 Diatom forms gametes (auxospore) that fuse to become a specialized cell which swells to the maximum (original) size for the species.  Then divides asexually and initiates the asexual phase of the life-cycle again. GROWTH & REPRODUCTION Asexual phaseSexual phase Diatom life-cycle  Cell divides into two copies of itself, one of which is the same size as the mother cell, the other of which is slightly smaller.  Gradual reduction in size occurs  Life-cycle continues from one to several years.  Enters the sexual phase for original size preservation or by a trigger from the environment.

 Do these changes in the life cycle in a diatom (one species), impose changes in the size of valves?  Do these changes in the size of valves of a diatom, make it difficult to recognize the species?  Whatever occur in the sizes of the valves of diatoms, each species is unique and can be recognized on the basis of its valves alone. ³ Note: reduction in the diatom- size during the life-cycle (asexual phase). Length reduces faster than width

1. Diatoms constitute a major component of the marine (open sea) phytoplankton, living only in the upper 200 m of the surface water. 2. Autotrophic and form the basis of food chains in many aqueous ecosystem. Diatom distribution and Ecology Distribution of diatoms frustules in the surface sediments of the Indian and Pacific Oceans, in millions per gram of sediments, from Brasier (1980). Antarctic

3. Inhabit all aquatic environments. They occur in the body of the water as plankton or on the bottom where they may be attached to plants or rocks or sand particles, or may be free living and able to move between particles of the substratum. 4. In general, members of both classes (Centric and Pennate) may be found in either fresh or salt water, though Centric forms tend to predominate in marine habitats, while Pennate diatoms are more typical of freshwater environments. 5. Can achieve very high cell densities in nature. In the same time, cell density can be decreased by reducing the concentration of heavy ions (K+, Na+, Cl-). 6. To maintain bouyancy, many species have elaborate spines that increase the cell's effective surface area, and consequently decrease the settling rate. The spines promote turbulence, thus reducing the width of the boundary layer, and promoting nutrient exchange.

Diatoms and dissolution Diatoms are so resistant to dissolution than any other shells. High deposition of diatoms forms DIATOMITE beds in the geologic history.

Centrate diatoms started first in Lower Cretaceous, then pinnate diatom evolved to dominate only at the beginning of Miocene age. The oldest certain fossil diatoms are Lower Cretaceous in age. Diatoms probably had a much longer history than this; there are reports of Precambrian and Triassic fossils that might be diatoms or diatom relatives. However, definite fossil diatoms older than the Cretaceous are not known. An older report of diatoms from the Upper Jurassic is now doubted by experts, since silica recrystallizes under pressure. So, any older diatom fossils may have been destroyed. Most diatom fossils known are from Eocene and Miocene rocks, such as the marine diatoms rich by Actinoptychus heliopelta and Sceptroneis caduceus. GEOLOGIC RECORD AND EVOLUTION

Actinoptychus heliopelta Sceptroneis caduceus

At areas of seasonal changes (lakes and oceans), diatom deposits occur in layers which correspond to annual cycles, which can be used to observe changes over many years, providing information about past climatic changes.

1.About 20 to 25% of all organic carbon fixation on the planet (transformation of carbon dioxide and water into sugars, using light energy) is carried out by diatoms. 2.Diatoms cover a vast area of oceans, so they are major players in the global carbon balance. 3.Important in fresh water primary productivity, although they have more competition in the freshwater environment. 4.Diatoms contain chlorophyll, thus they are a major source of atmospheric oxygen. 5.Form the basis of food chains for zooplankton in any a aqueous ecosystem. APPLICATIONS AND IMPORTANCE OF DIATOMS

6.Diatomacous rocks are widely used in commerce as: cleansers, paints, filtering agents, abrasives and many toothpastes. 7.Because each species of diatoms tends to have distinct requirements for temperature, salinity, acidity, oxygen, mineral concentration and disturbances, they are frequently used to evaluate the river health (water and habitat quality). 8.See rest of the applications in Brasier, 1980.