496 CHLOROPLAST E.R.: EVOLUTION OF TWO MEMBRANES

The external coccoliths can be removed by lowering the pH of the culture medium. Cells of Coccolithus huxleyi decalcified in this manner may acquire a complete coccolith envelope (about 15 coccoliths) within 15 hours of their being transferred back to a normal medium in the light. Complete recalcification in Cricosphaera sp. may require 40 hours. In both instances, cell division is not a prerequisite for the formation of new coccoliths. If the organisms are grown in artificial seawater, the coccoliths dissolve if the product of the concentrations of calcium and carbonate is appreciably smaller than the solubility product of calcite. In C. huxleyi, coccoliths are still formed inside cells even when the calcium content of the medium is reduced to levels where external coccoliths dissolve, although coccolith production is retarded at calcium concentrations less than half that of normal seawater. A photochemical process is apparently directly associated with coccolith production because when light is turned off, there is a sharp drop in coccolith production.

Pleurochrysis carterae has biflagellate cells surrounded by a coccosphere consisting of a single layer of 100 to 200 coccoliths. The cells incorporate calcium into extracellular coccoliths at a more or less constant rate throughout a 16-hour light : 8-hour dark cycle. The cells divide during the dark periods with a concomitant decrease in cell size during the dark period followed by an increase in cell size during the light period. The cells form coccoliths in the light as well as in the dark at a similar rate (van der Wal et al., 1987) (although Emiliania huxleyi (Fig. 22.16(b)) produces coccoliths only during the light period (Linschooten et al., 1991).

Although coccolithophorids constitute a minor part of recent calcareous oozes (bottom sediments composed of calcified remains of organisms) in the ocean, in the Cretaceous they dominated the calcareous nanoplankton (Tasch, 1973). This domination paralleled an increase in Ca2 in seawater at the time (Brennan et al., 2004). Coccolithophorids provided the major constituent of Mesozoic (Jurassic and Cretaceous) and Tertiary chalks and marls. The abundance of coccolithophorids in these chalks can be demonstrated by taking a piece of ordinary blackboard chalk, pulverizing it, mixing it with distilled

water in a test tube, and letting it stand for 20 minutes. Draw some of the solution into a pipette, dispose of the first four to five drops, and place the next few drops on a slide. Place a cover slip on the slide, and view it at a magnification of 400 to 500 . Many coccoliths and other remains will be seen.

Coccoliths in sedimentary rocks can be used as markers in the discovery and mode of deposition of oil deposits. For example, the oil shales of the Kimmeridge Clays in England are sandwiched between limestone bands that are composed mostly of coccoliths of one species,

Ellipsagelosphaera britannica (Gallois, 1976). Other oil-bearing rocks have similar characteristic coccoliths. Therefore petroleum geologists know that when a drill core shows certain coccoliths that are associated with petroleum, there is a good chance of finding oil in that stratum of rock.

Toxins

The prymnesiophycean alga Prymnesium parvum (Fig. 22.7) secretes the potent exotoxin prymnesin (Fig. 22.17). The toxin causes fish mortalities by increasing cell membrane permeability and disturbing cellular ion balance (Fistarol et al., 2003). The toxin is most effective against aquatic gillbreathing animals, such as fish and molluscs. In Amphibia, only the gill-containing tadpole stage is sensitive to immersion in solutions containing the ichthytoxin. The rapidity of the action of Prymnesium toxin on immersed fish suggests that the immediate target must be an exposed organ, probably the gill. Experiments have shown that the toxins affect the permeability of the gill, resulting in the increased sensitivity of the fish. In fish removed promptly from such toxin solutions, the gill damage is repaired within hours. In Israel, Shilo (1967) has found that it is possible to control P. parvum in fish breeding ponds by adding small amounts of ammonium salt, which causes the algal cells to lyse.

Secretion of prymnesin by Prymnesium parvum immobilizes prey organisms and enables P. parvum to more easily seize its prey (Skovgaard and Hansen, 2003). Prymnesium produces higher quantities of toxin when phosphorus is limiting

PRYMNESIOPHYTA 497

Fig. 22.17 The chemical structure of prymnesin-2. (After

Igarashi et al., 1996.)

(Legrand et al., 2001). Secretion of the toxin results in greater kill of prey organisms which are subsequently ingested by the Prymnesium cells. The phosphorus in the prey organisms alleviates the phosphorus deficiency in the Prymnesium cells, resulting in decreased production of toxin.

Some species of Chrysochromulina produce toxins that kill fish, mussels, and ascidians (Hansen et al., 1995; Moestrup, 1994; Simonsen and Moestrup, 1997). The best documented fish kills have occurred off the coast of Norway and Sweden. The large blooms of Chrysochromulina causing the fish kills have been associated with a lack of predation by the normal ciliate grazers of

Chrysochromulina. It appears that the long spines on the surface of the Chrysochromulina cells make them too large to be taken up by the ciliates (Hansen et al., 1995).

In the North Sea of Europe, and in the seas off Antarctica, blooms of the prymnesiophyte Phaeocystis (Figs. 22.4, 22.18(a)) occur as macroscopic lobed colonies or “bladders” in the spring and fall. Phaeocystis colonies are hollow, balloonlike structures with individual cells lying beneath a thin mucous skin (Hamm et al., 1999; Solomon et al., 2003). Grazing by invertebrates results in colonies of larger size, the larger size induced by chemicals released into the water by the grazing (Tang, 2003). Colony formation and enlargement is a defense mechanism that results in clogging of the filtration apparatus of the grazers (Haberman et al., 2002).

(b)

(d)

(c)

(a)

Fig. 22.18 (a) Motile cell of Phaeocystis poucheti.

(b) Isochrysis galbana. (c), (d) Imantonia rotunda. (F) Flagellum;

(H) haptonema; (L) leucosin vesicle; (N) nucleus;

(P) pyrenoid; (S) scale. ((d) after Reynolds, 1974.)