Diatoms (Bacillariophyceae) are microscopic unicellular algae that inhabit virtually all aquatic environments. They occur in the body of the water as plankton or on the bottom where they may be attached to plants or rocks or sand particles, or may be free living and able to move between particles of the substratum. Individually, diatoms are invisible to the naked eye, but large concentrations can often be seen as brown or golden-brown discolouration on, for example, the surface of rocks, sand or mud. The structure of a diatom is similar to that of a pillbox or a petri dish. The shell is made of silica and consists of two interlocking halves, one larger than the other (called "epitheca" and "hypotheca", respectively). The thecae each consist of a structure called the valve (the main element) and several connected bands attached to it (the cingulum; the two cingula between them are referred to as the girdle). Collectively, the valves and girdle are referred to as the frustule (Fig. 1). Diatom species vary in size over several orders of magnitude, but most fall within the range of 10 to 100 µm length. The global diversity of diatoms is considerable: although the number of published species currently only stands at somewhere between 10,000 and 20,000, there are estimates that the real figure is in the order of 200,000 or more. This difference can mostly be accounted for by differences in interpretation of existing species, but we can also anticipate the discovery of many completely new diatoms.