Title
Monterey Bay Aquarium Research Institute
Marine Botany

Diatom Size

Diatoms range in size from ten to 150 microns (millionths of a meter). Any individual's size has important consequences for its float-sink equilibrium and it's susceptibility to grazers among the zooplankton.

In order to grow very large, a diatom might require further modifications, such as test projections. A diatom's weight increases greatly with size largely because of the additional weighty silica in the test. Test projections, like spines and setae, however, increase the drag force felt by a diatom in the water without increasing its weight very much. This means it will take longer for a cell to sink out of the sunlit surface waters of the ocean. So test projections might have evolved as a way to allow diatoms to grow bigger.

Test projections might have allowed diatoms to live at larger sizes, but they did not cause diatoms to get larger. The evolutionary pressure that may have acted to increase diatom size may have been grazing by zooplankton. Diatoms may grow to a point where they are too large for copepods and other zooplankton to eat. Not unlike the peach in the shark attack episode in James and the Giant Peach, the diatom may be just too large for the predator's jaws to get a piece of.

Test projections may also have a role in protecting diatoms from grazers, effectively increasing cell size to a point where the grazer cannot engulf it.


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copyright John Becker 1996.

Last updated: Feb. 05, 2009