Arachnodiscus diatoms, photomicrograph, darkfield illumination
The diatom's silicified cell wall forms a pillbox-like shell (frustule) composed of overlapping halves containing intricate and delicate markings. Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons ( Fragilaria), fans ( Meridion), zigzags ( Tabellaria), or stellate colonies ( Asterionella). Diatoms are producers within the food chain. A unique feature of diatom cells is that they are encased within a cell wall made of silica (hydrated silicon dioxide) called a frustule. These frustules show a wide diversity in form, but are usually almost bilaterally symmetrical, hence the group name. The symmetry is not perfect since one of the valves is slightly larger than the other allowing one valve to fit inside the edge of the other. Fossil evidence suggests that they originated during, or before, the early Jurassic Period. Only male gametes of centric diatoms are capable of movement by means of flagella. Diatom communities are a popular tool for monitoring environmental conditions, past and present, and are commonly used in studies of water quality. There are more than 200 genera of living diatoms, and it is estimated that there are approximately 100,000 extant species. Diatoms are a widespread group and can be found in the oceans, in freshwater, in soils and on damp surfaces. Most live pelagically in open water, although some live as surface films at the water-sediment interface (benthic), or even under damp atmospheric conditions. They are especially important in oceans, where they are estimated to contribute up to 45% of the total oceanic primary production.[citation needed] Spatial distribution of marine phytoplankton species is restricted both horizontally and vertically. Usually microscopic, some species of diatoms can reach up to 2 millimetres in length.
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Photo credit: © Scenics & Science / Alamy / Afripics
License: Licensed
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Keywords: arachnodiscus, biodiversity, darkfield, diatoms, illumination, marine, micrograph, microscope, photomicrograph
