figs. 5.86, 5.87) is the most common green alga as a phycobiont in a lichen symbiosis. The genus also is found as a free-living alga, and, when free-living, it is usually twice the size of the alga in the lichen symbiosis (see Ahmadjian, 1993, for a review). Trebouxia has a massive chloroplast with a single pyrenoid. When it is growing in the lichen association, reproduction is normally by autospores, although under wet conditions zoospores may be formed. When the alga is grown in liquid culture, zoospores are formed. Sexual reproduction is isogamous or anisogamous by the

fusion of biflagellate gametes. Lichenized Trebouxia produces primarily sugar alcohols (80% ribitol) from photosynthetic processes, whereas free-living Trebouxia forms much smaller amounts of sugar alcohols (15% ribitol) and greater quantities of other carbohydrates (Green, 1970). Richardson (1973) has summarized the differences between algae growing as phycobionts (algae in the lichen association) and the free-living ones: the freeliving algae (1) synthesize less sugar or sugar alcohol, (2) form more polysaccharides, (3) develop cell sheaths not seen in phycobionts, and (4) release less photosynthate into the surrounding medium. Trebouxia is able to grow saprophytically in the dark in the absence of light (Ahmadjian, 1960).

Lichen mycobionts are able to discriminate between suitable and unsuitable algal partners. The mycobiont Xanthoria parietina secretes a protein that will bind only to the cell walls of species of Trebouxia and Pseudotrebouxia, algal genera that normally make up this lichen symbiosis (Bubrick and Galun, 1980). The cell walls of these algae are characterized by high levels of acidic polysaccharide and a protein coat on the cell wall surface. Members of other algal families do not bind the lichen protein. In the lichen Cladonia cristatella, compatible phycobionts have fungal haustoria in most of the phycobiont cells. Fungal haustoria are usually used by the fungus to transfer nutrients

218 EVOLUTION OF THE CHLOROPLAST

Fig. 5.87 The beginning of a lichen. (a) Scanning electron micrograph of the envelopment of a cell of the phycobiont

Trebouxia erici by the hyphae of the mycobiont Cladonia cristatella. (b) The mycobiont has completely enveloped the cells of the phycobiont, and the hyphae have formed the thallus of the lichen. (From Ahmadjian and Jacobs, 1981.)

from a host to the fungus when the host is a parasitized organism. However, in lichens, transfer of metabolites to the fungus is minimal (Collins and Farrar, 1978; Hessler and Peveling, 1978). This is probably a case of controlled parasitism (Ahmadjian and Jacobs, 1981).

Sphaeropleales

These filamentous Chlorophyceae have a mature lateral wall that consists of segments, rather than having a continuous structure. In Microspora, there are H-shaped wall segments that arise from two separate phases of wall secretion (Fig. 5.88) (Pickett-Heaps, 1973). During interphase, cell expansion is accommodated by the interlocking H-segments moving apart. At the same time, a new cylindrical wall is secreted inside these segments. Then during cytokinesis, the newly formed cross wall turns this cylinder into the typical wall segment. A similar wall occurs in the Oedogoniales, but the algae in the Sphaeropleales lack the unusual structure of the Oedogoniales.

The wall structure in the Sphaeropleales is similar to that in the Xanthophyceae, and it is probable that the filaments in both algal groups evolved from a unicellular alga, although independently of each other. The Sphaeropleales probably evolved from an alga in the Chlorellales (Cáceres et al., 1997).

Algae in the Sphaeropleales (Figs. 5.88, 5.89) typically occur in shallow freshwater habitats that are inundated only periodically. The ephemeral vegetative stage (often lasting a few weeks or less) occurs during short intervals of flooding, whereas the thick-walled, resistant zygotes persist in the soil through long dry periods, thereby enabling long-term survival of these algae. The zygotes are ornamented and sometimes have cirri (Fig. 5.90), curled appendages composed of organic material, on their surface (Hoffman and Buchheim, 1989).

Sphaeroplea has multinucleate cells arranged end to end in unbranched filaments (Buchheim et al., 2001) (Fig. 5.89). The alga is freshwater, occurring on periodically wet ground, completing its life cycle within 4 to 5 weeks. Within a cell the cytoplasm is restricted to a number of transverse bands, each band separated by a vacuole. Each cytoplasmic band has several nuclei and a bandshaped chloroplast with several pyrenoids, or numerous discoid chloroplasts. There is also a

CHLOROPHYTA 219

(b)

(a)

Fig. 5.88 Microspora crassior. (a) Filament. (b) Diagrammatic representation of wall deposition. Initially the interphase cell is enclosed by two H-shaped segments and a central cylinder. At cytokinesis, a cross wall is deposited in the middle of the cylinder. During cell expansion, the two H-shaped segments move apart while a new cylinder is secreted inside the H-shaped segments. ((b) after Pickett-Heaps, 1973.)

Fig. 5.89 Sphaeroplea annulina. (a) Part of a vegetative cell. (b) Portion of an oogonium in which the eggs are being fertilized, and a portion of an adjoining antheridium. (After Smith, 1955.)

220 EVOLUTION OF THE CHLOROPLAST

Fig. 5.90 Scanning electron micrograph of a zygote of

Sphaeoplea fragilis showing the sculptured exterior and a long curved cirrus. (From Hoffman and Buchheim, 1989.)

(a)

		(c)
(b)

Fig. 5.91 Chlorosarcina sp. (a) Packet of cells dividing by desmoschisis to produce daughter colonies, each enclosed within the cell wall of the parent cell. (b) Colony-forming zoospores. (c) Zoospore.

thin layer of cytoplasm without chloroplasts between the vacuoles and the side walls. Asexual reproduction occurs by fragmentation of the filaments. Sexual reproduction is usually oogamous, with eggs and spermatozoids formed in alternate cells of the same filament or in different filaments. The spindle-shaped biflagellate spermatozoids escape through pores in the side walls of the antheridial cell and swim to the oogonial cells. The eggs, when first formed, are multinucleate, with all the nuclei but one disintegrating. The oogonial cells have pores in their walls through

which the spermatozoids enter, swim about the eggs, and eventually fuse with them. Zygotes form a thick ornamented cell with a reddish protoplast, and are released by the decay of the oogonial cell. Zygotes germinate by forming four biflagellate ovoid zoospores, which settle and germinate into a new filament.

Chlorosarcinales

The algae in this order have a type of cell division (desmoschisis) that results in the formation of a number of cells, each with its own cell wall, within the cell wall of the parent cell. The genera in the Chlorosarcinales lack the plasmodesmata and complexity found in the Chaetophorales and Oedogoniales.

Chlorosarcina occurs free-living in the soil, or it can occur as an endophyte within the epidermis of aquatic vascular plants. Each cell contains a parietal chloroplast and has a cell wall that separates the cell from the other cells inside the old parental cell wall (Fig. 5.91). During desmoschisis, the protoplasm of a cell divides at successively perpendicular planes to build up cuboidal packets of cells. Cell walls are formed around the newly divided protoplast, next to the parent cell wall which remains intact. Eventually the colonies fragment to distribute the alga. Asexual reproduction can occur by the formation of four to eight biflagellate zoospores per cell,