A frustule is the hard and porous cell wall or external layer of
diatoms. The frustule is composed almost purely of
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is o ...
, made from
silicic acid, and is coated with a layer of organic substance, which was referred to in the early literature on diatoms as
pectin
Pectin ( grc, πηκτικός ': "congealed" and "curdled") is a heteropolysaccharide, a structural acid contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal, chemical component o ...
, a fiber most commonly found in cell walls of
plant
Plants are predominantly Photosynthesis, photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all curr ...
s.
This layer is actually composed of several types of
polysaccharide
Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with ...
s.
[Progress in Phycological Research: v. 7 (1991) by F.E. Round (Volume editor), David J. Chapman (Volume editor)]
The frustule's structure is usually composed of two overlapping sections known as
theca
In biology, a theca (plural thecae) is a sheath or a covering.
Botany
In botany, the theca is related to plant's flower anatomy. The theca of an angiosperm consists of a pair of microsporangia that are adjacent to each other and share a commo ...
e (or less formally as valves). The joint between the two thecae is supported by bands of silica (girdle bands) that hold them together. This overlapping allows for some internal expansion room and is essential during the reproduction process. The frustule also contains many pores called areolae and slits that provide the diatom access to the external environment for processes such as waste removal and
mucilage
Mucilage is a thick, gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion. The direction of their movement is always opposite to that of the secretion of ...
secretion.
The microstructural analysis of the frustules shows that the pores are of various sizes, shapes and volume. The majority of the pores are open and do not contain impurities. The dimensions of the nanopores are in the range 250–600 nm.
Thecae
A frustule is usually composed of two identically shaped but slightly differently sized thecae. The theca which is a bit smaller has an edge which fits slightly inside the corresponding edge of the larger theca. This overlapping region is reinforced with silica girdle bands, and constitutes a natural "expansion joint." The larger theca is usually thought of as "upper," and is thus termed the epitheca. The smaller theca is usually thought of as "lower," and is thus called the hypotheca.
[ As the diatom divides, each daughter retains one theca of the original frustule and produces one new theca. This means that one daughter cell is the same size as the parent (epitheca and new hypotheca) while in the other daughter the old hypotheca becomes the epitheca which together with a new and slightly smaller hypotheca comprises a smaller cell.
]
Pseudoseptum
Some genera of diatoms develop ridges on the internal surface of the frustules which extend into the inner cavity. The ridges are commonly termed Pseudoseptum with the plural pseudosepta.
Diatoms
Diatoms have varied life strategies including floating in the water column (phytoplankton
Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'.
P ...
), colonising submerged surfaces and living within the surface of deposited sediments. Some cells are essentially cylindrical (centric) while others have an elongated "boat-like" shape. Since they are algae belonging to the division Bacillariophyta they require light for photosynthesis. Perhaps the most studied group of diatoms belong to the phytoplankton. Phytoplanktonic diatoms rely on ocean currents and wind to keep them in the upper oceanic levels as their cell wall is denser than water around them. They would naturally sink otherwise.
Diatom skeletons and their uses
When diatoms die and their organic material decomposes, the frustules sink to the bottom of the aquatic environment. This remnant material is diatomite or "diatomaceous earth
Diatomaceous earth (), diatomite (), or kieselgur/kieselguhr is a naturally occurring, soft, siliceous sedimentary rock that can be crumbled into a fine white to off-white powder. It has a particle size ranging from more than 3 μm to ...
", and is used commercially as filters, mineral fillers, mechanical insecticide, in insulation material, anti-caking agents, as a fine abrasive, and other uses.Diatom Frustule 2
/ref> There is also research underway regarding the use of diatom frustules and their properties for the field of optics, along with other cells, such as those in butterfly scales.[
]
Frustule formation
As the diatom prepares to separate it undergoes several processes in order to start the production of either a new hypotheca or new epitheca. Once each cell is completely separate they then have similar protection and the ability to continue frustule production.[
# The newly formed nucleus and the pre-existing nucleus each move to the side of the diatom where the new hypotheca will be formed.
# A vesicle known as the silica deposition vesicle forms near the plasma membrane.
# This forms the center of the pattern and silica deposition can continue outward from that point, till the frustule is produced.
]
References
External links
Frustule
on Britannica
on astrographics.com
by Christina Brodie, UK
Regarding the Super formula
on microscopy-uk.org.uk
A Geometric Primitive for Computer Aided Design, by Paul Bourke, January 1990
by Paul Bourke, March 2002
{{Protist structures
Algal anatomy
Diatom biology