Bacillariophyceae: Occurrence and Vegetative Body | Algae | Botany

In this article we will discuss about:- 1. Salient Features of Bacillariophyceae 2. General Characters of Bacillariophyceae 3. Occurrence and Distribution 4. Vegetative Body 5. Reproduction 6. Economic Importance.

Contents:

1. Salient Features of Bacillariophyceae
2. General Characters of Bacillariophyceae
3. Occurrence and Distribution of Bacillariophyceae
4. Vegetative Body of Bacillariophyceae
5. Reproduction in Bacillariophyceae
6. Economic Importance of Bacillariophyceae

1. Salient Features of Bacillariophyceae:

The members of this class are either unicellular or form colonies. All are characterized by the presence of golden-brown chromatophores which, besides the usual pigments, contain some brown pigments of obscure nature. The cell wall is made up of both pectic substances and silica, and usually consists of two halves with rich ornamentations on their surface.

The cells are either radially or bilaterally symmetrical. The members of Pennales show special type of sexual union between protoplasts of vegetative cells. They are probably all diploid. Diatoms are widely distributed both in fresh and salt water of all kinds. Some are also terrestrial.

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2. General Characters of Bacillariophyceae:

The Bacillariophyceae, popularly known as diatoms, constitute a very large assemblage of unicellular algae. Owing to the colour of their chromatophores they appear to be brown algae, but they are usually considered as a separate and distinct group.

They are readily distinguished from other algae by the following salient fea­tures:

(a) The plant is usually unicellular and always non-flagellate, the silicified wall consisting of two overlapping halves;

(b) The photosynthetic reserve materials are fats and volutin but never starch;

(c) The photosynthetic pigments of the chromatophores are masked by the presence of a golden-brown pigment known as dia­tomin;

(d) Special type of large rejuvenescent cells (auxospores) are formed, either directly from the vegetative protoplast or as a result of sexual union, a feature not found in other groups of algae.

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3. Occurrence and Distribution of Bacillariophyceae:

Diatoms are inhabitants of both fresh and salt water, being usually associated together in great abundance. Some diatoms are either strictly marine or strictly freshwater. Both freshwater and marine diatoms may be sessile in habit or free-floating (plankton); when sessile, they are attached to other types of algae or affixed to rocks or other solid inanimate objects under water.

After the death of individual cells, the silicified wall of a dia­tom remains unaltered and great numbers of these walls are de­posited at the bottom of ponds, lakes or any other body of water in which diatoms thrive. If the conditions are favourable, deposition goes on unimpeded for ages, and as a result, accumulation may at­tain a considerable thickness. Deposits of such fossil diatoms are known as the diatomaceous earth, which are found distributed all over the world.

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4. Vegetative Body of Bacillariophyceae:

Diatoms are usually unicellular and display a great diversity of forms, but sometimes cells are united together to form simple filamentous or branched colonies. Each cell of a living diatom or its wall alone is called a frustule, which consists of two overlapping halves or valves that fit together like a pair of Petri dishes. The outer valve is called epitheca and the inner, hypotheca. Each frustule thus presents two different views, namely girdle view and valve view, according to the position in which it is observed.

Both the valves of a frustule have their outer walls strongly silicified and made up of an organic matrix chiefly consisting of pectin, there being no micro chemical reaction for cellulose. The siliceous deposit on a valve is not uniform but in the form of nu­merous fine transverse markings or ribs and also as small protu­berances and areolae. The areolae may consist of minute vertical pores as incomplete canals which do not perforate the wall com­pletely.

They are seldom perforated by minute pores through which mucilaginous secretion comes out. The ornamentation is either radially symmetrical with reference to a central point (Centric dia­toms), or bilaterally symmetrical or asymmetrical with respect to a long axis (Pennate diatoms).

Lying internal to the cell wall, there is a thick layer of cytoplasm as a primordial utricle surrounding a conspicuous central vacuole and containing one or more chromatophores, with or without one to several pyrenoids.

There is only one spherical to ovoid nucleus of the normal type embedded in the peripheral layer of cytoplasm of it is suspended in a mass of cytoplasm at the centre of the vacuole and is connected to the peripheral layer by means of broad cytoplasmic strands.

The chromatophores are mostly rich golden brown in colour and vary in shape and number from species to species. This parti­cular colour of the chromatophores is due to the presence of a special pigment, known as diatomin, which masks the photosynthe­tic pigments, chlorophyll and the associated carotenoids. Food reserves are in the form of fatty oil accumulated as droplets of variable size in the cytoplasm or in the chromatophores.

Many planktonic unicellular diatoms -exhibit spontaneous movements, either as a series of jerks directed along the long axis of the cell or as forward and backward progression and retrogres­sion, and these are also seen in some of the colonial forms.

The common Indian species of Centric diatoms are Melosira varians, M. jurgensii, Coscinopiscus radiatus, etc., while those of Pen­nate ones are Synedra ulna, S. acus, Navicula stauroptera, N. viridis, Gomphonema acuminatum, Rhopalodia gibba, Asterionella formosa, etc.

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5. Reproduction in Bacillariophyceae:

Diatoms reproduce vegetatively by cell division. During the process the protoplast of the mother cell swells up and the valves are slightly pushed apart. The nucleus now divides by mitosis into two daughter nuclei. Cytoplasm undergoes cleavage in such a way that each valve receives a daughter nucleus and a portion of cyto­plasm. Then a new valve is secreted outside the daughter proto­plast within each old valve.

In this way two daughter cells are formed. These two daughter cells are unequal in size and one of the valves belongs to the parent cell and the other is newly formed. In the subsequent cell divisions, the sizes of the daughter cells suc­cessively become smaller and smaller.

When a certain definite minimum size is reached, a special mode of reproduction takes place by the formation of a special large rejuvenation takes place by the formation of a special large rejuvenating cell called auxospore such that the original size of the cell is regained.

In Centric diatoms, the protoplast becomes about three times bulky by the absorption of water and the valves are widely pushed apart. This increased bulk of protoplast now secretes two over­lapping valves and the entire structure thus formed is known as an auxospore. The auxospore divides vegetatively and gives rise to vegetative cells.

In Pennate diatoms, the auxospore is formed from the zygote which is the product of fusion of gametes produced by a single of two vegetative cells.

Two conjugating diatoms come to lie close together and be­come enveloped by a common mucilaginous sheath. The diploid nucleus of each now divides by meiosis to form four haploid nuclei of which a single or two haploid daughter nuclei persist in each vegetative cell. Cytoplasm now collects around each of these daughter nuclei and in this way each vegetative cell gives rise to a single or two gametes.

The gametes fuse sexually and give rise to a single or two zygotes and function as auxospores. Sometimes, two gametes are produced from a single vegetative cell, which fuse together to form zygote and auxospore.

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6. Economic Importance of Bacillariophyceae:

The uses of the diatomaceous earth are very varied, viz. as a filtering material, as a substance used in insulation of boilers, blast furnaces, etc., as a cementing material, as a mild abrasive in metal polishes and toothpastes, as an absorbent for liquid nitrogly­cerine (the explosive material of the dynamite), and as a material for painting the ship to protect it against the attack of sea barna­cles.