A frustule is the hard and porous cell wall or external layer of
diatom
A diatom (Neo-Latin ''diatoma'') is any member of a large group comprising several Genus, genera of algae, specifically microalgae, found in the oceans, waterways and soils of the world. Living diatoms make up a significant portion of Earth's B ...
s. The frustule is composed almost purely of
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundant f ...
, made from
silicic acid
In chemistry, a silicic acid () is any chemical compound containing the element silicon attached to oxide () and hydroxyl () groups, with the general formula or, equivalently, . Orthosilicic acid is a representative example. Silicic acids are ra ...
, and is coated with a layer of organic substance, which was referred to in the early literature on diatoms as
pectin
Pectin ( ': "congealed" and "curdled") is a heteropolysaccharide, a structural polymer contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal chemical component of pectin is galact ...
, a fiber most commonly found in cell walls of
plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
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 wat ...
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 (: 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 common ar ...
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, with the direction of their movement 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.
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
Diatomaceous earth ( ), also known as diatomite ( ), celite, or 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 ...
or "diatomaceous earth
Diatomaceous earth ( ), also known as diatomite ( ), celite, or kieselguhr, is a naturally occurring, soft, siliceous rock, siliceous sedimentary rock that can be crumbled into a fine white to off-white powder. It has a particle size ranging fr ...
", 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.[
# Following mitosis, two daughter cells form inside the parent cell, with the nucleus of each daughter cell moves to the side of the diatom where the new hypotheca will form.
# A microtubule center positions itself between the nucleus and the plasma membrane above which the new hypotheca will be placed.
# A vesicle known as the silica deposition vesicle forms between the plasma membrane and the microtubule center. This forms the center of the pattern and silica deposition can continue outward from that point, forming a huge vesicle along one side of the cell.
# A new valve is formed within the silica deposition vesicle by the targeted transport of silica, proteins, and polysaccharides. After formation the valve is exocytosed by fusion of the silica deposition vesicle membrane (the silicalemma) with the plasma membrane.
# The daughter cells fully separate, with the inner face of the silicalemma becoming the new plasma membrane.
# Following separation, the daughter cells generate girdle bands, allowing the cells to expand unidirectionally along the axis of cell division.
]
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