Diatom forms gametes auxospore that fuse to become a specialized cell which swells to the maximum or

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GROWTH REPRODUCTION
Diatom life-cycle
Asexual phase
Sexual phase

• 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.

• 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.

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• 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.

• Note reduction in the diatom-size during the
  life-cycle (asexual phase). Length reduces faster
  than width

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Diatom distribution and Ecology

• Diatoms constitute a major component of the
  marine (open sea) phytoplankton, living only in
  the upper 200 m of the surface water.
• Autotrophic and form the basis of food chains in
  many aqueous ecosystem.

Distribution of diatoms frustules in the surface
sediments of the Indian and Pacific Oceans, in
millions per gram of sediments, from Brasier
(1980).
Antarctic
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• 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.
• 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.
• 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-).
• 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.

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Diatoms and dissolution

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

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GEOLOGIC RECORD AND EVOLUTION

• 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.

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• 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.

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APPLICATIONS AND IMPORTANCE OF DIATOMS

• 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.
• Diatoms cover a vast area of oceans, so they are
  major players in the global carbon balance.
• Important in fresh water primary productivity,
  although they have more competition in the
  freshwater environment.
• Diatoms contain chlorophyll, thus they are a
  major source of atmospheric oxygen.
• Form the basis of food chains for zooplankton in
  any a aqueous ecosystem.

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• Diatomacous rocks are widely used in commerce as
  cleansers, paints, filtering agents, abrasives
  and many toothpastes.
• 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).
• See rest of the applications in Brasier, 1980.