CRYPTOPHYCEAE

Fig. 9.2 Drawing of the most common type of flagellar

scale found in freshwater cryptophytes. (From Lee and

Kugrens, 1986.)

Fig. 9.1 Drawing of a cell of the Cryptophyceae as seen in the light and electron microscope. (CE) Chloroplast envelope; (CER) chloroplast endoplasmic

reticulum; (CM) Corps de Maupas;

(D) dorsal; (E) ejectisome; (L) lipid;

(M) mitochondrion; (N) nucleus; (NM) nucleomorph; (P) pyrenoid; (PP) periplast plate; (S) starch;

(V) ventral.

and Gibbs, 1982). The only cryptophyte that is known to lack a nucleomorph is Goniomonas (Figs. 9.8, 9.9(c)), a colorless cryptophyte that lacks a plastid. A second colorless cryptophyte, Chilomonas (Fig. 9.9(b)), is a reduced form of a photosynthetic cryptophyte and contains a leucoplast and a nucleomorph (McKerracher and Gibbs, 1982).

In the chloroplast, the thylakoids are grouped in pairs (Fig. 9.3), and there are no connections between adjacent thylakoids. The Cryptophyta is the only group to have this arrangement of thylakoids. Chlorophylls a and c2 are present. The major carotenoid present is -carotene, and the major xanthophyll, diatoxanthin. There are three spectral types of phycoerythrin and three spectral

Fig. 9.3 Transmission electron micrograph of part of a chloroplast of Chroomonas mesostigmatica. The thylakoids are grouped in pairs. The dense contents of the thylakoids represent the phycobilisomes. Also present are lipid droplets

(l) and a large starch grain (s). 50 000. (From Dodge, 1969.)

types of phycocyanin, all of which are different from the phycobiliproteins found in the cyanobacteria and red algae (Hill and Rowan, 1989). The phycobiliproteins are in the intrathylakoid space (inside the thylakoids (Fig. 9.3) (Gantt et al., 1971; Spear-Bernstein and Miller, 1984), and are not on the stromal side of the thylakoids in phycobilisomes as occurs in the cyanobacteria and red algae. Each photosynthetic cryptophyte has only one species of phycobiliprotein – either a phycoerythrin or a phycocyanin – but never both. No allophycocyanin is present (Gantt, 1979). Allophycocyanin acts as a bridge in the transfer of light energy from phycoerythrin and phycocyanin to chlorophyll a of the reaction center in red algae and cyanobacteria. The presence of allophycocyanin may not be necessary in the cryptophytes because the greater absorption range of cryptophycean phycobiliproteins in conjunction with chlorophyll c overlaps the chlorophyll a absorption spectrum. There is a variation in

the amount of pigments under different lightintensity conditions. Cells of Cryptomonas grown under low light-intensity conditions (10 E m 2 s 1) contain twice as much of chlorophylls a and c2, and six times as much phycoerythrin per cell, as those grown under high light-intensity conditions (260 E m 2 s 1) (Thinh, 1983). Under low light-intensity conditions there is a higher concentration of phycoerythrin and the thylakoids are thicker.

The reserve product (similar in appearance to starch grains) is appressed to the pyrenoid area outside of the chloroplast envelope but inside the chloroplast E.R. The cryptophytes are the only algae that form their storage product in this area. The starch is an -1,4-glucan composed of about 30% amylose and amylopectin. Cryptophycean starch is similar to potato starch and starch found in the green algae and dinoflagellates (Antia et al., 1979).

Some of the Cryptophyceae have eyespots. The eyespots that have been reported consist of lipid granules inside the chloroplast envelope. In

Chroomonas mesostigmatica, the red eyespot is in the center of the cell (Fig. 9.4) and is an extension of the chloroplast beyond the pyrenoid (Dodge, 1969). In Cryptomonas rostella, the eyespot is

Fig. 9.4 Transmission electron micrograph of a cell of

Chroomonas mesostigmatica showing the eyespot (E) present in the chloroplast. (Ey) Ejectisome; (F) flagellum; (G) Golgi;

(N) nucleus; (Nu) nucleomorph; (S) starch. (Micrograph provided by Paul Kugrens.)

beneath the chloroplast membrane near the depression. Some of the Cryptophyceae exhibit positive phototaxis, with maximum sensitivity of the colorless Chilomonas being in the blue at 366 nm (Halldal, 1958).

The Cryptophyceae have projectiles called ejectisomes, which are of different structure from the trichocysts of the Dinophyceae and which are probably closely related to the R-bodies of the kappa particles of the ciliates (Hovasse et al., 1967; Kugrens et al., 1994). Within a cell there are usually large ejectisomes near the anterior depression and smaller ejectisomes around the cell periphery (Figs. 9.1, 9.4, 9.8). Both sizes of ejectisomes have the same structure; they are made up of two unequal-sized bodies enclosed within a single membrane (Fig. 9.5). Each of these bodies is a long tape curled up on a very tight spiral. The tape is tapered, with the greatest width being on the outside of the ejectisome. The

smaller body is joined to the first and sits at an angle within the V-shaped portion of the larger body. The two bodies actually constitute one long tape with two spirals. The bodies are always arranged so that the smaller body is near the surface. The ejectisomes discharge when the organism is irritated (Fig. 9.5), the discharged ejectisome being a long tubular structure with a short portion at an angle to the long portion. The discharged small ejectisome from the cell periphery is 4 m long, whereas that of a larger ejectisome from under the anterior depression is about 20 m long. The discharge of the ejectisome results in a movement of the organism in the opposite direction. The discharge of the ejectisome could function as an escape mechanism, or it could be a direct defense mechanism causing damage to an offending organism. Ejectisomes originate in vesicles in the area of Golgi bodies.

The Corps de Maupas is a large vesicular structure in the anterior portion of the cell (Fig. 9.1). Its main function is probably that of disposing of unwanted protoplasmic structures by digestion (Lucas, 1970a,b).