What are diatoms?

Diatoms are algae with distinctive, transparent cell walls made of hydrated silica.


Morphology.  Diatoms are algae with distinctive, transparent cell walls made of silicon dioxide hydrated with a small amount of water (Si02 + H20). Silica is the main component of glass and hydrated silica is very like the mineral opal, making these algae, often called "algae in glass houses" more like "algae in opal houses". The cell wall is called a frustule and consists of two halves called valves. Since silica is impervious, diatoms have evolved elaborate patterns of perforations in their valves to allow nutrient and waste exchange with the environment. These valve patterns can be quite beautiful and are also helpful for classifying diatoms. Diatoms grow as single cells or form filaments and simple colonies.

Ecology. Diatoms are abundant in nearly every habitat where water is found – oceans, lakes, streams, mosses, soils, even the bark of trees. These algae form part of the base of aquatic food webs in marine and freshwater habitats. Assemblages of diatom species are often specific to particular habitats and can be used to characterize those habitats.

Classification. As algae, diatoms are protists. Diatoms have been placed in the division Bacillariophyta, which is distinguished by the presence of an inorganic cell wall composed of hydrated silica. There are an estimated 20,000 to 2 million species of diatom on Earth. This range is so large because scientists are still working to understand basic aspects about "what is a diatom species" and because new and diverse forms are still being discovered and described in scientific publications.

Physiology. Diatoms use the pigments chlorophyll a and c to collect energy from the sun through photosynthesis. They also contain the accessory pigments fucoxanthin and β (beta) carotene, which give them a characteristic golden color. Diatom cells store energy from photosynthesis in the form of chrysolaminarin (a carbohydrate) and lipids (fats in the form of oils). The high production of lipids in many diatom species has created great interest in diatoms as a source of biofuels. Indeed, as one of the important global sources of carbon fixation, diatoms already are an important biofuel for aquatic food webs. It is estimated that the photosynthetic activity of diatoms produces between 20 and 40% of the earth’s oxygen (02).

Diatom Forms

Diatom frustules are characteristically highly ornamented, forming an amazing range of forms. The shape of the diatom frustule is species specific. In other words, the evolutionary relationships of diatoms and their names (diatom taxonomy) has been based on the silica frustule, at least until recently (although there are exceptions). Two major groups are recognized within the diatoms: 1) Coscinodiscophyceae, or centric diatoms, cells with radial symmetry (about a point) and 2) Bacillariophyceae, or pennate diatoms, cells with bilateral symmetry (about a line). The centric diatoms are not able to move, but some pennate diatoms may move across surfaces or up and down within sediments. Cells are able to move by a structure termed the raphe.

The navigation of this site is organized around shape categories (morphological groups), into which we have grouped all of the genera.

How big are diatoms?

Nearly all diatoms are microscopic - cells range in size from about 2 microns to about 500 microns (0.5 mm), or about the width of a human hair (note that one micron is equal to 10-6 meter). Scientists use light microscopes (LM) or scanning electron microscopes (SEM) to view diatom structures. When diatoms are viewed with a light microscope, the frustules appear clear (we are seeing through glass). When diatoms are viewed with a scanning electron microscope, the frustules appear opaque.

Life History

For most of their life history, diatom cells divide by vegetative division, also called vegetative reproduction. That is, a single cell divides and forms two new cells. But, the new valve (cell) walls are formed inside the parent cell and are smaller than the parent cells. That is, because new cells must be formed within the parent and the rigid, inorganic cell walls of silica cannot expand, so the daughter cells are constrained to be smaller than the parent. Furthermore, daughter cells have one valve of the parent and one valve that is newly formed and smaller in size. This biological constraint has important implications: with each cell division diatom cells become progressively smaller. In addition, as the cells of many species within a population become smaller, their relative dimensions change. The range of size and shape of a population is termed a "size reduction series". In this website, we demonstrate the size reduction of each species. We try to include a complete range, but if we find larger or smaller specimens (or users tell us about them), we expand the size range reported.

Diatoms regain their maximum size through the formation of auxospores, which may be formed through sexual or asexual reproduction. An auxospore is a unique type of cell that possesses silica bands (perizonia) rather than a rigid silica cell wall. The perizonium allows the cell to expand to its maximum size, then produces a frustule of the normal cell morphology.

Habitats and Adaptations

Diatoms live in aquatic and semi-aquatic habitats. Some diatoms live as free floating cells in the plankton of ponds, lakes and oceans. Planktonic species often have morphological adaptations that allow them to remain suspended in water. These adaptions to prevent sinking include forming long chains, linked by silica spines. Other species form zig-zag or stellate (star-shaped) colonies that resist sinking.

Other diatom species grow attached to surfaces. They may lie attached to a rock or aquatic plant. Many frustules of these species are shaped in such a way to aid in attachment. Their frustules may be arched or curved to fit nicely to the stem of a piece of aquatic moss.

Some diatom species form a stalk which is attached to a surface. While some species form short stalks, or mucilage pads, others form long branching stalks. The stalks function to hold the cells in place and are resistant to waves or high flow in rivers. Stalks also appear to function to obtain nutrients from the water.

Diatoms that have a raphe system are able to move over benthic surfaces, whether the surfaces are fine grains of sand, or within the mud of a tidal zone, or even on other diatoms. Some diatoms form mucilage tubes and move up and down inside the tubes. Diatoms have differing abilities to move, depending on the species. They are able to travel at different maximum speeds, related to the degree to which the raphe system is developed.

Diatoms as Environmental Indicators

In general, diatom species are very particular about the water chemistry in which they live. In particular, species have distinct ranges of pH and salinity where they will grow. Diatoms also have ranges and tolerances for other environmental variables, including nutrient concentration, suspended sediment, flow regime, elevation, and different types of human disturbance. As a result, diatoms are used extensively in environmental assessment and monitoring. Furthermore, because the silica cell walls do not decompose, diatoms in marine and lake sediments can be used to interpret conditions in the past. Paleoecology is a field that utilizes both living and subfossil diatom valves that are preserved in marine and freshwater sediments. Scientists use living cells to understand the environmental factors that determine the modern presence and abundance. Then, scientists can apply the knowledge of species preferences in modern conditions to interpret the diatom species from the past, and the historical conditions that those species imply.

Learn More About Diatoms

An excellent DVD with stunning video and still images of diatom biology is available. It is appropriate for more advanced students:

Pickett-Heaps, Jeremy D.and Pickett-Heaps, Julianne. 2003. Diatoms: Life in Glass Houses. Cytographics, 58 minutes. ISBN: 0-9586081-6-4

Useful references include:

Round, F.E., Crawford, R.M. and Mann, D.G. 1990. The Diatoms, Biology and Morphology of the Genera. Cambridge University Press. 747 p.

Smol, J.P. and Stoermer, E.F. 2010. The Diatoms: Applications for Environmental and Earth Sciences. Second Edition, Cambridge University Press. 667 p.