The Chlorophyta, or green algae, have chlorophylls a and b, and form starch with the chloroplast, usually in association with a pyrenoid. The Chlorophyta thus differ from the rest of the eukaryotic algae in forming the storage product in the chloroplast instead of in the cytoplasm. No chloroplast endoplasmic reticulum occurs around the chloroplasts.

The Chlorophyta are primarily freshwater; only about 10% of the algae are marine, whereas 90% are freshwater (Smith, 1955). Some orders are predominantly marine (Caulerpales, Dasycladales, Siphonocladales), whereas others are predominantly freshwater (Ulotrichales, Coleochaetales) or exclusively freshwater (Oedogoniales, Zygnematales). The freshwater species have a cosmopolitan distribution, with few species endemic in a certain area. In the marine environment, the green algae in the warmer tropical and semitropical waters tend to be similar everywhere in the world. This is not true of the Chlorophyta in the colder marine waters; the waters of the Northern and Southern hemispheres have markedly different species. The warmer waters near the equator have acted as a geographical barrier for the evolution of new species and genera.

Cell structure

In the Chlorophyta, microtubular hairs do not occur on the flagella, although fibrillar hairs (Chlamydomonas, Fig. 1.7(b)) and Golgi-produced scales (Pyramimonas (Fig. 5.10), are present in some genera.

Cell walls usually have cellulose as the main structural polysaccharide, although xylans or mannans often replace cellulose in the Caulerpales (Huizing et al., 1979). The primitive algae in the Prasinophyceae have extracellular scales, or a wall derived from interlacing scales, composed of acidic polysaccharides (Becker et al., 1996). Algae in the Volvocales have walls composed of glycoproteins (Goodenough and Heuser, 1985). Chloroplast pigments are similar to those of higher plants; chlorophyll a and b are present. The main carotenoid is lutein. The siphonaceous genera, as well as the unicells Tetraselmis and Mesostigma, are the only green algae to have siphonoxanthin (Fig. 5.1) and its ester siphonein

(Yoshi et al., 2003).

Accumulation of carotenoids occurs under conditions of nitrogen deficiency, high irradiance or high salinity. This is particularly true in Dunaliella (Figs. 5.62, 5.63) where -carotene accumulates between thylakoids in the chloroplast, and Haematococcus (Fig. 5.63), where astaxanthin (Fig. 5.1) accumulates in lipid globules outside the chloroplast (Hagen et al., 2000; Wang et al., 2003). Hematochrome is a general term for these carotenoids. Accumulation of hematochromes color the cells orange or red, with hematochrome accumulating up to 8–12% of the cellular contents in Dunaliella (Orset and Young, 1999). Animals can not synthesize carotenoids and they acquire the pigments through the food chain from primary producers. Hematochromes are responsible for the coloring in fish, crustaceans and birds (such as the pink in flamingos).

Chloroplasts are surrounded only by the double-membrane chloroplast envelope, with no chloroplast endoplasmic reticulum (Fig. 5.2). The thylakoids are grouped into bands of three to five thylakoids without grana. In some of the siphonaceous genera (e.g., Caulerpa), amyloplasts

containing starch grains and a few thylakoids occur in the chloroplasts.

Starch is formed within the chloroplast, in association with a pyrenoid, if one is present (Fig. 5.2). The starch is similar to that of higher plants and is composed of amylose and amylopectin. The

photosynthetic pathways are similar to those of higher plants, many of these pathways first being worked out in green algae such as Chlorella.

Contractile vacuoles are present in vegetative cells of most Volvocales. Usually in biflagellate genera there are two contractile vacuoles at the base of the flagella. When there are two contractile vacuoles, they contract alternately with a rapid contraction and slow distention. The contractile vacuoles may control the water content of the cells where the protoplasm has a higher concentration of solutes than does the medium, leading to a total inflow of water that is compensated by the water pumped out by the contractile vacuoles. The contractile vacuoles may also function in removing wastes from the cells. Contractile vacuoles are sometimes called pulsating vacuoles because of their alternate filling and emptying action.

Phototaxis and eyespots

There are two types of phototactic movement in the Chlorophyta: movement by flagella and movement by the secretion of mucilage.

Most of the flagellated cells that show phototactic movement have an eyespot. In the Chlorophyta, the eyespot or stigma is always in the chloroplast, usually in the anterior portion near the flagella bases (Fig. 5.2). The eyespot consists of one to a number of layers of lipid droplets, usually in the stroma between the chloroplast envelope and the outermost band of thylakoids. The eyespot is usually colored orange-red from the carotenoids in the lipid droplets. The phototactic response varies with light intensity; Strasburger in 1878 (Bendix, 1960) observed that organisms with positive phototaxis at moderate light intensities exhibited negative phototaxis at very high light intensities. He also noted that at a given light intensity, temperature has an effect on phototaxis. Haematococcus zoospores (Fig. 5.63) at a given light intensity will be negatively phototactic at 4 °C, positively phototactic at 16–18 °C, and very strongly phototactic at 35 °C. Similar results were obtained with Ulothrix (Fig. 5.31) and Ulva (Fig. 5.33).

