CHAPTER FOUR

GENERAL CHARACTERISTICS

4.1Morphology/Planktonic Habit of Diatoms

Diatoms are generally 2 to 200 micrometres in size (Hasle et al, 1996) with few larger species. Their yellowish-brown chloroplasts, the site of photosynthesis, are typical of heterokonts, having four membranes and containing pigments such as the carotenoid fucoxanthin. Individuals usually lack flagella but are present in male gametes of the centric diatoms.

Diatoms are often referred to as “jewels of the sea” or “living opals” due to their optical properties (Parker et al, 2007). The biological function of this structural coloration is not clear, but it is speculated that it may be related to communication, camouflage, thermal exchange and/or UV protection (Gorden et al. 2009).

Diatoms build intricate hard but porous cell walls called frustules composed primarily of silica (Horner, 2002). This siliceous wall (The Corning Museum of Glass, 2013) can be highly patterned with a variety of pores, ribs, minute spines, marginal ridges and elevations; all of which can be used to delineate genera and species. The cell itself consists of two halves, each containing an essential flat plate, or valve and marginal connecting, or girdle band. One half, the hypotheca, is slightly smaller than the other half, the epitheca.

Figure 4: Representative details of a diatom cell (culled from https://en.m.wikipedia.org)

Figure 4: Representative details of a diatom cell (culled from https://en.m.wikipedia.org)

Nucleus; hold the genetic material

Nucleolus; Location of the chromosomes

Golgi complex; modifies proteins and sends them of the cell

Cell wall; Outer membrane of the cell

Pyrenoid; Centre of carbon fixation

Chromatophore; pigment carrying membrane structure

Vacuoles; vesicle of a cell that contains fluid bound by a membrane

Cytoplasmic strands; hold the nucleus

Mitochondria; create ATP (energy) for the cell

Valves/Striae; allow nutrients in, and waste out, of the cell.

Figure 5: Pennate diatom: Girdle view structure and components

Figure 6: The structure of a diatom cell. Schematic representation of typical shapes of centric(A) and pennate (B) diatoms (modified after Tikkanen 1986).

The morphological appearance and features of the diatom are very useful in their preliminary identification. Their general shape and superficial appearance are variable and six of these are correlated with the planktonic habit.

Stellate colonies of Asterionella and Tabellaria

Needle shape of some spp like Rhizosolenia and Synedra

Long sometimes coiled filaments of some spp of Melosira

Elongated bristles as in Stephanodiscus and horns from the edge of the valve, for example,

Chaetoceros

Flat discoid shape of many centric forms e.g., Cyclotella/ Mucilaginous envelope e.g.,

Cyclotella planctonica

a

b

d

e

c

a

b

d

e

c

Figure 7: Morphological appearances of diatoms. (a) Asterionella; (b) Rhizosolenia; (c) Melosira; (d) Chaetoceros; (e) Cyclotella planctonica.

4.2Colony Formation and the Secretion of Mucus

The attachment of cells to form colonies is accomplished by mucus secreted by the cells themselves. A layer of mucus or gelatinous strands may be formed between the valve faces resulting in ribbon-shaped or threadlike colonies. The cells of zigzag colonies are united by small gelatinous cushions at the corners of the valves. In a few species cells are united by their girdle sides. In certain pelagic species of Nitzschia, contact is at the ends of the cells only.

Some diatoms secrete simple or branched gelatinous stalks, by which they are attached to other plants, animals, or the substrate. Some species secrete a large amount of mucus so that many individuals are embedded in a common mass. Simple or branched gelatinous tubes may be formed to enclose many cells.

Special pores, situated in definite localized positions in the valves of different genera, often secrete mucus. They occur near the middle of the valve in some genera, near the ends of the valves in others. This varying position of the secretory pores is mainly responsible for the many different shapes of colonies. A gelatinous membrane has been described as stretching between the radiating cells in plankton species of Asterionella. This would be a special adaptation for flotation.

Secretion of mucus, however, is not the only cause of colony building. Many diatoms are joined into chains or various-shaped colonies by processes such as spines, knobs, or long setae.

In Stephanopyxis palmeriana the cells are bound into chains by spines at right angles to the valve face. In some others, a fine canal is present in the spines and through this canal the cementing substance is secreted. A similar arrangement is present in Skeletonema costatum. In Chaetoceros and Bacteriastrum the basal arts of the processes or setae, outgrowths on the valves, are joined together and are often more or less strongly twisted, the free ends then running out into the surrounding water. In the general Lauderia and Thalassiosira a cytoplasmic thread runs between cells from the centre of the valves, and often a row of spines or cytoplasmic rays project from the margins of the cells as well. Cytoplasmic connections arising from various parts of the cell hold together the chains formed by many truly planktonic species.

4.3Description of Diatoms:

Diatoms are a large group of algae consisting of around 1,000 to 1,300 described genera and over 30,000 species of which no shell is the same. The diatoms are the most beautiful microscopic algae due to the structure and sculpturing of their walls. They have been referred to as perfect architects, spinning themselves intricate houses of opal in the sea. They have been described as exquisite and most vital plants on earth. They bob, drift, and sometimes glide through most waters of the world in incredible numbers (Hoover, 1979). Like all plants, these golden-brown algae thrive wherever there is light, water, carbon dioxide, and the necessary nutrients.

They occur in various habitats like fresh water, saline water and in terrestrial condition on or within the soil. Sometimes they also occur as epiphytes along with algae, on the leaf of forest trees, mostly in tropical rain forests. Depending on the mode of nutrition they may be photosynthetic autotrophs or photosynthetic symbionts or heterotrophs.

4.4Diatom characteristics

The important characteristics of the class Bacillariophyceae are:

They are commonly unicellular and free- living but some members form colonies of various shapes like filaments, mucilaginous colonies etc.

Microscopic cells are of different shapes. They may be oval, spherical, triangular, boat- shaped etc.

Plant bodies are either bilateral or radial in symmetry.

The cells are surrounded by a rigid cell wall, called frustules, consisting of upper epitheca and lower hypotheca; arranged in the form of a box with its lid.

The cell wall is composed of pectic substances impregnated with high amount of siliceous substance.

The wall may have secondary structures like spines, bristles etc.

Vegetative cells are diploid (2n).

The cells generally have many discoid or two large plate-like chromatophores. Some cells possess stellate chromatophore.

The photosynthetic pigments are chlorophyll a, chlorophyll c along with xanthophylls like fucoxanthin, diatoxanthin and diadinoxanthin.

Reserve food is oil, volutin and crysolaminarin.

Some vegetative cells show gliding movement.

Motile structure (antherozoid) has single pantonematic flagellum.

Vegetative multiplication takes place by cell division, which is very common. Some of the cells become very much reduced in size.

They produce characteristic spore, the auxospore which develops to regain the normal size.

Sexual reproduction takes place by isogamy and oogamy.

4.5Movement of Diatoms

Movement in diatoms is associated with the raphe. Diatoms with raphe are called Raphidae while those without raphe are termed Araphidae. All diatoms with raphe are motile. Most of the members of the order Pennales contain raphe and perform gliding movement. The gliding movement is caused by the circulation of cytoplasm within the raphe by the release of mucilage. The rate of movement varies from 02–25 µm/sec. The locomotion is affected by temperature, light etc.