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| Fig. 1 - Rotifers |
The
best method for preparing pond water suitable for observation under the
microscope is to make a hay infusion. Here, a sample of natural water from a
pond, creek or other source is collected and poured into a glass jar or beaker.
A handful of cut hay or grass, which will provide nutrients for any microbes
present, is then added to the jar.
Organisms Visible in Pond Water Using
the Light Microscope
If
left for several weeks, a succession of microbes can be observed as one species
replaces the next in this mini ecosystem. Initially, a prepared drop from the
surface of the pond water should reveal several types of bacteria (spiral, rod
shaped or spherical) swimming around.
After
around a week, protozoans such as Paramecium (see figure 2) and Euglena
may be visible. Further examples of this group of microbes, including ciliates
such as Stentor and Vorticella and algae such as Volvox, diatoms
and Chlamydomonas ,should appear in the next four or five weeks.
Students
should be encouraged to research the Monera and Protista kingdoms, as most of
the organisms they observe will belong to these groups. The Monera , which
include bacteria and blue green algae, are much smaller than other microbes
(around 2 micrometres in diameter) and do not possess a true membrane bound
nucleus or ‘organelles’ (small structures within the cell that carry out
specialised functions).
The
Protista are essentially a loose grouping of single celled organisms that do
possess a membrane bound nucleus and organelles. They are much larger than the
Monera, averaging around 40-100 micrometres in diameter.
An
interesting multicellular microbe that may be found among decaying vegetable
matter in the pond water is the Rotifer (see figure 1). These organisms are
around 0.1-0.5mm long and possess a distinctive corona of small hairs (cilia)
at their apex. These cilia sweep food into the rotifer’s mouth in a motion that
resembles a wheel rotating (giving rise to its common name of ‘wheel animal’).
Observing Pond Water – Materials and
Teaching Method
A
standard light microscope with magnification of up to 400x is required in this
activity. Students may prefer to observe their pond water at 100x
magnification, as this provides greater resolution and covers a larger field of
view.
The
following materials and equipment are required per group of around four
students:
 |
| Figure 2 - Paramecium |
•
light microscope and microscope lamp
•
glass slide and coverslip
•
Pasteur pipette or eyedropper
•
glass jar of pond or creek water
•
handful of hay or cut grass
Students
should be instructed to copy down the following directions, which should be
followed by a teacher-led explanation.
•
Place the hay or grass in the pond water and leave for a
week in a warm place.
•
Use the eyedropper or pipette to draw up a sample of water
from the surface of the jar.
•
Place this drop on a glass microscope slide and cover with
a cover slip.
•
Place the slide on the microscope stage and focus first
using the low power objective (10x). This will give a total magnification of
100x when combined with the magnification of the lens in the microscope’s
eyepiece.
•
Observe and draw any organisms visible – if bacteria are
the only things present the higher power objective may need to be used.
•
Repeat this procedure for water in the middle of the jar
and the detritus at the base of the jar, as different types of microbes are
often found in each region.
The
following questions could be written on the board after students write up the
experiment and their observations:
1.
Use resource material to name as many of the microbes you
have drawn.
2.
Identify any microbes you observed that belong to the
kingdom Monera.
3.
Identify any microbes you observed that belong to the
kingdom Protista.
4.
Rotifers are multicellular organisms. To what kingdom do
they belong?
5.
Research information on rotifers to describe their method
of locomotion, eating habits and reproductive behaviour.
If
the pond solution contains a reasonable number of Paramecia, students
could add a drop of yeast that has been stained with congo red dye to the
microscope slide. This will allow them to observe the Paramecia ingesting
the yeast and to track the movement of the yeast through the cell.
Interestingly, the yeast may change to a blue colour as the pH changes in the
organism’s gullet.
References
Microbus,
2007, 'Pond water
Critters That You Can See With a Microscope' microscope.org
Proscope
Digital Microscope, 2010, 'Biology
Experiment: Microbes From a Hay Infusion', bodelin.com
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