Diatoms class Bacillariophyceae

The majority of diatoms are unicellular, uninucleate plants with a size range of about 15 ¡m to 400 ¡m in maximum dimension, although some smaller and a few considerably larger forms exist. The largest known diatom is a tropical species, Ethmodiscus rex, up to 2 mm in diameter. The diatom cell, known as a frustule, has a cell wall of unusual composition and structure. It is impregnated with siliceous material giving a glassy quality and consists of two parts, the valves. At its simplest, for example in Coscinodiscus, the cell wall is like a transparent pillbox (Figure 2.1), the larger valve or epitheca overlapping the smaller hypotheca much as the lid of a pillbox overlaps the base.

The valves are often very elaborately ornamented with an intricate sculpturing of minute depressions, perforations or tiny raised points which are sometimes arranged in beautiful symmetrical patterns of great variety. In some, the cell wall has larger projections forming spines, bristles and knobs. Ornamentation increases the surface area and also strengthens the cell wall, which in the majority of planktonic diatoms is very thin. In some species, growth occurs by elongation of the valves at their margins forming a number of intercalary bands, for example Guinardia (Figure 2.2a). Internal thickenings of these bands may form septa which partially divide the interior of the frustule.

Diatoms Cytoplasm
Figure 2.1 Diagrammatic section of a pillbox diatom.

The cytoplasm usually lines the cell wall and contains numerous small, brown chromatophores. There is a large central vacuole containing a cell sap. The nucleus with an enclosing film of cytoplasm is often suspended within the vacuole, supported by cytoplasmic threads extending from the peripheral layer. In planktonic diatoms the cell sap is probably lighter than seawater and may confer some buoyancy to support the heavier protoplasm and cell wall. In many diatoms the cytoplasm is not confined to the interior of the frustule, but exudes through small perforations to cover the surface or form long thin threads, and these may join the cells together in chains.

Planktonic diatoms present a considerable variety of shape, each in its way well adapted to provide a large surface/volume ratio which improves their photosyn-thetic efficiency. They may be grouped into four broad categories as follows:

(a) Pillbox shapes - Usually circular and radially symmetrical when seen in top or bottom view, for example Coscinodiscus (Figure 2.2f), Hyalodiscus. Sometimes they are connected by protoplasmic strands to form chains, for example Thalassiosira (Figure 2.2h), Coscinosira.

(b) Rod or needle shapes - The division between the valves may be at right-angles to the long axis of the cell, for example Rhizosolenia (Figure 2.2e), and these are often joined end to end to form straight chains. In others the division runs lengthways, for example Thalassiothrix, Asterionella (Figure 2.2d and g), and these may be joined to form starlike clusters or irregular zig-zag strands.

(c) Filamentous shapes - Cells joined end to end by the valve surfaces to form stiff, cylindrical chains (Guinardia, Figure 2.2a) or flexible ribbons (Fragilaria, Figure 2.2c).

(d) Branched shapes - Cells bearing various large spines or other projections, and sometimes united into chains by contact between spines, Chaetoceros (Figure 2.2i), or by sticky secretions, Biddulphia (Figure 2.2b).

In addition to the planktonic forms there are numerous benthic species of diatom occurring on the shore or in shallow water. These often grow on the surface of sediments, particularly in sheltered areas such as sea lochs. Here they

Asterionella Japonica
Figure 2.2 Some common diatoms from shallow seas around the British Isles. (a) Guinardia flaccida, (b) Biddulphia sinensis, (c) Fragilaria, (d) Thalassiothrix, (e) Rhizosolenia, (f) Coscinodiscus concinnus, (g) Asterionella japonica, (h) Thalassiosira, (i) Chaetoceros decipiens.

form a clearly visible, thin brown layer. On rocks and stones, they may form a slimy covering. Some project above the surface of the substrate on short stalks. Diatoms are also commonly found attached to the surface of other plants or animals. Benthic diatoms usually have appreciably thicker and heavier cell walls than the planktonic species. Certain benthic species living on sediments have some powers of motility, gliding within the interstices of the deposit so as to move to or from the surface with changing conditions (see page 292).

The usual method of reproduction is by simple asexual division. Under favourable conditions this may occur three or four times a day, so that rapid increase in numbers is possible. The protoplast enlarges and the nucleus and cytoplasm divide. The two valves become gradually separated, the daughter cells each retaining one valve of the parent cell. The retained valve becomes the epitheca

Diatom Auxospore Microspore

Figure 2.3 Reduction of mean cell size in diatoms following cell division.

Figure 2.3 Reduction of mean cell size in diatoms following cell division.

of each daughter cell, and a new hypotheca is secreted, the margin of which fits inside the old valve. The new cell formed within the parent epitheca is therefore the same size as the parent cell, but the cell formed inside the original hypotheca is smaller. Because of this, it is a peculiarity of diatoms that the average size of the individuals in a population tends to decrease as division continues (Figure 2.3). This process of size reduction does not go on indefinitely. Eventually the valves of the smaller individuals separate, the protoplasm flows out and the valves are shed. The naked protoplasm, known as an auxospore, enlarges and grows new, larger valves.

Some diatoms can form resistant spores to carry them through unfavourable periods, for example, during the winter months in neritic water when the temperature falls and salinity may fluctuate appreciably. The cell vacuole disappears and the protoplasm becomes rounded, secreting a thick wall around itself. Probably many resistant spores sink to the bottom and are lost, but in shallow water some may be brought to the surface again later by wave action, currents and turbulence, and then germinate. In high latitudes, diatom spores become enclosed in sea ice during the winter months and germinate the following year when the ice melts.

