There are two possible mechanisms to explain chloroplast movement. In the first or “active movement” the chloroplast moves relative to the rest of the protoplasm, whereas in the second or “passive movement” the protoplasm moves, carrying with it the chloroplast and other organelles. In Vaucheria, passive movement occurs. If chloroplast movement is followed at high or low intensity, it can be seen that not only chloroplasts, but also other organelles and inclusions are rearranged by light. Furthermore, if only a small area of the filament is irradiated with a spot of light, a strong accumulation of cytoplasm plus inclusions can be observed at this place (Fig. 19.10(d)). The organelles are actually trapped in a portion of the cytoplasm as they stream through the cell. Illumination of an area of Vaucheria results in the formation of an actin fiber network that acts as a trapping mechanism (Blatt, 1983).

REFERENCES

Al-Kubaisi, K. H., and Schwantes, H. O. (1981). Cytophotometrische Untersichungen zum

Fig. 19.10 (a) Position of chloroplasts of Vaucheria in the dark, and in low and high light intensities. (b) Action spectrum of chloroplast orientation movement in Vaucheria under low light (lower curve) and high light (upper curve) intensities. Vertical axis is relative quantum efficiency. (c) Diagrammatic representation of the lens effect of light passing through a Vaucheria coenocyte. (d) Movements of single chloroplasts in Vaucheria under low light intensity. A beam of light (dashed area) causes accumulation of chloroplasts in the illuminated area. (After Haupt and Schönbohm, 1970.)

Generationswechel autotropher und heterotrophen siphonaler Organismen (Vaucheria sessilis und

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(Synurophyceae) and Tribonema aequales (Xanthophyceae). J. Phycol. 27:428–36.

Bailey, J. C., and Andersen, R. A. (1998). Phylogenetic relationships among nine species of the Xanthophyceae inferred from rbcL and 18S rRNA gene sequences. Phycologia 37:458–66.

Birckner, V. (1912). Die Beobachtung von Zoosporenbildung bei Vaucheria aversa Hass. Flora 104:167–71.

Blatt, M. R. (1983). The action spectrum for chloroplast movements and evidence for blue- light–photoreceptor cycling in the Vaucheria. Planta 159:267–76.

Cleare, M., and Percival, E. (1973). Carbohydrates of the freshwater alga Tribonema aequale. II. Preliminary photosynthetic studies with 14C. Br. Phycol. J. 8:181–4.

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Vaucheria. J. Exp. Bot. 8:71–86.

Haupt, W., and Schönbohm, E. (1970). Light-oriented chloroplast movements. In Photobiology of Microorganisms; ed. P. Halldal, pp. 283–307. London: Wiley-Interscience.

Hibberd, D. J. (1981). Notes on the taxonomy and nomenclature of the algal classes Eustigmatophyceae and Tribophyceae (synonym Xanthophyceae). Bot. J. Linn. Soc. 82:93–119.

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PHAEOTHAMNIOPHYCEAE

Recent nucleotide sequencing has uncovered an evolutionary line of golden-brown algae not related to other golden-brown algae (Bailey et al., 1998). These algae have been placed in the class Phaeothamniophyceae, a class that is most closely

related to the Xanthophyceae and Phaeophyceae. The cytology of these three classes is similar (Fig. 20.1). The cells have two membranes of chloroplast endoplasmic reticulum with the outer membrane of chloroplast E.R. continuous with the outer membrane of the nuclear envelope. The chloroplasts have a ring-shaped genophore and girdle lamellae. The flagella are inserted

Fig. 20.1 A filament and

zoospore of Phaeothamnion

polychrysis. Also included is the fine

structure of a zoospore.

laterally into the motile cells. The anterior tinsel flagellum has tripartite hairs that lack lateral filaments. The posterior flagellum lacks hairs. New daughter cells are formed by eleutheroschisis (parent cell wall is completely cast off and new daughter cell walls are formed). Vesicles under the plasma membrane appear similar to the physodes that occur in the Phaeophyceae. The Phaeothamniophyceae is the only class of algae where fucoxanthin and heteroxanthin occur together. Endogenous siliceous cysts (statospores) are not produced by these algae.

Phaeothamnion is a filamentous brown alga that produces zoospores that settle to produce new filaments (Fig. 20.1) (Andersen et al., 1998). Tetrachrysis occurs in environments such as peat ponds and has cells embedded in a common mucilage (Dop et al., 1980) (Fig. 20.2). Tetrasporopsis is a colonial freshwater alga that consists of a brown, gelatinous, bladdery sac (Entwisle and Andersen, 1990) (Fig. 20.2). Phaeoschizochlamys occurs among detritus or suspended between other freshwater algae. The cells occur in mucilage of different shapes up to 0.5 mm in

diameter (Fig. 20.2). The cells divide to form four autospores containing two chloroplasts.

REFERENCES

Andersen, R. A., Potter, D., Bidigare, R. R., Latasa, M., Rowar, K., and O’Kelly, C. J. (1998). Characterization and phylogenetic position of the enigmatic golden alga Phaeothamnion confervicola: ultrastructure, pigment composition and partial SSU rDNA sequence. J. Phycol. 34:286–98.

Bailey, J. C., Bidigare, R. R., Christensen, S. J., and Andersen, R. A. (1998). Phaeothamniophyceae classis nova: a new lineage of chromophytes based upon photosynthetic pigments, rbcL sequence analysis and ultrastructure. Protist 149:245–63.

Dop, A. J., Kosterman, Y., and van Oers, F. (1980). Coccoid and palmelloid benthic Chrysophyceae from the Netherlands. Acta Bot. Neerl. 29:87–102.

Entwisle, T. J., and Andersen, R. A. (1990). A reexamination of Tetrasporopsis (Chrysophyceae) and a description of Dermatochrysis gen. nov. (Chrysophyceae): a monostromatic algae lacking cells walls. Phycologia 29:263–74.