Introduction to some aquatic protozoa in Essex
Figure 2 above shows the common protozoan Amoeba proteus, belonging to the class Sarcodina. A
proteus is typically found in freshwater ponds where there is an abundance of decaying organic
material. A. proteus varies between 250mm and 750mm depending on the availability of food. It is
found on the surface of materials as it prefers a solid surface on which to move. A. proteus is a
predator that captures and ingests a variety of other microbes, including bacteria, protozoa and
algae.
Amoebae are a successful group of microbes that inhabit a wide variety of niches including freshwater
lakes/ponds, sewage, moist soil, salt water, hot springs, chemically polluted water, animal intestines
and even the human mouth. A number of amoeba are pathogenic to humans and other animals.
Amoeba are notable for their ability to alter their shape rapidly; the term amoeboid means "to readily
change shape".
Amoebae consist of an inner fluid, called the endoplasm, and an outer region, the ectoplasm, which
is more rigid than the endoplasm. Many amoebae, but not all, both move and capture prey by sending
out pseudopodia (false feet) in the desired direction. Once the prey is detected, pseudopodia extend
and engulf the prey into a food vacuole.
Human phagocytic immune cells resemble amoebae both morphologically and physiologically. The
bacterial agent of Legionnaire's Disease {Legionella pnuemophillia) is able to survive and grow
within certain species of freshwater amoebae because they are able to avoid being digested by the
amoeba's digestive system. The fact that a comparable mechanism is also used by the L.
pneumophillia to avoid destruction by human phagocytic cells suggests a similarity in the two
evolutionarily diverse hosts.
Many freshwater amoebae can be found very easily in just a small drop of pond water that has been
placed on a microscope slide and left to settle for a short while. For most school children this is
usually their first view of life at the microbe level. Many school and college text books go on to say
that the Amoeba proteus is the ideal microbe to examine under the microscope to observe how
single-celled animals move and eat; they do not tell you how difficult it can be to actually find one
of these creatures!
Most amoebae will round up into a ball when placed onto a glass slide. However, they soon recover
and this is when you can watch these remarkable animals going about their business. Freshwater
amoebae usually reveal themselves by a tentative movement of a lobe, which very quickly turns into
full-blown pseudopodia. Pseudopodia are extensions of the cytoplasm that come in various types
depending on the species.
A. proteus puts out lobopodia, which tend to be blunt, while some species will only have a single
lobopodium that the cell will flow into. Under a microscope the cytoplasm can be seen flowing from
the main cell body into the pseudopodia easily even with transmitted light because the cytoplasm
usually contains many food vacuoles that are in various stages of digestion. It must be remembered
that these single-celled organisms can carry out the basic functions of multi-cellular organisms,
which may consist of billions of cells.
Essex Naturalist (New Series) 18 (2001)
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