The following points highlight the two types of Chrysophytes. The types are: 1. Diatoms 2. Desmids.
Chrysophytes: Type # 1. Diatoms:
i. Habitat and Habits:
(i) Diatoms occur in all aquatic and moist terrestrial habitats.
(ii) They may be free floating or bottom dwellers. The free floating forms remain suspended on the surface of water by mucilage secretion and presence of light weight lipids.
(iii) Diatoms may show gliding type of movement with the help of mucilage.
(iv) The siliceous frustules of diatoms do not decay easily. They pile up at the bottom of water reservoirs and form big heaps called diatomite or diatomaceous earth. It may extend for several hundred metres in certain areas from where the same can be mined.
ii. Structure:
(i) The body is covered by a transparent siliceous shell (silica deposited in cell wall) known as frustule. The frustule is made of two valves, epitheca and hypotheca. The two valves fit together like a soap box. The frustule possesses very fine markings, pits, pores and ridges.
(ii) Diatoms are microscopic, variously coloured and of diverse forms protists which do not possess flagella except in the reproductive state.
(iii) They are basically unicellular but can form pseudo-filaments and colonies.
(iv) Depending upon the symmetry, the diatoms are of two types namely, pennate and centric. The pennate diatoms show bilateral symmetry e.g. Navicula and the centric diatoms have radial symmetry, e.g., Melosira.
(v) Each cell has a large central vacuole. The single large nucleus is commonly suspended in the central vacuole by means of cytoplasmic strands.
(vi) Chloroplasts or chromatophores are yellowish brown to greenish brown. They contain chlorophyll a and chlorophyll c.
(vii) Diatoms contain fucoxanthin (typical of brown algae) that provides brownish tinge.
Reserve food:
The food is reserved in the form of oils and leucosin (polysaccharide). Volutin globules (proteinaceous in nature) are also present.
iii. Reproduction:
(i) The common mode of multiplication is by binary fission.
(ii) Resting spores or statospores are formed in some cases.
(iii) Meiosis is gametic. Sexual reproduction varies from isogamy to zoogamy. In the latter case, male gametes are motile and uniflagellate. Fertilization produces a zygote which grows in size and forms a rejuvenascent cell called auxospore in that their vegetative cells are typically diploid.
Examples:
Triceratium, Pleurosigma, Navicula, Cymbella, Amphipleura.
iv. Economic Importance:
(i) Diatoms are very important photosynthesizes. About half of all the organic matter synthesized in the world is believed to be produced by them. Though microscopic, diatoms are an important source of food to aquatic animals. A 60 tonne blue whale may have 2 tonne of plankton in the gut which is mostly diatoms.
(ii) The oils extracted from some fishes and whales are actually the ones produced by diatoms.
(iii) Diatomite deposits are often accompanied by petroleum fields. Much of the petroleum of today is probably due to decayed bodies of the past diatoms.
(iv) Diatomite is porous and chemically inert. It is, therefore, used in filtration of sugar, alcohols oil, syrups and antibiotics.
(v) Diatomite is employed as a cleaning agent in tooth pastes and metal polishes.
(vi) Diatomite is added to paints for enhancing night visibility.
(vii) Diatomite is employed as insulation material in refrigerators, boilers and furnaces.
(viii) Diatomaceous earth is added to make sound proof rooms.
(ix) Diatomite is a good industrial catalyst.
(x) Diatomite is a source of water glass or sodium silicate,
(xi) Diatoms are very good pollution indicators.
Chrysophytes: Type # 2. Desmids:
Desmids are unicellular green algae. Like Spirogyra, they have an elaborate chloroplast. Their cells have two distinct halves. The outer wall of the cell has various protuberances covered with mucilaginous sheath which is thought to play a role in the cell’s slow gliding movement.
Sexual reproduction occurs by ‘conjugation’ similar to that of Spirogyra. They are mainly found in fresh water and are usually indication of clean (unpolluted) water.