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
- 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)
Pennales are also known as Bacillariales and
have the following characteristics:
- Valve triae arranged in
relation to a point
- Pen shaped
- Bilaterally symmetrical
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
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
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.
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
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.
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.
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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Return from Diatoms to MicroscopeMaster Fun Research Home
Pickett-Heaps, Jeremy D.and Pickett-Heaps,
Julianne. 2003. Diatoms: Life in Glass Houses. Cytographics, 58 minutes. ISBN:
Diatom life history
and ecology, Microfossil Image Recovery and Circulation for Learning and
Education (MIRACLE), University College London