Diatoms
Diatoms are golden brown photosynthetic protists and are called Chrysophytes (including both diatoms and desmids). They are both and terrestrial. Some are marine. They support much of marine life. Their important characters are
1.These are microscopic organisms possessing varying colours.
- They are basically unicellular , but may form pseudofilament and colonies, lacking flagella except in the reproductive stage. They may be free floating (phytoplanktonic ) , remaining afloat on surface of water due to presence of light weight lipids.
- The cellulosic cell wall is impregnated with silica to form transparent siliceous shell, known as frustules. Depending upon the symmetry , diatoms may be pinnate type , having bilateral symmetry (e.g., Navicula) and centric type, having radial symmetry (e.g., Melosira).
- the cell wall is characteristic, made up of two halves, one half covering the other (epitheca over hypotheca ) resembling a soap box.
- The cell wall encloses the peripheral layer of cytoplasm (primordial utricle ) surrounding a large central vacuole.
- Nucleus lies in the central vacuole, suspended with the help of cytoplasmic strands.
- Mode of nutrition is holophytic (phtoautotrophic) , photosynthetic pigments are chlorophyll a , chlorophyll c, β-carotene and special carotenoids containing fucoxanthin; xanthophylls like diatoxanthin, diadinoxanthin,
- the reserve food is oil and a polysaccharide called leucosin (chrysolaminarin) , vlutin granules are also present.
- They are responsible for almost 50% of the total organic matter synthesized in the biosphere.
- movement occurs by mucilage propulsion.
- they mainly undergo asecxaul reproduction. The common mode of asexual reproduction is binary fission.
- During binary fission, one half of the cell wall is retained by each of the daughter cells formed. The other half of the cell wall is secreted afresh.
13. Resting spores are called statospores (centric diatoms) - They reproduce sexually as well. Sexual reproduction varies from isogamy to oogamy. It involves gametic meiosis as diatoms are generally diploid (diplontic life cycle).
Did You Know?
1.Silica shells of dead diatoms are nearly indestructible and thus, get accumulated at the sea bed. Such huge rock- like deposits of hard shells of diatoms constitute diatomaceous earth, which is mined to obtain a whitish powder called diatomite or kieselguhr or diatomaceous earth. Diatomite is rough and gritty. Because of these features, it is used in filters in brewing industry, sugarcane refineries, in polishes for metals, tooth pastes, for making insulating bricks, in insulation of refrigerators, houses and for making the latter sound proof , in the manufacture of dynamite, water glass or sodium silicate and strong acids. This is added to paint to increase their night visibility.
2.They are very good indicators of water pollution.
Common examples of diatoms are Triceratium , Melosira, Navicula, Cymbella.
