Diatom specimens

Diatom captions

These are all scanning electron microscope (SEM) images except for the Stauroneis specimen, which is taken from a light microscope.

Species of Actinoptychus

This circular, marine diatom has an interesting surface topography, with three raised and three lowered sectors. Other species in the genus can have larger numbers of pairs of sectors, four, five, six, or eight. We are interested in what controls the numbers of sectors.

Species of Odontella

This marine diatom can be found in the inshore plankton or can be associated with seaweeds. This cell has fallen open during preparation leaving one valve and a large girdle band (at bottom). Cells attach to each other by secreting a sticky substance, mucilage, through the tips.

Species of Clipeoparvus

This small diatom has recently been described as a new species, belonging to a new genus. It occurs in Turkish lakes and the sediment record shows that it has been present for over 1700 years. The type material (reference specimen) is in the Museum collection.

Species of Gomphonema

Here are two cells of a freshwater diatom that have an interesting shape, with one end fatter than the other. There are also special pores at the narrower end through which the diatom cell secretes a sticky substance called mucilage to attach itself to rocks or other solid surfaces.

Species of Hydrosera

This triangular-shaped diatom can form long filaments. Individual cells link together by secreting a sticky substance called mucilage at their corners. This specimen was collected from the River Thames where it grows on shady sea walls, looking like strands of orange-brown hair.

Species of Stauroneis

This light microscope image shows a species of Stauroneis. Its name comes from the Greek word ‘stauros’ meaning cross. That’s because the clear area across the middle of its wall forms a cross-like pattern.

Species of Psammodictyon

This picture shows a cell that has almost fallen apart during preparation so that it is possible to see what the outside and the inside of the half wall looks like. It is a marine diatom that occurs on soft sediments around coastlines.

Species of Parlibellus

This is a marine coastal diatom that can grow and move inside tubes made of a sticky substance, mucilage, which it produces itself. The picture shows that it has numerous girdle bands linking the two valves and emphasises the box-like structure of diatom cells.

Species of Pseudostaurosira

Cells of this small freshwater diatom are linked by spines that develop at the edges of adjacent valves to form filaments. The spines develop simultaneously on each valve, and as they become broader toward their tips they ‘zip’ the two valves together.

What are diatoms?

Diatoms are a major group of single-celled algae. They are found throughout the world from the poles to the tropics, in fresh waters and the oceans, and live by photosynthesis.

Diatoms are tiny. Most are microscopic, and the largest only reaches 2mm in length. But they are incredibly numerous – scientists estimate there may be more than 100,000 species.

Why are diatoms important?

Due to their enormous numbers, diatoms produce more oxygen each year through photosynthesis than the rainforests. They are also extremely important for many animals as they form the basis of aquatic food chains – everything else in seas, lakes, rivers and streams either eats them directly, or eats something that eats diatoms.

Diatoms have precise ecological requirements, so they can also be used as environmental indicators, telling us what is happening in the environment. The cell walls of diatoms may be preserved for long periods of time in sediments and provide a record of past changes in lake systems.

Diatom features

Diatoms have cell walls made of silica. Each species has a distinct pattern of tiny holes in the cell wall (frustule) through which they absorb nutrients and get rid of waste. Zoom into the slides above to see these.

Viewed under the microscope, diatoms show a huge variety of shapes with many interesting and beautiful patterns. Their shapes and structure are usually highly regular and symmetrical, and these features are used to identify and classify them.