Diatoms:
Living Snowflakes
By Susan Sobehrad
Fall 97
Description:
Scientists estimate that there may be as many as 12,000
species of diatoms, each identifiable by their delicate, ornate
shells. Diatoms are a dominant species of phytoplankton, and are
known scientifically as the class Bacillariophyceae and may be
grouped in one division (Chrysophyta) of the kingdom Protista
along with the golden-brown and the yellow-green algae. Diatoms
are one of the major producers in the ocean, and serve as a major
component of the ocean food web. At home in the sunlit layer,
diatoms are autotrophic--they require energy from sunlight,
nutrients like nitrates and phosphates, and carbon dioxide for
photosynthesis. These tiny, one-celled creatures are abundant in
temperate water, and they can grow up to 1 millimeter across,
although most are smaller. Diatoms may thrive in both fresh and
marine waters systems, and on moist soil. In the water, diatoms
may live attached to rocks, sand, or plants, or they may float
freely.
Diatoms are divided into two basic types: the elongated forms
called pennate diatoms which are most often found in shallow
areas and fresh water, and the centric diatoms which are
circular, triangular, or irregular in shape and are most abundant
in the ocean. Diatoms have thin, double shells of silica called
frustules that fit together, one of top of the other, like the
two parts of a pillbox. The top of the frustule, the epitheca, is
perforated with many holes, arranged in a pattern characteristic
of the species The holes permit the cell's plasma membrane to
maintain close contact with the environment, and allows for the
diffusion of materials into and out of the cell. Numerous pores
also reduce the weight of the floating diatom Some diatoms also
have longitudinal slots in the wall of the base, or hypotheca,
through which the cell can make contact with a solid surface and
crawl about. In the center of the frustule is the nucleus which
contains the genetic materials and the chloroplasts, or
photosynthetic organelles. Diatoms reproduce through cell
division--one cell divides into two cells. After a diatom cell
divides each new cell retains one part of the parent shell and
builds a new part to fit into it.
Movement:
Diatoms cannot move under their own power, but have
developed adaptations that keep them afloat. Some diatoms have
pairs of thin spines or setae projecting from the ends of the
cells. These setae fuse with other cells to form long chains,
thereby increasing the buoyancy of the chained groups or
colonies.
Surface Blooms:
Massive surface blooms of buoyant diatoms have been
observed where the cold waters of polar seas converge with warm
equatorial currents. The blooms are marked by a narrow line where
the water changes from blue to green. These blooms are detectable
by radar and even by satellites in orbit around the earth.
Diatomaceous Earths:
When diatoms die, their hard, silicate shells remain in
tact. The shells sink to the bottom of the sea, and over long
periods of geologic time, the layer of diatom shells may become
very deep. Diatoms began to accumulate 100 million years ago, and
reached a peak of abundance in the middle part of the Cenozoic,
the Miocene, where there are thick deposits composed largely of
their skeletons. The beds are called diatomites, or diatomaceous
earths, and are of commercial value. This fine, crumbly substance
is used in insulating materials, abrasives, ceramics, and in
filtering and filling materials. More than 270,000 metric tons of
it are extracted annually from a quarry near Lompoc, California.
The Facts:
Diatomic Structure
Uses of Diatoms:
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Revised: December 22, 1999