Monterey Bay Aquarium Research Institute
Marine Botany

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General Information 

Defining Characteristics

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 reproduction.

Macroscopic Appearances

  • 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.

Microscopic Appearance

  • 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)

Cell Contents

  • 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 cell wall.
  • 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 the seafloor).

copyright Jennifer Shin 1999.

Last updated: Feb. 05, 2009