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In this article we will discuss about:- 1. Features of Bacillariophyta 2. Classification of Bacillariophyta 3. Relationship 4. Phylogeny.
Features of Bacillariophyta:
The Bacillariophyta, commonly known as the diatoms, are ubiquitous algae of both fresh- and salt-water and of damp places including aerial habitats as, old walls, rocky cliffs, bark of trees, and damp soils. Some are submerged in water growing attached to various substrata like aquatic algae and submerged objects.
They play a very important role, particularly in the aquatic vegetation of the world forming an important part of the plankton, the basic food of aquatic fauna. They occur in countless individuals in even small quantities of water. Diatoms have been very appropriately named the ‘jewels of the plant world’, for their diverse forms and very delicately designed wall markings being the objects of great beauty (Fig. 98).
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These are unicellular, sometimes colonial microscopic algae. They are of innumerable shapes, some being boat-like in outline, while others are rods, disks, triangles, and wedges. A special branch of phycology known as diatomology, the science of diatoms, has developed dealing with the various aspects of the huge number of diatom species and their unique structure.
The diatom cell, known as frustule, is composed of two overlapping halves, or valves very nicely fitted one upon the other like an old fashioned pill-box, or a pair of petri dish. Of the two halves, the outer one is the epitheca and the inner the hypothec (Fig. 99B). The edges of the two halves are more or less incurved, together forming the girdle.
When one or the other valve is seen, it is known as valve-view, and in girdle-view the girdle is visible. The diatom cell wall is composed of pectic substances which are impregnated with silica.
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The siliceous material instead of being deposited as a smooth layer, forms different patterns which vary according to the genus and species. But usually the ornamentation follows two general patterns, for example, in the centric diatoms (Centrales) radially symmetrical about a central point, or drum-like, and in the pennate diatoms (Pennales) bilaterally symmetrical with respect to the long axis of the cell.
On the back of each valve of most elongated types there are linear perforations forming longitudinal slit, called the raphe (Fig. 99A & C). But when without longitudinal slit is the pseudoraphe. There are also structural modifications of the cell wall forming thickenings which are known as nodules, appearing at both ends of raphe—polar nodules, and interrupting it near the mid-point—the central nodule (Fig. 99A).
The markings of the wall, raphe and nodules constitute taxonomic criteria. The protoplast of a diatom cell forms a lining layer closely adpressed to the cell wall and generally surrounding a large central vacuole.
In species of elongated cells, the vacuole is frequently bridges in the middle by a band of cytoplasm in which is embedded the large nucleus (Fig. 99B & D). Depending on species, the cell may contain one to many discoid or irregularly shaped plastids which are yellow to brown in colour.
Pigments of a diatom cell are: chlorophyll a, chlorophyll c, fucoxanthin and a special pigment—diatomin casting a golden- brown hue to the diatom cell. Starch is never formed in diatoms; on the contrary, the reserve foods are fats and volutin. Diatom cells of some species exhibit movement which is slow and jerky and confined to forward and backward directions.
Cause of movement has been a topic of considerable interest. The movement may be due to the flow of currents of water or by the circulation of cytoplasm.
After the death of diatom cells, their siliceous walls, or empty frustules drift to the bottom of the body of water in which they occur and tinder favourable conditions there accumulates a large deposit of diatom walls, known as diatomaceous earth. During tertiary and quaternary ages the formation of diatomaceous earth was so great that the great deposition was formed in ocean.
Such deposition is known as diatomite or kisegurh. Diatomite because of its porosity, is light in weight and can stand high temperature. Its melting point is 175°C. Diatomite is used in various industries.
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Some of them are: used in polishes for silverware, metals, etc.; preparation of tooth powders; insulation of steam pipes and blast furnaces; to filter oils; as an absorbent for the liquid nitro-glycerine in dynamite for safety in transportation; used in sugar- refineries; in brewing industry; and in manufacturing antibiotics to remove waste mycelium.
At first Alfred Nobel used diatomite as an absorber of highly explosive nitro-glycerine when he manufactured dynamite.
Statospore Formation in the Centrales:
Many of the Centrales have been found to produce in their frustules, thick-walled resting spores known as statospores. The statospores are formed during unfavourable seasonal condition. The details of their development are not known. The statospores are very frequent in marine species, but are also found in fresh-water ones.
Classification of Bacillariophyta:
The Bacillariophyta include the single class Bacillariophyceae which has been subdivided into two orders:
I. Order Centrales:
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Valves circular, polygonal or irregular in outline and with an ornamentation on the wall; ornamentation radial or concentric about a central point; valves never have a raphe or pseudoraphe; protoplast with many chromatophores; statospores or microspores may be formed; auxospore formation mainly by oogamy.
II. Order Pennales:
Valves bilaterally symmetrical or asymmetrical in surface view; wall ornamentation bilateral with respect to a line; valve always with a raphe or pseudoraphe; protoplast with one or two chromatophores; statospores or microspores are never formed; auxospore formation mainly by isogamy.
Some Indian species:
Cymbella affinis Kutz. Eunotiapectinalis var. gibbulosus Venkat. Eunotia pectinalis var. neglecta Gandhi; Navicula cryptocephaloides Hust.; Neidium gracile Hustedt.; Pinnularia graciloides Hustedt.
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Special features:
1. Unicellular pill-box type of plant body composed of two valves.
2. Silica impregnated cell wall with beautiful markings.
3. Cell wall deposition produces diatomaceous earth which has great economic importance.
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4. Specialized vegetative cell division.
5. Auxospore formation.
Relationships of the Bacillariophyta:
The cell structure suggests that the diatoms are associated with the Conjugales particularly with the desmids.
The presence of fucoxanthin and chlorophyll a and chlorophyll c suggests closer association with the Phaeophyta.
Pascher suggested a relationship with the Xanthophyta and Chrysophyta and classified diatoms under Chryosophyta for having golden-brown pigment and similar assimilation product.
Since diatoms are largely associated with the Peridineae forming plankton flora of the ocean, some relationship has been drawn by the phycologists between the Bacillariophyta and the Peridineae.
Phylogeny of the Bacillariophyta:
Because of the siliceous skeleton an extensive fossil record of diatoms is known and a discussion of phylogeny based on the fossil evidence is possible with diatoms to a much larger extent than with other algae. Centric forms appear to be the most primitive appearing first in the Jurassic, whereas the pennate forms do not make their first appearance until the Tertiary period.
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The evidence also suggests that the pennate forms were derived from the centric forms.
