occurrence of diatoms

Diatoms

The class of Bacillariophyceae is a large group of algae, consisting of over 10,000 species belonging to 200 genera. The members are popularly known as diatoms. Fine geometrical expressions and intricate sculpturing in the cell wall make these organisms one of the most beautiful microscopic objects. They occur in almost every aquatic habitat as free living photosynthetic autotrophs, colourless heterotrophs Or photosynthetic symbionts. Daitoms are also found in terrestrial habitats and form a substantial proportion of the soil flora. In tropical rain forests, they grow in association with blue green algae on leaves of trees. In India the group is represented by 569 species distributed over 92 genera. Some common Indian species of diatoms are Pinnularia graciloides, Nielosira jurgensii and Coscinodiscus radiatus. 

Important Distinguished characters of Bacillariophyceae are as follows

• Diatoms are mostly unicellular but some colonial species are form filaments, loose chains or mucilagenous colonies. 
• The cells are microscopic and are variable in shape. They may be triangular(Tricertatium), boat shaped(Cymbrella), rod shaped(Bacillaria), oval(coccaneis placentula) or spherical(coscinodiscus excentricus). 
• The cell is surrounded by a rigid two part overlapping cell wall called Frustule. It is made up of pectic substances but is impregnated with high amount siliceous substances. 
• The vegetative cell is uninucleate and diploid. It usually has two large parietal or many discoidal plastids. 
• It is olive green or golden in color, and it's main pigments are chlorophyll a, c and xanthophyll. 
• The storage products of photosynthesis are oil and chrysolaminarin. 
• Motile cells have one pantonemotic flagellum. 
• The cells shows gliding movements. 
• Vegetative reproduction takes place by cell divisio. 
• Sexual reproduction is isogamous Or oogamous. 
• The zygote formed as a result of gametic union develops into a special type of spore called auxospore. 
• Continued reproduction by cell cell division results in the reduction of cell size and auxospores formation is a restorative or rejuvenation process. 

Occurance:

Diatoms are found in every aquatic environment. Genera like Synedra, Melosira, Navicula and Asterionella are strictly freshwater and Sceletonema, Eulcampa, Biddulpia, Fragilaria, Hemidiscus, Corethron etc. Are found in marine water habitat. Some aquatic forms occur as plankton of ponds and lakes. The algal flora of roadside ponds and pools always includes some diatoms. Species of pinnularia, Navicula, Frustulia, Amphora and colonies are found in terrestrial habitat. Such forms are able to withstand dessications for a very long period. Some algae are epiphytic on other algae. Licmophora is an endozoic diatom.