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This is a very successful group of organisms, with over 12,000 recognised species. It is believed that this is grossly under exstimated and there are probably a million or more.

Assortment of diatoms

Assortment of diatoms; rectangular, lower dominant species is Biddulphia sinensis




They dominate the cooler waters of the ocean. They have a very resistant silica wall or test. Coupled with their abundance it does mean that they help in the buildup of marine sediments. Some, called diatomite, are mined. They survive in most aquatic situations and coat rocks and weeds. Strong wave action on the coast may create a brownish discoloration from a specialist group of diatoms: the surf diatoms.

One of the main structural feature is the frustule made from silica. It is essentially a lidded box and has amazing patterns on it, used to help in the identification of the different species Shapes vary enormously, see the centric diatoms like Coscinodiscus compared with the pinnate forms. Reproduction is via cell division. In many cases this is primarily mitosis creating two vegetative cells. These vegetative cells are diploid but through meiosis they can form gametes.

Diatoms are such a diverse and incredible group it is impossible to cover much more here. Try the web and here are a few to try:

Baccilaria paxilifera

Baccilaria paxilifera - the sliding diatom: this species can be found in huge numbers and actually moves by sliding cells against each other through the water.


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