Microbial Discovery Activity
Pond Scum:
Investigating Microogranisms
Abstract
Little else can stimulate a student’s interest in biology like a drop of pond water teaming with
invisible life viewed with a microscope. This activity describes two means of observing pond water
other than the traditional hanging drop or temporary wet mount slide preparation.
Intended Audience
K-4
5-8
9-12
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Learning Objectives
At completion of this activity, learner will be able to:
• appreciate (acknowledge) the previously unseen life in pond water.
• prepare a biofilm slide.
• draw and describe a microorganism.
Necessary Student Background
Student must be familiar with the workings of a microscope.
Keywords
Aquatic, Environment, Protozoa, Biofilm, Water quality, Classification
American Society for Microbiology
Education Department
1752 N Street, NW
Washington, DC 20036
Education [email protected]
General Information
Author
Mark Gallo, Ph.D.
Associate Professor of Biology
Niagara University, NY 14109
[email protected]
National Science Education Standards Addressed
Unifying Concepts and Processes – This activity incorporates evidence of microscopic life.
Standard 1: Science as Inquiry – In completion of this activity student performs directed, inquiry
based observations of microorganisms.
Standard 3: Life science - In completion of this activity student will have had a chance to view
living organisms from an environment and determine the characteristics of the microorganisms.
Standard 5: Science and Technology – Biofilms will be noted as constructs by living organisms,
microscopes will be used as a tool to recognize the microbes, students will be asked to determine
what defines a living organism.
Standard 6: Science in Personal and Social Perspectives – In this activity learner monitors the
quality of fresh water ecosystems.
Life Science Concepts: may incorporate responses of unicellular organisms
Scientific Thinking: student can make predictions about water quality, temperature, pH
Scientific Tools and Technologies: microscopes, centrifuges, slide preparation
Scientific Communication: Sharing observations with classmates and teachers
Scientific Investigation: hands-on manipulative experiment created from a known water source
Activity Specifications
Classroom setting
Requires special equipment
Uses hands-on manipulatives
Requires mathematical skills
Can be performed individually
Requires group work
Requires more than one (45 min) class period
Appropriate for special needs student
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Teacher Handout
Pond Scum: Investigating Microorganisms
Class Time
Approximately 45 minutes of class time is required.
Teacher Preparation Time
Time must be allotted for specimen gathering and slide preparation:
• Biofilm slide preparation should be performed at least 24 hours prior to class.
• Pond water, lake water, river water (etc) needs to be collected or brought in by students.
• Order Demoslides and slide holders from: Connecticut Valley Biological Supply Company,
Inc. 1-800-628-7748
• You may also want to order some live cultures of mixed protozoa. These typically last for
several weeks or more and are excellent in that they show a robust, diverse population of
microbes.
Background
The diversity of life becomes self-evident in the microscopic world and has fascinated individuals
since the first glances at it through the microscopes of Antonie van Leeuwenhoek. Students can
explore this world of numerous single-celled and multi-cellular organisms. One can explore the
questions of what is living/non-living, how are cells organized, what are the size limitations of life,
how do microbes move, sense their world, interact with other microbes and the world around them.
Students may want to peruse two outstanding websites to investigate pond water life:
• “Water World” at the Microbe Zoo:
http://commtechlab.msu.edu/sites/dlc-me/zoo/zwpmain.html
• Florida State University’s Molecular Expressions Digital Video Gallery, Streaming Video,
and Downloads of protozoa commonly found in ponds:
http://micro.magnet.fsu.edu/moviegallery/pondscum.html
Materials and Equipment
For the biofilm preparation:
Have the following for each team
• 2 - 4 microscope slides
• 22 x 40 coverslips (if available - other coverslips will do)
• small container such as a deli or fast food container that will hold water to a depth of
approximately 3 cm.
• microscope
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For the demoslide preparation:
Have the following for each team
• 1 demoslide tube with lid
• 1 demoslide holder
• microscope
• centrifuge or a length of string, about 2 feet long
Demoslides: From http://www.southernscientific.com/demoslides.asp
“Designed to simplify the preparation of living protozoa for microscopic examination in the
classroom. Culture chamber and examination chamber combined into one unit. No more
transferring of materials for study. The
growth chamber holds 5 ml. of algae or
protozoan culture and can be maintained
there for up to a week or longer, depending on
the type of culture. The unique Microscope
Stage Adapter allows for easy positioning of
the Demo Slide and the Demo Slide Tube
Rack holds 12 tubes either horizontally or
vertically. Demo Slides are based on the CenSlide® system from DAVSTAR)”
Methods
Biofilm preparation
1. Place 2 - 4 glass slides on the bottom of a small container.
2. Pour pond water over the slides and rearrange the slides if they moved. Slides should not
overlap. The water should be about 3 cm deep.
3. Cover the container if it will be more than 24 hours before you can view the biofilm.
4. Set the container in a spot where it will not be disturbed.
5. After 18 - 24 hours (or longer if necessary), carefully remove a slide and wipe the BOTTOM
with a paper towel.
