Number of species in the collection: 2.

Back to Phylum: Heterokontophyta

 

Pictures of Fragilariophyceae:                     

 

 

Characteristics of Fragilariophyceae:         

 

The Fragilariophyceae are a group of microscopic algae found worldwide, inhabiting both salt and freshwater environments, as well as surfaces with periods of moisture. It is a very common and numerous group, although it often goes unnoticed as it is composed only of microscopic species.

They are unicellular algae, sometimes colonial, whose cells are enclosed by a shell made of two silica valves (hydrated glass) that fit together. These glassy covers have various ornamentations, leaving spaces where the cell communicates with the exterior. This ornamentation often exhibits bilateral symmetry, meaning the pattern is replicated on both sides of the organism. Unlike other groups, these diatoms lack a central linear suture called a raphe. They often secrete sticky substances that allow them to adhere to a substrate or to other cells, forming colonies in these cases. When the cells die, their shells can accumulate, forming large underwater sediments that over time give rise to a type of rock called diatomite. Their earliest fossils date back to the Jurassic, making them a relatively modern group.

They live in both freshwater and saltwater, either floating or attached to a substrate, forming an important part of the plankton and therefore serving as food for a large number of animals. Occasionally, they can appear massively, but their growth is severely limited by the availability of silicon in the water, as they cannot form their shells without it. During cell division, each daughter cell inherits one of the two valves from the mother cell, but since one valve is smaller than the other (because one valve fits inside the other), the resulting cells have different sizes. Thus, there are some individuals that decrease in size with each division, being limited by the size of the valve they inherit, and therefore they form the second valve based on the first one. Size recovery occurs with sexual reproduction, where two cells shed their shells, fuse, and create a new shell of the largest possible size so that subsequent asexual divisions take longer to produce smaller cells.
 

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