PHYCOLOGICAL SOCIETY OF AMERICA
Diatoms are unicellular eukaryotes that are microscopic in appearance.
For the most part, they are photosynthetic micro-organisms, although
some may live heterotrophically. They may be simple or branched, filamentous,
and even enveloped in a gelatinous envelope or tube. All diatoms are
enclosed by a frustule that is made up of two valves fitted together
by a connective zone called a girdle. Growth occurs by ordinary mitotic
cell division as well as through the formation of an auxospore by sexual
- Color: Individually, diatoms do not tend to have a strong,
highly visible color. Though when diatoms are visualized in groups,
there is a yellow tint that can be seen due to the presence of
photosynthetic plastids. When observed as larger and larger masses,
the color takes on a darker color, often deep brown or black. Some
diatoms also have a blue or green color, but these are the exceptions.
- Size & Shape: The size of these very diverse micro-organisms
varies greatly, spanning a large range of values (usually measured
in micrometers). Diatoms can form small colonies, and filaments
have been seen to measure over two feet. With respect to shape,
diatoms are grouped as either centric (with concentric markings
on its valves) or pennate (markings are separated by a median).
- Growth: The various manners in which diatoms grow and
exist are definitely an indication of their diversity. Some diatoms
grow singly, while others may exist as clusters or filaments that
anchor them to surfaces. Some are epiphytic, while others exist
as parasites. Though some diatoms may be sessile, there are many
that are free due to structures that allow movement. Those diatoms
that do form colonies are usually liked by siliceous, mucilage,
or polysaccharide structures.
- Cell Wall: The cell wall is composed, for the most part,
of silica (SiO2). This term refers to the frustule and the organic
material that coats the valves and girdle.
- Frustule: The frustule consists of two valves that
fit within each other (one valve is slightly smaller than the
other. The frustule can vary greatly in shape, ranging from
box-shaped to cylindrical, symmetrical as well as asymmetrical.
Epitheca: This term is used to refer to both the larger,
older valve (epitheca) of the frustule as well as the girdle
elements (epicingulum) that are connected with it.
Hypotheca: This term is used to refer to the smaller,
younger valve (hypotheca) of the frustule as well as the girdle
elements (hypocingulum) that are connected with it.
- Connective Zone: This zone is composed of the overlapping
girdle elements of the diatom (the epicingulum and hypocingulum).
It acts to connect the valves, forming sutures that still allow
the two valves to move apart or towards one another.
- Septa: This word is used to refer to the partitions
that are formed within the valves. Both the septa and valve
markings (on both the inner and outer surface) can be used
to characterize diatoms.
- Raphe: This is a structure that is found within pennate
diatoms. A pseudo-raphe is a blank space while a true raphe
is a space that actually divides the valve with the exception
of a connection that is usually at the center of the valve
(in effect, connecting the two sides of the valve that is divided
by the raphe)
- Protoplast: There is nothing particularly unusual within
the diatoms' "living matter" when compared to other eukaryotic
macroalgae. It is completely contained within the silicified frustule.
A nucleus, mitochondria, plastids, and other various organelles
are present within the protoplast. More specifically, here are
a few examples.
- Cytoplasm: This is a colorless plasma that may be
found within the cell, on the inner side of the frustule and
- Nucleus: The nucleus is usually located near the
center of the diatom. It is often spherical or lenticular and
encloses the chromosomes and nucleoli.
- Plastids: These are the organelles that are similar
to chlorophyll in their photosynthetic abilities. They can
occur in bands or as a random distribution of granular masses.
Plastids constitute a large portion of the protoplast.
- Volutin: Volutin molecules are nitrogen reserves.
They are found throughout the cytoplasm and vary in their shape,
size, and arrangement from species to species.
- Oil Drops: These drops are actually sugar and starch
in globular form. They are located in the cytoplasm and, like
volutin, vary in shape and size.
- Habitat: Diatoms can be found in both fresh water and
marine environments. Generally, diatoms inhabit most bodies of
water in all parts of the world (if provided with sufficient amounts
of nutrients). In fresh water habitats, diatoms prosper throughout
the year, especially in the spring and fall months. Not only do
diatoms exist in streams, lakes and other bodies of fresh water,
but they can be found on the rocks, plants, and mud that are present
within or at the borders of water. In marine environments, diatoms
have the capacity to exist within animals' digestive tracts, in
their shells, on macroalgae, and even on ice floes!
- Habitat Determination: The currents can serve as
a method of moving organisms to new habitats. The Gulf Stream,
for example, has the capacity to move organisms from the East
Indies up the eastern coast of the United States. Since diatoms
can also become attached to the legs of insects and birds,
as well as the scales of fish and the sides of ships, they
are able to travel far and wide in order to inhabit new places.
- Ocean Location: Diatoms can be found in the littoral
(within the range of the tides) and pelagic (water that covers
the ocean bottom) regions. They are classified as either pelagic
(existing within the water column) or benthic (existing on
copyright Jennifer Shin 1999.