Observing Compost Microorganisms
Introduction
Observe the microbial communities in your compost over the
course of several weeks or months as the compost heats up and
then later returns to ambient temperature. Can you identify differences
in microbial communities at various stages of the composting process?
Materials
- compound microscope
- .85% NaCl (physiological saline)
- methylene blue stain (Prepare stain by adding 1.6 g methylene
blue chloride to 100 ml of 95% ethanol, then mixing 30 ml of
this solution with 100 ml of 0.01% aqueous solution of KOH)
Procedure
- Make a wet mount by putting a drop of water or physiological
saline on a microscope slide and transfering a small amount of
compost to the drop. Make sure not to add too much compost or
you will not have enough light to observe the organisms.
- Stir the compost into the water or saline (the preparation
should be watery) and apply a cover slip.
- Observe under low and high power. You should be able to find
many nematodes (they should be very wiggly), flatworms, rotifers
(notice the rotary motion of cilia at the anterior end of the
rotifer and the contracting motion of the body), mites, springtails
and fast-moving protozoans. Pieces of fungi mycelia can be seen,
but might be difficult to recognize. Bacteria can be seen as
very tiny, roundish particles, which seem to be vibrating in
the background.
- If you want to observe the bacteria directly, you can prepare
a stained slide and observe the slide using a 100X oil immersion
lens. To prepare a stained slide, mix a small amount of compost
with a drop of physiological saline on a slide. Spread with a
toothpick. Let the mixture air dry until you see a white dried
film on the slide. Next fix the bacteria to the slide by passing
the slide through a hot flame a few times. Stain the slide using
methylene blue stain. Flood the slide with the methylene blue
stain for one minute and then rinse with distilled water and
gently blot dry using blotting or filter paper.
- Fungi and actinomycetes may be difficult to recognize with
the above technique because the entire organism (including the
mycelium, reproductive bodies and cells) will probably not remain
together. Fungi and actinomycetes will be observed best if you
can find fungal growth on the surface of the compost heap. The
growth looks fuzzy, powdery, or like a spiderweb. Lift some compost
with the sample on top, and and prepare a slide with cover slip
to view under the microscope. You should be able to see the fungi
well under 100X and 400X. The actinomycetes can be heat-fixed
and gram-stained to view under oil immersion at 1000X.
- To separate nematodes, rotifers, and protozoans, a continuous
column of water leading from the compost to the collection vial
is necessary, and the following adaptation of the above method
should be used: The compost is put into a beaker with the screen
stretched across the top and taped in place. The beaker is then
turned over into the funnel. Plastic tubing is placed at the
end of the funnel stem and a screw clamp is placed a few inches
below the end of the funnel stem on the pastic tubing.
The plastic tubing should lead into a collection vial or small
beaker. The clamp is closed and water is poured into the funnel
until the beaker is about 1/2 filled. After a few days the clamp
is slightly and slowly opened and organisms which have concentrated
at the end of the tubing should fall into the vial.
Techniques for Detailed Study
of Compost Microorganisms
This protocol was written by Elaina Olynciw, biology teacher
at A. Philip Randolph High School, New York City, while working
in the laboratory of Dr. Eric Nelson at Cornell University as
part of the Teacher Institute of Environmental Sciences.
Credits
Cornell Waste Management Institute ©1996
Cornell University
Bradfield Hall
Ithaca, NY 14853
607-255-1187
cwmi@cornell.edu
