Illustrated here are two
different kinds of diatom, class Diatomeae, phylum Chrysophyta.
These organisms belong to the plant Kingdom.
Both of these are planktonic, marine specimens.
The discoid forms (above
left), Actinocyclus sp., belong to a group of diatoms that
are renowned for their birefringent colours. These specimens were
mounted upon a victorian microscope slide.
living chain diatom (above right) was shot in 3-D and in doing
so, it became obvious that the cells are triangular in
cross-section and that the yellow nuclear mass within is suspended
in the centre of the cell by cytoplasmic strands.
Diatoms are unicellular organisms
that lack a flagellum. They range from 0.0015 to 2 mm in size
and their siliciceous tests may be cylindrical, elliptical, polygonal
or lancet shape, some forming long chain-like colonies. Diatoms
inhabit both freshwater and marine environments and belong to
both the planktonic and benthic communities. Some planktonic forms
can regulate their buoyancy, and many benthic species are able
to crawl slowly over the substrate. It is thought that some diatoms
secrete a mucus-like substance that aids their locomotion. Though
the majority of diatoms are free-living and autotrophic, some
are symbionts or parasites upon, for example, foraminifera,
corals and algae.
At present about 70,000 species,
both fossil and recent, have been described. Diatoms are both
abundant and of vital importance. They are thought to comprise
approximately a quarter of plant life by weight and produce at
least a quarter of the oxygen we breathe. In life they provide
high quality nutrition for a wide range of animals, from small
protozoans to large baleen whales. In death they fall to the ocean
floor and create an oil-rich plasma layer that is, over thousands
of years, transformed into petroleum. Their silicieous tests are
commonly mined and used as filters and abrasives. In addition
to their economic value diatoms are important environmental indicators.
Extant diatoms enable biologists to accurately assess the pollution
levels in certain ecosystems, whereas extinct diatoms aid palaeontologists
to reconstruct palaeoclimates and interpret the depositional environment
of certain strata.
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