Diatoms Under the Microscope

Classification and Characteristics  

What are Diatoms?

Diatoms are photosynthetic organisms referred to as algae with a length/diameter of between 2 and 500 microns. They have a transparent cell wall (frustule) made of silicon dioxide, which is itself hydrated with a little amount of water. Therefore, diatoms are simply aquatic organisms, which can be found in such environments as fresh and marine (salty) waters and moist soils.

The hydrated silica that makes the cell wall of these organisms looks more like opal, which is transparent, forming what resembles a glass house for the algae. The cell wall (frustule) is composed of two halves (valves) that fit into each other like a pill capsule. Because silica is impervious (it does not let anything through) this system allows for the exchange of nutrients and waste in the environment where the organism resides.  

The valves also play an important role in the identification and their classification. Although they grow as single cells, they can also form filaments or simple colonies in a group.


As algae, diatoms are protists. This means that they are eukaryotic organisms that are not specifically defined as plants, animals or fungus. Formally, they are classified under Division Chrysophyta in Class Bacillariophyceae. This Class of organism is distinguished by the presence of an inorganic cell wall that is composed of hydrated silica.

Some of the other characteristics of this Division (Chrysophyta) include:

  • An endoplasmic cysts
  • They store oils rather than starch
  • A bipartite cell wall
  • Secretion of silica


Diatoms are also divided in to two main Orders, which include the Centrales and the Pennales.



Also referred to as Biddulphiales, Centrales have the following characteristics:

  • Valve striae arranged in relation to a point
  • An annulus (central areola)
  • Cylindrical


Pennales are also known as Bacillariales and have the following characteristics:

  • Valve triae arranged in relation to a point
  • Pen shaped
  • Bilaterally symmetrical

Life Cycle

Typically, diatoms divide and reproduce by a process referred to as vegetative division, which involves the division of a single cell into two new cells. During the reproduction cycle, the new cell is formed inside the parent cell. The new cell is smaller in size given that it forms within the mother cell that has a rigid cell wall that does not expand.

During this process, the daughter cell also takes a valve of the parent frustule as its epitheca before building its own hypotheca in a period of about 15 minutes. This process may be repeated a number of times a day (1 to 8 times). However, this largely depends on the availability of dissolved silica.

The process also results in reduced size of the cells with each division, which in turn results in a relative change in dimensions. This change in size and shape of a population is commonly referred to as Size Reduction Series. In this case therefore, one can expected to see a variation of shapes and sizes of a given population of diatoms under the microscope.

As a result of the reducing average size of the diatom frustule in a population, there comes a point where restoration of the size of the frustule is necessary. It's at this point that auxospores are produced.

These particular cells possess a different cell wall compared to the former generation and lack the siliceous frustule as well. This allows for the swelling of the frustule to the maximum size. The initial auxospore cell forms a new frustule of maximum size, which then forms following an active vegetative reproduction after nutrient levels have been depleted. Once the nutrient level increases, the cycle continues.

Habitats and Morphology

Different types of diatoms have different morphological adaptations that allow them to survive in their respective habitats. For instance, diatoms that live in such aquatic habitats as ponds, lakes and oceans possess morphological features that make it possible for them to remain suspended in water.

By forming long chains that are linked to each other by silica spines, these planktonic species are able to remain suspended on water. Others will form zigzag/stellate colonies that keep them afloat. These species are often star-shaped. 

Other species grow and multiply on such surfaces as rocks and other aquatic plants. For these species, their frustules are often arched or curved in a manner that allows them to fit on stems of aquatic moss. Other species need to attach them to surfaces and therefore form a stalk or mucilage pads that allow them to achieve this.

Depending on their habitats therefore, one can identify differences in their structures, which can help identify where they come from.

Diatomaceous Earth

When the aquatic diatoms die, they sink to the bottom of whatever habitat they are found in and collect to form what is known as diatomaceous earth. The shells (made of silica) cannot decay, and therefore collect together at the bottom of the lake. In some cases, they collect to form a soft, chalky light weight rock called diatomite.

This is commonly used as an insulating material as well as making explosives, filters and abrasives among other products.  Most of the diatomaceous earth available on earth is composed of silicon dioxide and may contain lower levels of crystalline silicon dioxide. It is used in a wide variety of products including wettable powders and pressurized liquids where it is used in farms, buildings, skin care products and pet kennels among others. 

In its dry form, Diatomaceous earth causes insects to dry out and die by absorbing their oils and fats from their cuticle. If a person is exposed to diatomaceous earth it can cause nasal irritation or cough and shortness when inhaled in very large amounts. Dust containing this substance can also be irritating to the eyes or cause skin irritation and dryness. However, it is not poisonous. 

See more information on Diatomaceous Earth here.

Diatoms Under a Microscope

Diatoms make for very interesting specimen under the microscope. They show complex patterns with very fine punctures on their surface. With some of the species, fine pores in the frustule are used for testing the resolving power of the lens of a microscope.




Diatoms can be easily prepared for viewing under the microscope by preparing wet mounts. Here, the sample is simply smeared on the slide using such liquids as water. The slide can then be placed on the microscope for viewing. This is the simplest method and can help determine how to proceed.

In some cases, hydrogen peroxide (or other oxidizing agents) may be used to remove the organic matter of the frustule for better viewing. Here, a small amount of hydrochloric acid (HCL) may be used for the purposes of removing calcium carbonate followed by rinsing in distilled water to remove all the acid. The sample can then be dried and placed on a slide for viewing.

To increase contrast, a mounting medium of higher refractive index can be used. Brightfield and phase contrast microscopy can be used for observing diatoms. Here, phase contrast is particularly preferred when viewing specimens that are lightly silicified. For a dry specimen, 40X and 100X are commonly used.

Different Diatoms

Take a look at our article in more depth about Pond Water Under the Microscope and Microorganisms. As well as reading about chalk under a microscope.

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




Diatom life history and ecology, Microfossil Image Recovery and Circulation for Learning and Education (MIRACLE), University College London


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