Tiny miracles - diatoms

Diatoms are a fascinating subject for microscopy. In the 19th century, it was the "salon slides" that made diatoms known to the wider public. With these elaborate preparations, people simply enjoyed the beautiful shapes and admired the skilful type plates, e.g. by J. D. Möller from Wedel, Germany. Today, diatoms are studied because of their ecological relevance. It is now known that it is not the rainforests but the phytoplankton in the oceans that produces a large part of the oxygen in the air we breathe. Diatoms actually account for the largest share of all oxygen production. However, they are not only found in plankton, but also in the “Aufwuchs” growth on stones or plants in rivers, lakes or in the sea. Diatoms therefore also form an important base in the food pyramid and serve as a food source for higher organisms like insect larvae, small crustaceans or fish. The brownish coatings of diatoms on glass panes and plants are also known to aquarists and are usually a nuisance that should be removed. But what is really hidden in these unsightly coatings?

Diatoms are algae that live in a shell made of silicon dioxide (silicate, silicic acid). These shells are built up by biomineralisation and are usually beautifully structured and ornamented. There are centric shapes (round, triangular or polygonal) or so-called pennate diatoms, i.e. elongated shapes reminiscent of a boat or artfully s-shaped curves or arches. Many forms have a fine surface structure of ribs or pores in common. In the history of microscope manufacturing, the extremely fine structures were therefore also sought after as test objects for the quality and resolving power of the optics, which led to the production of so-called "diatom test plates". These test plates can be used to test the quality of a microscope.

The shells or so-called frustules made of silicate are constructed like a Camembert cheese box with a bottom and a slip lid. Some diatoms sit firmly on surfaces, but many are also mobile in that parts of the cell plasma are stretched outwards through the shell pores. In this way, diatoms can glide on surfaces quickly colonise new ground. The habitats of diatoms are therefore also diverse. They live in marine plankton, in rivers, on seashores or even in the arctic sea ice. Diatoms can even be found in the deep sea or on leaves in the rainforest, in moist forest moss, etc. They only need water, nutrients and light. In the dark sediment, some species can even survive without photosynthesis by feeding on nitrate, for example.

The durable shells of silicate formed thick deposits after the diatoms died off in the course of the earth's history. These deposits are called diatomaceous earth. Today, it is used as an abrasive and polishing agent, as a filter medium for drinks or broth, as a binding agent or also as a mild insecticide powder.