Diatoms Bacillariophyceae

Diatoms (Fig. 18.14) are among the best-known and economically most important members of the phylum. These mostly unicellular algae occur in astronomical numbers in both fresh and salt water but are particularly abundant in colder marine habitats. In fact, as a rule, the colder the water, the greater the number of diatoms present, and huge populations of diatoms zoospore escaping after plug dissolved zoospore escaping after plug dissolved


Figure 18.13 Statospores, which often resemble apothecary bottles, are formed by many of the golden-brown algae. After G.M. Smith, 1950. The Freshwater Algae of the United States. 2d ed. The McGraw-Hill Companies. All rights reserved.

Figure 18.13 Statospores, which often resemble apothecary bottles, are formed by many of the golden-brown algae. After G.M. Smith, 1950. The Freshwater Algae of the United States. 2d ed. The McGraw-Hill Companies. All rights reserved.

Figure 18.14 A diatom. (Scanning electron micrograph courtesy J. D. Pickett-Heaps)

Stern-Jansky-Bidlack: I 18. Kingdom Protista I Text I I © The McGraw-Hill

Introductory Plant Biology, Companies, 2003

Ninth Edition

Chapter 18

are found on and within ice in both Antarctica and the Arctic. A major constituent of the foam that accumulates at the wave-line on beaches is an oil produced by diatoms.

Diatoms usually also dominate the algal flora on damp cliffs, the bark of trees, bare soil, or the sides of buildings. More than 5,600 living species are recognized, with almost as many more known only as fossils. Some can withstand extreme drought, and one species is known to have become active after lying dormant in dry soil for 48 years.

Diatoms look like tiny, ornate, glass boxes with lids. Half of the rigid, crystal-clear wall fits inside the other overlapping half. Many marine diatoms are circular in outline when viewed from the top, while freshwater species viewed the same way tend to resemble the outline of a kayak. As much as 95% of the wall content is silica, an ingredient of glass, deposited in an organic framework of pectin or other substances.

The diatom walls usually have exquisitely fine grooves and pores that are exceptionally minute passageways connecting the protoplasm with the watery environment outside the shell. These fine grooves and pores are so uniformly spaced they have been used to test the resolution of microscope lenses. When phycologists (scientists who specialize in the study of algae) want to examine the markings on diatom walls, they often obtain a better view by dissolving the cell contents with hot acid.

Each diatom may have one, two, or many chloroplasts per cell. In addition to chlorophyll a, the accessory pigments chlorophyll c1 and chlorophyll c2 are typically present. The chloroplasts usually are golden-brown in color because of the dominance of fucoxanthin, the brownish pigment also found in the brown algae. Food reserves are oils, fats, or the carbohydrate laminarin.

Freshwater diatoms that have a lengthwise groove, called a raphe, glide backward and forward with somewhat jerky motions, at a rate of up to three times their length in 5 seconds. It is believed that the movements occur in response to external stimuli such as light. Extremely tiny fibrils apparently take up water as they are discharged into the raphe or pores through which moving cytoplasm protrudes. When they come in contact with a surface, they stick and contract, moving the cell as the caterpillar treads on a tractor do and leaving a trail something like that of a snail.

Reproduction in diatoms is unique, with half of the cells becoming progressively smaller through several generations until, through a sexual process, the original cell size is restored. Before any cell division can occur, an adequate source of silicon must be present in the surrounding medium. In culture, the number of cells produced is directly proportional to the amount of silicon added to an otherwise nutrient-balanced medium. Division in culture is also rhythmic, with all the cells dividing at the same time. After a diatom's cell contents, which are diploid, have undergone mitosis and division, the two halves of the cell separate, with a daughter nucleus remaining in each half. Then a new half wall fitting inside the old half is formed. This occurs for a number of generations, with the result that half of the cells become progressively smaller. Eventually, however, when the ratio of cytoplasm to nucleoplasm reaches a critical point, a cell undergoes meiosis, producing four gametes, which then escape. These fuse with other gametes, becoming zygotes called auxospores. Auxospores are like any other zygotes except that the original size of the diatom is restored when the auxospores balloon rapidly before rigid walls are formed (Fig. 18.15).

Half the cells of each generation become progressively smaller.

The size remains constant in the other cells.

gametes gametes zygote


Figure 18.15 Reproduction in diatoms. The two rigid halves of each pillboxlike cell separate and a new rigid half shell forms inside each original half. This results in half the new cells becoming smaller with each generation. At one point, however, the diploid nucleus of a reduced-size cell undergoes meiosis, and four gametes are formed. The zygote produced when the two gametes unite becomes considerably enlarged. The enlarged cell, called an auxospore, develops into a diatom of the same size as the original diatom.

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