Sexual reproduction has been observed in certain diatoms. In some species it precedes auxospore formation, the protoplasts of two diatoms fusing to form a single auxospore. In other cases, fusion of protoplasts appears to give rise to two or more auxospores. The formation of microspores has also been observed, the protoplast dividing numerous times to form minute biflagellate structures which are thought to act as gametes.

When planktonic diatoms die, fragments of their valves sink down to the sea-bed. In some areas, notably beneath the Southern Ocean and the northern part of the North Pacific, this accumulation of diatomaceous material gives rise to a siliceous ooze (see page 217).

2.2.2 Dinoflagellates (class Dinophyceae)

These are unicellular, biflagellate organisms with a range of size similar to diatoms but with a larger proportion of very small forms that escape through the mesh of fine plankton nets (nanoplankton). The arrangement of the flagella is characteristic of the group. Typically (Figure 2.4) the cell is divided into anterior and posterior parts by a superficial encircling groove termed the girdle, in which lies a transverse flagellum wrapped around the cell and often attached to it by a thin membranelle. Immediately behind the origin of the transverse flagellum, a whip-like longitudinal flagellum arises in a groove known as the sulcus, and projects behind the cell. The longitudinal flagellum performs vigorous flicking movements and the transverse flagellum vibrates gently, the combined effects driving the organism forwards along a spiral path. There are various departures from this characteristic form. For example, Amphisolenia has a thin, rod-like shape; Polykrikos (Figure 2.5f), has several nuclei and a series of girdles and sulci, usually eight, each provided with transverse and longitudinal flagella.

Many dinoflagellates have no cell wall. In these non-thecate forms the cytoplasm is covered only by a fine pellicle. Others are thecate and covered with a strong wall of interlocking cellulose plates. In certain species the cell wall is elaborated into spines, wings, or parachute-like extensions, and these are especially complex in some of the warm water forms (for example, Dinophysis, Figure 2.5g), perhaps assisting flotation. Dinoflagellates from the British Isles are described in Dodge (1982, 1985).

Dinoflagellates are mainly a marine planktonic group occurring in both oceanic and neritic water. They are most numerous in the warmer parts of the sea, where they sometimes outnumber diatoms, but are also found in cold areas. Around the

Diatoms Epitheca Structure
Figure 2.4 A simple non-thecate dinoflagellate.
Dinoflagellates Amphisolenia

Figure 2.5 Some dinoflagellates of the north-east Atlantic. (a) Ceratium tripos, (b) Ceratium furca, (c) Ceratium fusus, (d) Peridinium depressum, (e) Noctiluca scintillans, (f) Polykrikos schwarzi, (g) Dinophysis.

Figure 2.5 Some dinoflagellates of the north-east Atlantic. (a) Ceratium tripos, (b) Ceratium furca, (c) Ceratium fusus, (d) Peridinium depressum, (e) Noctiluca scintillans, (f) Polykrikos schwarzi, (g) Dinophysis.

British coasts they are scarce in the winter months, and reach their greatest abundance in the midsummer period when the low concentration of nutrients seems to have less effect in limiting the growth of dinoflagellates than of diatoms. Dinoflagellates also occur in fresh and brackish water, and are sometimes abundant in estuaries. Some are found in sand in the interstitial water between the particles. There are also many parasitic dinoflagellates infecting a variety of planktonic organisms including radiolaria, copepods, pteropods, larvaceans and fish eggs.

Reproduction is by asexual fission, but is not as rapid as in diatoms. In thecate dinoflagellates the process is somewhat similar to fission in diatoms, each daughter cell retaining part of the old cell wall and secreting the other part, but the old and new cell plates do not overlap, and there is consequently no size reduction as occurs in diatoms. The daughter cells do not always separate completely and repeated divisions then form a chain. Resistant spores may be produced during adverse periods.

Many dinoflagellates contain small chromatophores and perform photosynthesis. The group is certainly important as primary producers of food materials. Some of them are highly pigmented, and are sometimes so numerous that the water appears distinctly coloured, different species producing green, red or yellow tints. When this happens it is often described as a 'red tide' (see below). There are also many colourless dinoflagellates. Such thecate forms without chromatophores are presumably saprophytes, feeding off decaying material. However, some of the non-thecate, colourless forms are holozoic, feeding in an animal-like manner on various small organisms including other dinoflagellates, diatoms, microflagellates and bacteria. Noctiluca (Figure 2.5e), for example, devours copepod larvae and other small metazoa. Some of these holozoic dinoflagellates possess tentacles, amoeboid processes or stinging threads for capturing their food.

Some species are luminous, and can be the cause of remarkable displays of phosphorescence in seawater. Noctiluca is an example which sometimes occurs in swarms around the British Isles, visible at night as myriads of tiny flashes of light when stimulated by agitation of the water. This can be seen to dramatic effect when scuba diving. Various species of Peridinium flash spontaneously in undisturbed water.

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Responses

  • Kia
    What material impregnates the cell wall of diatoms?
    9 years ago
  • lauri
    Why do diatoms form chains?
    9 years ago
  • aamu
    What is the two part pillbox cell wall surrounding a diatom called?
    6 years ago
  • RANDALL DAUB
    How does fragilaria move?
    6 years ago

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