Systematics: Class Bacillariophyceae in the division Chromophyta.

Closest relatives: Chrysophyceae and Xanthophyceae. (See Round et al., 1990, p. 122.)

Number of species: 10,000–12,000, approx 50,000 (Round & Crawford, 1984, p. 169), or in excess of 100,000 (Round & Crawford, 1989, p. 574); or in marine plankton approx 1400-1800 (Sournia et al., 1991, p. 1085).

Size: ca. 2 μm-ca. 2 mm.

Level of organization: Unicellular, often in colonies.

Cell covering: Siliceous wall and organic layer.

Flagella: Male gametes with one flagellum with stiff hairs.

Chloroplasts: Lamellae with three thylakoids, girdle lamella, and four membranes around the chloroplast.

Pigments: Chlorophylls a and c, betacarotene, fucoxanthin, diatoxanthin, and diadinoxanthin.

Mitochondria: Tubular type.

Storage products: Chrysolaminarin and oil.

Motility: Present in pennate diatoms with a raphe.

Biotopes: Marine and freshwater, plankton, benthos, epiphytic, epizoic (e.g., on whales and crustaceans), endozoic (e.g., in foraminifera), endophytic (e.g., in seaweed), on and in sea ice, and “air diatoms.”

Geological age: Centrics: Jurassic (a few species) and Early Cretaceous (Gersonde & Harwood, 1990). Araphid pennates: Late Cretaceous (Medlin et al., 1993, with references). Raphid pennates: Middle Eocene (Medlin et al., 1993, with references).

Reproduction (Figs, 1a and 1b)

Diatoms reproduce vegetatively by binary fission, and two new individuals are formed within the parent cell frustule. Each daughter cell receives one parent cell theca as epitheca, and the cell division is terminated by the formation of a new hypotheca for each of the daughter cells. This type of division, with formation of new siliceous components inside the parent cell, leads to size reduction of the offspring. The possible size range of the diatom cells seems to be species dependent, and the specific variation may be as large as 8 to 10 times the length of the apical axis or the diameter.

The considerable size variation is often accompanied by a pronounced size dependent change in cell proportions, normally in the form of an increase in the ratio between the length of the pervalvar axis and the apical axis or diameter. In addition, size variation often causes changes in valve ornamentation, like a reduction in the number of central clustered processes in Thalassiosira spp. (E. Syvertsen, personal observations), a loss of special structures like the pili of certain species of the Cymatosiraceae (Hasle et al., 1983), and an alteration of the valve outline of morphologically bipolar species from elongate toward almost circular, e.g., Fragilaria spp. (Hustedt, 1959) and Cymatosiraceae.

The decrease in the average cell size of a diatom population during vegetative growth implies a need for a means of restoring the cell size. This is made possible by auxospore formation, in which a cell sheds its siliceous theca, thereafter forming a large sphere surrounded by an organic membrane. Within this sphere, a new diatom frustule of maximal size is formed, and the cycle starts anew. The first cell formed inside the auxospore, the initial cell, may have a morphology deviating in girdle structure, valve outline, and process pattern from that of a “normal” vegetative cell (vide, Thalassiosira decipiens, Hasle, 1979, Fig. 41; Cymatosira lorenziana, Hasle et al., 1983, Fig. 19).

Auxospore formation is size dependent and normally takes place when the cell has reached about one-third of its maximal size (Drebes, 1977). Below this limit the diatoms seem unable to rejuvenate themselves, and they continue to divide until they reach a stage at which cell division is no longer possible. There are reports in the literature of taxa which do divide without a simultaneous size reduction (Drebes, 1977), and some species seem to be able to multiply at their lower size limit without further size reduction for an extended length of time (E. Syvertsen, personal observations). For most species, however, auxospore formation is a necessary and normal occurrence in their life cycle. It may take place as a vegetative event or as the result of sexual reproduction.

All diatoms are diplonts with a meiosis at the end of the gametogenesis. The zygote develops into an auxospore. In the centric diatoms, sexual reproduction is by oogamy with flagellated male gametes, while most pennate diatoms are morphologically isogamous lacking a flagellated stage. A few araphid species have been shown to be anisogamous (Drebes, 1977) and are considered to represent a transitional stage between centric...