* Most are free living and photosynthetic.
* Some species can undergo a stupendous increase in population size;
this event is called an algal bloom.
* Diatoms, the most abundant group, have sculptured cell walls composed
of silica and occur in a variety of shapes.
* The remains of diatoms have accumulated in the bottom of many lakes
and there are large areas of their remains in underground deposits.
* These deposits are called diatomaceous earth and are mined for use in
insulation and filters.