Planktonweb

Diatomes

Miljø - Sjø

Miljø - ferskvann

Diatoms are common in both fresh and salt water. The most striking feature of diatoms is a wall round the cell, composed of silica. Ever since microscopes became generally available, scientists and others have been fascinated by the beauty and intricacy of these siliceous walls. The wall consists of two parts, which here are called the upper and the lower shells. The walls are resistance to dissolution and a significant number of fossil diatoms are known.

The diatoms play a dominant role in primary production of organic material in marine water and also in fresh water. Most obvious in size and numbers are the blowing of diatoms in coastal water in early spring. There are seasonal variations in temperature, illumination and availability of nutrients in both seawater and in lakes, which give rise to fluctuations in production and composition of the plankton.

The colour of the chloroplasts is more golden brown. The photosynthetic pigment includes chlorophyll a and other pigments are covering the green colour. The diatoms live singly or are attached to one another in various ways to form colonies. The cells have no flagella.

The reproduction is based on vegetative cell divide where each part of the two shells, departs into two new cells. The one shell, the lower one, is always a little bit smaller than the upper one, and the size of the cell is decreasing for each dividing.

The occurrence of diatoms is dependent of silicon concentration in the water. The identification of diatoms is based on cell shape and the detailed structure of the siliceous wall. There are two major groups of diatoms differing in cell symmetry. In centric diatoms the cell symmetry is radial (symmetric about a point) and in the oblong diatoms the basic cell symmetry is bilateral (symmetric along a line). The most marine diatoms are centric.

Some are making outgrowth of the wall in spines or long arms, specially the ones, which are making long chains of cells. Greater surface increase the ability of diatoms to stay afloat.