Occurrence of Diatoms:
Diatoms are found in all possible habitats. Commonly they
are found in fresh water (Denticula tenuis, Navicula pupula, Meridion
circulare, Cymbella ventricosa, Melosira variens, Amorpha ovalis etc.), sea
water (Corethron, Biddulphia, Sceletonema, Fragilaria, Tropido- nensis etc.)
and soil (Pinnularia, Navicula, Frustulia etc.).
The terrestrial species (Amorpha, Navicula, Pinnularia
etc.) are able to withstand desiccation for a long period. Some diatoms
(Gomphonima, Cymbella etc.) can grow as epiphyte on other algae (Enteromorpha,
Cladophora etc.) and higher plant. Licmophora, a member of diatom, grows
endozoically.
Plant Body of Diatoms:
Plant body is unicellular, generally moves singly. The
cells are of different shapes viz. round, oval, elongated, rod-shaped,
triangular, disc-shaped etc. Sometimes they become aggregated and get embedded
in a gelatinous matrix, but they do not behave like multicellular organisms.
In colonial form the cells may be present as uniseriate
row (e.g., Melosira), like a branched body (e.g., Licmophora flabellate) or
other forms also.
Cell Structure of Diatoms:
The cell consists of cell wall and protoplast (Fig. 3.101
A, B, C). The cells are covered by a siliceous wall, the frustule. It consists
of two overlapping halves, the theca. The upper one is epitheca and lower one
is hypotheca.
Both the theca consist of two
portions:
(a) Valve — the upper flattened top and (b) Connecting
band or cingulum (pl. cingula) — the incurved region.
The common region of the connecting bands, where both
the theca remain fitted together, is the girdle. [When the diatoms are observed
from the valve side i.e., valve side is uppermost, called the valve view, but
when viewed from the connecting band, it is the girdle view]. Depending on
symmetry, the cells are divided into two orders: Pennales (bilaterally
symmetry) and Centrales (radially symmetry).
In some pinnate diatoms (Cybella cistula, Pinnularia
viridis etc.) an elongated slit is present on their valves, called raphe. The
raphe is interrupted at its midpoint by thickening of the wall called central
nodule. Similar thickening is also present at the ends called polar nodules.
Some members like Tabellaria fenestrate etc. of the order Pennales, do not have
raphe, called pseudoraphe.
Besides raphe or pseudoraphe, the cell walls have other
types of openings, called pores and locules.
The cell wall is mainly
made up of pectic substances, impregnated with silica. The content of silica
varies from 1% (Phaeodactylum tricornutum) to about 50% on the basis of dry
weight of the cell.
Protoplast:
The entire content
present inside the cell wall is the protoplast. The cell membrane encloses a
large central vacuole surrounded by cytoplasm. The cytoplasm contains single
nucleus, mitochondria, golgi bodies and chloroplasts. The chloroplasts may be
of different shapes like stellate, H-shaped, discoid etc. In some species the
chloroplasts contain pyrenoids.
The photosynthetic
pigments are chlorophyll a, c1 and c2, β-carotene, fucoxanthin, diatoxanthin and
diadinoxanthin. The latter two are present in small quantity. (The
golden-brown colour of diatom cells is due to the presence of xanthophylls like
fucoxanthin, diatoxanthin and diadinoxanthin.
The term diatomin is
used for the mixture of chlorophyll and carotenoids, particularly carotene and
several brown xanthophylls pigments.) The reserve food of diatoms is
chrysolami- narin and oil droplets (they do not store in the form of starch).
Locomotion:
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.
Reproduction of Diatoms:
Diatom reproduces by
vegetative and sexual means.
1. Vegetative Reproduction:
Vegetative
reproduction performs with the help of cell division (Fig. 3.102). It takes
place usually at midnight or in the early morning.
During cell division
the protoplast of the cell enlarges slightly, thus the cell increases in volume
and slightly separates both the theca (epitheca and hypotheca). Then the
protoplast undergoes mitotic division and gets separated along the longitudinal
axis through the median line.
Thus one half of
protoplast remains in epitheca and the other one in hypotheca. One side of the
protoplast thus remains naked. Now both the theca i.e., epitheca and hypotheca
of mother cell behave as epitheca of the daughter cells.
Thus new silicious
valves are deposited towards the naked sides of the protoplast and always
behave as hypotheca of the daughter cells. Connecting bands are developed
between the theca. Later on, the daughter cells get separated.
During cell division,
both the theca i.e., epitheca and hypotheca of the mother cell behave as
epitheca of the daughter cells. So at the side where the hypotheca behaves as
epitheca, the cell becomes reduced in size. Thus with continuous cell division
some cells gradually become reduced in size.
Economic Importance of Diatoms:
The diatoms are used in various purposes either directly
or indirectly.
The different uses of diatoms
are:
1. Diatomite:
After the death of diatom cells the outer coverings i.e.,
the silicified walls become accumulated at the bottom of water. The
accumulation may be thicker during favourable conditions. These deposits are
called diatomaceous earth, diatomite or keiselghur.
It is very suitable for use in
different industries:
a. As Filter
It is used as filter in different industries like sugar
(to filter microorganism), oil and chemical industry. Diatomite is also used
as filter for battery boxes.
b. As Insulator:
It is used as insulator in boilers and blast furnaces for
its heat- resistant ability.
c. As Absorbent:
It is used as absorbent of liquid nitroglycerine.
d. Other Uses:
Diatomite is used as abrasive (i.e., capable of rubbing
or grinding down) substance for the manufacture of metal paints, polish,
varnish, toothpaste etc. It is also used with bake-lite for electrical fuse
and switch boxes.
2. Petroleum:
Much of the petroleum is considered to be of diatom
origin as they are found in association with large oil deposits.
3. Food:
Due to their great abundance in the different seas and
their use as food by marine animals, they are called the ‘grasses of the sea’.
Those animals may be consumed as food by man and maintain the food chain.
4. Testing of Microscopic Lenses:
Due to the fine markings on shell, the diatom cells are
used to test microscopic lenses.
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