The diatoms of the freshwaters


The diatoms : microscopic algae

Diatoms are unicellular algae (many are around one twentieth of millimeter long), often in colonies. They possess the same cellular organelles as other eukaryotic photosynthetic algae and especially one or several plastids.

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.

The freshwater diatoms  

In the fresh waters of western Europa, either current (brooks, rivers) or still (swamps, peat-bogs, puddles, ponds, lakes), the pennate diatoms dominate. Only about thirty species of centric diatoms are present. Most of the species proliferate on the bottom (benthic species), either free or fixed to various mineral or organic supports. Planktonic species are present mostly in still and stagnant waters.

While in puddles and ponds, three to five tens of taxa can be found, in large lakes, until several hundredth of taxa can be present. In the Lake of Neuchatel, 408 taxa (111 planktonic taxa and 297benthic taxa) have been collected (Wuthrich, 1960).

While most of the species of fresh waters do not support the presence of salt (stenohaline species), some ones accept a moderate salinity (euryhaline species) allowing them to be also present in brackish waters

The gallery of pictures :

The species presented here have been collected mostly in Brittany and in the center of France, at altitudes between some meters and 1000 meters.

After removal of the protoplasm with sodium hypochlorite (10-12 %, 1 to 2 days) and cleaning of the siliceous wall (several washes with desionised water), the frustules/valves were permanently mounted in
Melmount© (ND = 1,704 ; Cargille labs, USA) and coloured micrographs were taken.

The work of Germain (1981) and additionally the works of Hustedt (1959) and Round, Crawford & Mann (1990) were used to determine the species.

Measures are indicated in micrometers (µm) ; one micrometer is equal to one thousandth of millimeter.
The taxa which are also present  in the french diatomites are indicated by (F).

Centric genera and araphid genera (1)
Raphid genera (3) (Craticula- Cymbella)

Raphid genera (4) (Diploneis - Mastogloia)

Raphid genera (5) (Navicula)

Raphid genera (6) (Navicula)

Raphid genera with canal raphe (2)

Some species are also presented on  Diatomées du port de Rouen (site "Diatomées marines").

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