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Life in Freshwater

Diatoms

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Diatoms highly magnified

Diatoms magnified 1000x

Diatoms are very small and most have to be magnified hundreds of times. The diatoms' walls are impregnated with silica and are like glass. This often gives the cell a jewel-like appearance. The diatom cell wall consists of two overlapping halves, like a petri dish. They are extremely diverse in shape and form; some are single cells whilst other form chains.

Diatoms are an easily overlooked microscopic component of all water environments, particularly the plankton. However, they can just as easily be found by scraping material off the surfaces of immersed stones and rocks to be viewed under a microscope. Found in any body of water, from fast flowing mountain streams to lowland lakes.

ECOLOGY: Diatoms are among the primary photosynthetic producers of foods in large bodies of water. Their short life span is matched by their ability to multiply very rapidly and through most seasons, although spring and autumn are the main periods. This is especially so for large lakes when wind and gales stir up the bottom sediments bringing silica into the water column encouraging diatoms to grow. See plankton blooms.

In asexual reproduction, cell division occurs, and each half of the original parent becomes the upper half of each daughter cell. A new lower half is formed inside each of the two halves of the parent cell walls. The daughter cell receiving the lid of the parent is like the parent in size, whereas the other daughter is slightly smaller. The continued diminution in size is halted eventually, when the small cells form auxospores that expand to the original size.

Most diatoms live singly; common genera are Pinnularia and Naviculara. Others are joined together by a sheath e.g. Asterionella , which is a star-shaped colony. Found in lakes. When diatoms die, they sink and form deposits of diatomaceous sediment. Ancient deposits of this are extracted and used in preparing such commercial products as detergents, fertilisers, insulating and soundproofing materials, polishes, and paint removers. One important export from Iceland is diatomite.

One of the easiest ways of studying diatoms is to immerse some microscope glass slides in a pond or stream. They may need to be secured so that they will not drift away. Within several days diatoms will have colonised the glass surface and can be placed under a microscope. The longer the slides are left the greater the variety. In fact it is possible to see a sequence of colonisation and succession.

 


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