6. Carefully lay one long coverslip over the top of the slide. If you don’t have long coverslips, lay
two smaller coverslips side by side.
7. Observe the biofilms using the 4X, 10X and 45X objectives.
Demoslide preparation
1. Pour well mixed pond water into a demoslide tube and cap tightly.
2. Centrifuge, if possible, at a slow speed for 10 minutes.
3. If a centrifuge is not available, tie a string tightly around the top of the tube and whirl
overhead for a minute to concentrate the pond water microbes in the bottom of the tube
4. Affix the tube to the slide holder and place it on the microscope stage.
5. Observe the tube using the 4X, 10X and 45X objectives.
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Microorganisms
Microorganisms are present in this activity, the type varies based on the sample. It is important to
stress to the students to not bring in samples from water sources known to be contaminated with
human or animal waste.
Safety Precautions
Remind students to wash their hands after handling pond water samples.
Assessment
Below are possible assessment techniques.
•
Have students identify 5 different organisms using an identification guide and ask them
to answer the following questions about each organism:
o
Into what Kingdom is this organism classified?
o
Is this organism prokaryotic or eukaryotic?
o
If this organism is motile, how does it move?
o
How does this organism obtain necessary nutrients?
o
How does this organism reproduce?
•
Have the students draw a picture of the organism and write a paragraph about the life
and times of the organism for a museum exhibit.
•
Create a poster of the web of life of the different microorganisms noted by the class.
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Supplementary Information
Tips / Suggestions:
•
Demoslides are excellent for inquiry-based experiments. The environment can be altered
(temperature, pH, light) and the tubes can be observed without disruption over long
periods of time.
•
Undoubtedly the teacher will be called upon to answer the question “What is this???”
many, many times. There are two ways to address this. One is to allow the students to
be the “discoverers” of these creatures and let them create appropriate names for the
organisms. The class can then use this as a means to form a classification system.
Another option is to allow them to compare what they have seen with what is known and
described in identification guides. The following are available through bookstores or
online vendor sites:
•
ƒ
How to Know the Freshwater Algae (George W. Prescott)
ƒ
How to Know the Protozoa (Theodore Jahn)
ƒ
Guide to Microlife (Kenneth Rainis)
ƒ
A Guide to Common Freshwater Invertebrates of North America (J. Reese
Voshell, Jr.)
Set up your own mini-ponds by following the “Puddles: exercise from the Meet the
Microbes” projects published by the National Association of Biology Teachers. The
activity can be found at: http://www.microbeworld.org/htm/mam/activities.htm
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Student Handout
Pond Scum: Investigating Microorganisms
Student Name: _________________________________
Introduction
Fresh water is teeming with living organisms. This experiment will allow you to be able to see and
identify many living organism with the aid of a microscope and keys.
Terms
Biofilm- Biofilms form when bacteria attach to surfaces exposed to water, and begin to
excrete a slimy, glue-like substance.
Protozoan- single celled organism such as an amoeba that can move and feed on organic
compounds of nitrogen and carbon.
Demoslide- Combination collection/storage/and microscopic observation tubes for Protozoa
and other cell cultures.
Motile- having the ability to move.
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Activity
Safety Considerations
1. As with any experiment, it is important to gather your materials before starting, be familiar with
their purpose, and READ THE DIRECTIONS.
2. Glass slides may be slippery and may break. If breakage occurs, please tell your teacher so the
broken glass can be properly disposed of is not a danger to others.
3. After your teacher asks you to clean up, it is very important to wash your hands with soap and
water.
Materials Checklist
microscope
demoslide + chamber
biofilm slide + cover slips
water sample
centrifuge or capped tube + string
Procedure
Biofilm preparation
1. Place 2 - 4 glass slides on the bottom of a small container.
2. Pour pond water over the slides and rearrange the slides if they moved. Slides should not
overlap. The water should be about 3 cm deep.
3. Cover the container if it will be more than 24 hours before you can view the biofilm.
4. Set the container in a spot where it will not be disturbed.
5. After 18 - 24 hours (or longer if necessary), carefully remove a slide and wipe the BOTTOM
with a paper towel.
6. Carefully lay one long coverslip over the top of the slide. If you don’t have long coverslips, lay
two smaller coverslips side by side.
7. Observe the biofilms using the 4X, 10X and 45X objectives.
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Student Results
Name: ______________________________________________________________________________________
1. Draw your observations below and describe what you see. Describe color of objects
and motility, if noted.
Biofilm slide
Demo slide
Pond water slide
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2. Please answer the following questions.
1. Which source had the most organisms?
2. Which source had the most diversity?
3. How would time of slide in the water affect the end results for the biofilm?
4. Do all your water samples contain the same organisms? Propose a mechanism for your
observations.
5. What may happen to the number and diversity of microorganisms throughout the year in an
aquatic environment?
6. How would you identify producers, consumers, and degraders in the microbial world?
7. Make a list of six (6) factors that could influence a water environment. Explain how each
would impact the biota.
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