Phylum- Chrysophyta / Bacillariophyta
Chief producers of Oceans
Mainly aquatic (both freshwater and marine)
Cellulosic cell wall having depositions of Silica called Frustule.
Frustule- transparent, glass-like, and made up of two halves i.e. Epitheca and Hypotheca.
Body has a soapbox like structure.
Absent in the vegetative phase.
May be present in Gametes
Floats passively with water currents.
Few can show mucilage propulsion.
A large central vacuole is present.
Chlorophylls- a and c
Xanthophyll- Diatozanthin, Fucoxanthin, Diadinoxanthin.
Asexual- New cells formed by Binary fission take each half of the frustule and use it as Epitheca and the hypotheca is newly synthesized. Thus, the size of the cells keeps decreasing in each division.
Sexual- Diatoms enter the sexual stage when the size of the cell becomes too small. As a result, gametes are produced which fuses to form a diploid zygote called Auxospore or also called Rejuvenescent spore (restores the original size of diatom).
Centrales- have radial symmetry eg. Melosira, Cyclotella.
Pennales- have bilateral symmetry eg. Navicula, Pinnularia.
The indestructible cell wall of diatoms deposits in the sea bed and forms Diatomaceous earth or Keisel guhr.
Diatoms are present in huge numbers and are the chief producers of the aquatic ecosystem.