Afrustule is the hard and porous cell wall or external layer ofdiatoms. The frustule is composed almost purely ofsilica, made fromsilicic acid, and is coated with a layer of organic substance, which was referred to in the early literature on diatoms aspectin, a fiber most commonly found in cell walls ofplants.[1][2] This layer is actually composed of several types ofpolysaccharides.[3]
The frustule's structure is usually composed of two overlapping sections known asthecae (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 andmucilage 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.[4][5][6]
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| Biomineralization |
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Teeth, scales, tusks etc |
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.[1] 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.
Some genera of diatoms develop ridges on the internal surface of the frustules which extend into the inner cavity. The ridges are commonly termedPseudoseptum with the pluralpseudosepta.[7] In the familyAulacoseiraceae, the ridge is more specifically called aringleist orringleiste.[8]
When diatoms die and their organic material decomposes, the frustules sink to the bottom of the aquatic environment. This remnant material isdiatomite or "diatomaceous earth", and is used commercially as filters, mineral fillers, mechanical insecticide, in insulation material, anti-caking agents, as a fine abrasive, and other uses.[9] 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.[2]
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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.[10]
A brief and extremely simplified version can be explained as:[10]
