The diatoms : microscopic algae
are unicellular algae (many are around one twentieth of millimeter
long), often in colonies. They possess chloroplasts and the same cellular organelles as other eukaryotic photosynthetic algae.
The hallmark of a diatom is its cell wall named frustule. This wall, heavily
impregnated with silica, is highly differentiated. It consists always
of two large intrically sculptured units called valves, together with
several linking structures termed girdle elements. Each species is
characterized by a particular shape and decoration of the valves.
According to the shape of the valves, the arrangement of the valve
striae and the type of sexual reproduction, the diatoms are divided in centric diatoms and pennate diatoms. The two valves or only one of most of the pennate diatoms have a longitudinal slit, the raphe, interrupted in the middle by a bridge of silica. This raphe is either simple or as a complex structure, the raphe-canal, the interior of which is divided by bridges of silica named the fibulae.
After removal of the protoplasm and cleaning of the siliceous wall of the collected diatoms,
coloured micrographs were taken. They allow to discover the beauty and
the diversity of the diatoms :