Bacillariophyta: Class, Order and Family!
Class: Bacillariophyceae
1. This class is represented by approximately 200 genera and 600 species.
2. Members are commonly known as diatoms and are commonly found in fresh water, water, in air or on soil.
3. Thallus is unicellular, uninucleate diploid and show radial or bilateral symmetry.
4. Cell wall is silicified. It shows characteristic secondary structures. It is often called the she or frustule.
5. Frustule is made up of two overlapping halves.
The upper larger half is called as the epitheca, and the lower smaller overlapped half is called hypotheca.
6. Cells generally possess many discoid or two plate like chromatophores.
7. Members of this class are also called as golden brown algae because of their characteristic pigments which include carotenoids, fucoxanthin, diatomin (diatoxanthin, diadinoxanthin), beside chlorophyll a and chlorophyll C.
8. The stored food products are in the form of oil, volutin chrysolaminarin.
9. Cell shows gliding movement.
10. Reproduction occurs by cell division and auxospore formation.
11. Motile stages possess a single, anterior pantonematic flagellum.
Classification of Bacillariophyta:
Class Bacillariophyceae has been divided into two orders. Pennales and Centrales.
Order: Centrales:
1. Thallus radially symmetrical.
2. Gliding movement absent.
3. Sexual reproduction anisogamous or oogamous.
4. Gametes are motile.
Order: Pennales:
1. Members are bilaterally symmetrical.
2. Cells show gliding movement.
3. Sexual reproduction is amoeboid.
Order Pennales is further divided into:
Family: Naviculoideae:
(i) Members are fresh water in habitat.
(ii) Valve view is boat shaped.
(iii) Raphe is present in both the valves.
Navicula:
Systematic Position:
Class: Bacillariophyceae
Order: Pennales
Family: Naviculoideae
Genus: Navicula
Occurrence:
Navicula (Latin for smallness), commonly known as pinnate diatom, is a fresh water genera. A few species may be terrestrial. The most common Indian species is N. halophila.
Thallus Structure:
Thallus is represented by a iso-bilaterally symmetrical diploid unicells.
Structurally, it can be differentiated into two parts:
A siliceous cell wall (called frustule) and the protoplast. The cell wall is made of pectic substances which are impregnated with silica (SiO2). Cell wall consists of two overlapping halves called epitheca and hypotheca. Epitheca remains fitted over the hypotheca as a lid over the box.
Each theca is further divided into two parts:
The main surface called valve and the incurved margin known as connecting band or cingulum. The two connecting bands of the two thecas are fitted together. The connecting band of the epitheca overlaps that of the hypotheca and the two bands remain united in the overlapping region (called girdle) by a connecting cement present between the (Fig. 1).
A frustule can be seen in two views:
Valve view and girdle view. In top view or valve view it appears as boat shaped (Fig. 2A) and in girdle view or side view more or less rectangular (Fig. 2B). The valve view shows marking or striations which spread out laterally in two parallel series, one on either side of the axial strip. The axial strip pears a longitudinal cleft known as raphe (Fig. 2B).
The raphe extends from one end of the valve to the other. It also bears three enlargements or rounded nodules, one central nodule and two polar nodules. Due to presence of raphe Navicula shows gliding movement. This movement is caused “by streaming cytoplasm by circulation within the raphe, and by the extrusion of the mucilage.”
Just inner to the cell wall is present a plasma membrane which encloses the cell protoplast It is differentiated into a single nucleus and cytoplasm. The cytoplasm forms a thick layer just below the cell wall and encloses a large central vacuole.
The cytoplasm includes mitochondria, Golgi bodies, and two large parietal brownish yellow chromatophores. Pyrenoids are absent. The photosynthetic pigments are chlorophyll-a, Chlorophyll-c, p-carotene, fucoxanths, diatoxanthin and diadinoxanthin. Reserve food material is in the form of chrysolaminarin and oil droplets.
Reproduction:
Navicula reproduces by two methods:
Vegetative and sexually.
Vegetative Reproduction:
It takes place by the mitotic cell division or fission. Successive cell division takes place very rapidly at night. Presence of aluminium-silicate in water is essential for cell division to occur. As the cell division starts, the cell protoplast increases in diameter. The cell also increases in size.
The diploid nucleus divides mitotically and produces two daughter nuclei (Fig. 3A-C). Two chromatophores divide. The single chromatophore spits longitudinary in such a manner that one chromatophore comes to lie in each half. Now the protoplasm cleaves into two uninucleate portions by division in longitudinal plane parallel to valve surface (Fig. 3C).
One daughter protoplast now lies in epitheca and the other in hypotheca.
Now both the daughter protoplasts with one daughter nucleus secrete the new siliceous wall on the two fresh protoplasmic surfaces exposed along the cleavage plane. The new valves developed always become hypotheca while the older theca (which may be epitheca or hypotheca of the parent cell) becomes the epitheca of the new or daughter cell.
When this cell again divides, it produces a daughter ceil which is again smaller than the present parent. Thus, in a population of diatom cells during successive divisions there is normally a progressive decrease in the average cell size (Fig. 4). It is called Macdonald-Pfitzer law. The smaller cells of later series of division loose their vitality and capacity of division.
Sexual Reproduction:
It takes place by the formation of auxospores. The successive decrease of cell size in vegetative reproduction is prevented by the auxospore formation. The auxospore formation is actually a ‘restorative process’ because the reduction in the original size of the cells, during the cell division is restored. During the process only those cells which have diminished sufficiently in size can act as ‘sex cells’ or conjugating cells.
Those cells which do not decrease in size by cell division apparently do not show sexual reproduction. Majority of the species of Navicula are monoecious but, N. haplophila is dioecious. Two sex cells come together, pair up longitudinally (called gamontogamy) and secrete a common mucilaginous envelope (Fig. 5A, B).
The diploid nucleous of each cell undergoes meiosis to form 4 haploid nuclei. Out of these two nuclei degenerate and only two remain functional (Fig. 5C). The protoplasm of each cell now cleaves into two portions each obtaining one haploid nucleus. The functional nuclei ultimately metamorphose into gametes (Fig. 5B-D).
The parent cell fuses (cytogamy) and the fusion of gametes occurs in a copulatory jelly. In N. haplophila the two gametes formed in one cell (conjugant) are amoeboid and the two gametes formed in the other are passive or immobile. The amoeboid gametes emerge through the open valves of the parent frustule and dip into open shell of the other to fuse with opposite gametes to form two zygotes (Fig. 5 E. F) in one shell.
The other is empty. Thus, N. haplopila shows physiological anisogamy. Two diploid fusion cells or zygotes escape from the enclosing pustules. They remain doranant for some time. Later the zygote elongates (more in the longitudinal plane) and functions as auxospore (Fig. 5G-I), which develops a silicified membrane called perizonium around its protoplasm.
It may be secreted by the auxospore or by the remains of the zygotic membrane. The auxospore secretes new pustules around itself around the perizonium. The reconstituted new cell is of normal size and after sometime begins to divide vegetatively to form new generations. The valves of the old pustules are often seen attached to the newly formed pustules (Fig. 5G).
