The upper images are of pennate diatoms, while the long one and four of the lower images are of more disc-shaped centric species. All diatoms are enclosed in a siliceous (glass!) shell – a frustule. This is perforated by various pores, and strengthened by ridges. Most diatoms have two shells (valves) that enclose the organism, and are joined by thin belts that go round between the valves.
The shells of most pennate diatoms are drawn out towards the tips, and a thin slit runs through the centre of the cell. This is best shown in the electron-micrograph (black and white) image of a single valve. This is a simple ‘naviculoid’ form of a pennate diatom. Several other images of naviculoid frustules demonstrate that the pattern of perforations and ridges vary from one type of diatom to another. Striatella, almost rectangular in profile, clearly reveals the supporting ridges of the frustule. These sculptings are used to identify diatoms. Several images show the contents of living cells. They include the plastids that can be large and almost form a layer close to the frustule (but within the cell). They are a golden or off-green color because they contain chlorophylls a and c – and so differ from the brighter green of land plants. Also visible are the nucleus, oil droplets (a result of photosynthesis) and in one cell sausage-like mitochondria.
A few images illustrate the different forms that pennate diatoms adopt. One, Bacillaria, is colonial – in which many cells join face to face. They can still move, and their sliding causes the colony to extend and retract. Tabellaria looks like a series of boxes that join by their corners to make filaments. Two, Asterionella and Thalassionema, form star-shaped colonies in which the pennate diatoms join at one end. Two other pennates have one end embedded in a ‘stalk’ of mucus, and with this they are attached to immersed surfaces – including the surfaces of other algae (in which case they are referred to as epiphytes -o n the outside (epi) of plants (phytes)).
Five images are of centric diatoms. Bottom right is of a valve of the Talassiosira frustule. Perforations form patterns, they radiate from the centre, but unlike the poennate diatoms, there is no polarity, nor a raphe. One side view shows how cells are joined end to end to make a filament, and the plastids within each cell have an uirregular and lobed shape. Achnanthes and Pauliella are centric diatoms in which many cells are joined face to face to make long filaments. Bacteriastrum, the final centric diatom, has delicate setae that radiate from the frustule. Setae occur in many planktonic diatoms. It is believed the setae behave a little like the feathery seeds of some plants, to make them more buoyant. For diatoms, this means that they won’t fall to the deep ocean or lake floor, and away from the light that they need to gorw and thrive.