Green algae use a two-instant mechanism for perceiving light (i.e., successive measurements of

light are performed by one receptor as the cell changes its position in relation to the light source) (Boscov and Feinleib, 1979). Such a mechanism can operate only if the cell frequently changes its position with respect to the light source. The photoreceptor then compares the light intensity at two different time intervals. The photoreceptor in Chlamydomonas is in the plasma membrane above the eyespot (Melkonian and Robenek, 1980b) and consists of a chromophore (colored substance) linked to a protein (opsin or apoprotein) that is embedded in the plasma membrane. The chromophore is 11-cis -retinal (the aldehyde of vitamin A) (Fig. 5.3). Light excitation causes isomerization of 11-cis-retinal into all-trans, triggering a conformational change that initiates the signaling process. The 11-cis-retinal is restored by an enzymatic process. The 11-cis -retinal is linked to a protein that varies from alga to alga. In

Chlamydomonas reinhardtii, the protein is chlamyopsin, while in Volvox carteri the protein is volvoxopsin (Ebnet et al., 1999; Hegemann et al., 2001). Combined, the chromophore 11-cis-retinal and the protein produce a rhodopsin, a general class of compounds that absorb light maximally around wavelengths of 500 nm. Chlamydomonas has two rhodopsins: Chlamydomonas sensory rhodopsin A and Chlamydomonas sensory rhodopsin B

(Sineshchekov et al., 2002). Chlamydomonas sensory rhodopsin A absorbs light maximally at 510 nm (Fig. 5.3), saturates at high light intensity, and mediates a fast photoreceptor current that is involved in the photophobic response (Fig. 5.3). The photophobic response causes the alga to stop and prevents the cell from crossing a light/dark border. Chlamydomonas sensory rhodopsin B absorbs light maximally at 470 nm, saturates at low light intensities, and generates a slow photoreceptor current that is involved in phototaxis.

The eyespot acts as an interference filter by reflecting blue and green light back onto the photoreceptor in the plasma membrane (Kriemer and Melkonian, 1990). Different amounts of light are reflected onto the photoreceptor as the alga swims through the medium. This results in changes in membrane potential involving rhodopsin. Entry of calcium into the cell is affected by the membrane potential of the plasma membrane, and, in turn, the concentration of calcium ions in the cytoplasm

Fig. 5.3 (a) The structure of 11-cis-retinal, part of the photoreceptor molecule in Chlamydomonas. (b) Action spectra showing the sensitivity of the photoreceptors Chlamydomonas sensory rhodopsin A (CSRA) and Chlamydomonas sensory rhodopsin B (CSRB) to different wavelengths of light.

(c) Scheme of light signal transduction initiated by the two sensory rhodopsin pigments in Chlamydomonas. ((b),(c) adapted from Sineshchekov et al., 2002.)

affects the rate of beating of the flagella. The swimming direction of the cell is affected by the rate of beating, because at one concentration of calcium ions, each flagellum beats differently (Kamiya and Witman, 1984). Therefore, changing the cytoplasmic calcium concentration differentially changes the beat of each flagellum, causing the cell to swim in a different direction. Melkonian and Robenek (1980b), using freeze-fracture replicas, have shown that the plasma membrane and outer membrane of the chloroplast envelope have more particles in the areas of the membranes over the eyespot than in other areas. The portion of the plasma membrane overlying the eyespot of Chlorosarcinopsis gelatinosa has 8200 protein particles m 2 in

the half of the plasma membrane next to the cytoplasm (Melkonian and Robenek, 1980a). The plasma membrane not over the eyespot has only 2100 particles m 2. These protein particles in the membranes over the eyespot probably represent a part of the photoreceptor system because they disappear during flagellar retraction and before cell wall secretion, a time when the photoreceptor system would be of no use.

A second type of phototactic movement in the Chlorophyta uses secretion of mucilage. In 1848, Ralfs, in his monograph on desmids, described their movement to the surface of mud brought into the laboratory, and presumed this movement to be due to the stimulus of light. Braun (1851) noticed that young cells of Penium curtum quickly aligned their long axis and moved toward the light, accumulating on the lighted side of the vessel they were growing in. The movement in desmids is brought about by the extrusion of slime through cell wall pores in the apical part of the cell (Domozych et al., 1993; Nossag and Kasprik, 1993).

Green algae can also show geotactic responses to gravity. Chlamydomonas (Fig. 5.55) exhibits nega-