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Mysterious Life of Caves

Classroom Activity

Objective
To understand that microorganisms can survive in many different environments and that microorganisms live in places where conditions are suitable for their growth.

• copy of "Microbial Townhouse" student handout (PDF or HTML)
• hard-boiled egg
• 5 cups of mud
• 1 cup of de-chlorinated water
• bucket
• large stirring spoon
• 1 tablespoon powdered chalk
• 2-liter clear plastic bottle with top cut off
• paper cup
• plastic wrap
• rubber band
• sheet of red cellophane or acetate (for one group only)

1. Tell students that they will be studying how microorganism populations grow in different mediums and how they change the environment in which they live. They will be creating three mud columns to study how different microbes will inhabit different environments, depending on the conditions that exist.

2. Have students collect mud. The best mud comes from the margins of fresh or saltwater ponds or marshes. Moist field soils can be used, but are less likely to produce easily observed microorganisms.

3. Cut off the plastic bottle tops. Hard boil the eggs. Organize students into three groups and distribute a copy of the "Microbial Townhouse" student handout and a set of materials to each group.

4. Have students follow student handout instructions to create their columns. Have one group add red cellophane or acetate around its column. After the columns are made, ask students to predict what, if any, changes they think will occur over time and why.

5. Students will study the columns under three variables: light, darkness, and filtered light. Have one group place its column in a well-lit place, but not in direct sunlight. Have the group making the red-acetate-wrapped column do the same. Have the final group place its column in the dark.

6. Have students observe the columns daily for six weeks. Students should record and describe their observations. Ask students what might have caused the layers they see. Why might the layers be different? Why might different microorganisms grow in different places? What are the differences among the three columns? What might be responsible for those differences?

7. As an extension, have students re-create the experiment using moist soils from other locations, such as their backyard, forest area, or garden.

What Is That Smell?
Is that rotten eggs? The columns may smell that way after a few weeks once microbial colonies create a sufficient amount of hydrogen sulfide. Keep the bottles in a well- ventilated space to disperse the odor. Keep the bottles sealed and prepare students for a strong smell when they are reopened.

Students developed a mud column known as a Winogradsky column. It is named after Sergei Winogradsky who devised it in the 1880s to study microorganisms in the soil. Different microorganisms will grow in each column depending upon their environmental needs.

Column in sunlight
Many of the microorganisms that developed in this column are photosynthetic; that is, they use light to give them energy to make food. However, the colonies throughout the column differ by their light, oxygen, and nutrient needs. Those at the top (cyanobacteria and any green algae) use visible light wavelengths to survive while those below use sulfur from the egg yolk or carbon from the newspaper and chalk as their energy source.

After the columns are set up, the metabolic activity of the original microorganisms in the mud soon reduces the oxygen level throughout the column. At the top, enough oxygen diffuses through the plastic wrap and water to sustain a high oxygen zone. This creates an oxygen gradient in the column; high oxygen near the top, low oxygen near the bottom. This oxygen gradient favors the growth of oxygen-dependent organisms near the top, while the growth of bacteria that don't need oxygen is favored near the bottom. Non-oxygen dependent bacteria that use sulfur for energy also produce smelly and toxic hydrogen sulfide gas. These bottom bacteria create a second, opposite, gradient of hydrogen sulfide; high near the bottom and low near the top. These gradients produce specific zones of opportunity for different microorganisms. Students will observe these zones, and the growth of the microorganisms in them, as colored bands.

Column in sunlight with red cellophane
Students likely will see some of the types that grow in the sunlight column because, like some of the microorganisms that grew in the clear plastic bottle, the ones that grow in this column need red light wavelengths to survive. (The red cellophane will absorb all other light wavelengths.)

Column in dark
Students may see types of non-photosynthetic bacteria growing in this environment.

Books

Taylor, Michael. Lechuguilla: Jewel of the Underground. Huntsville, AL: National Speleological Society, 1998.
Discusses the biological research and mapping efforts taking place in Lechuguilla.

Wolfe, David. Tales from the Underground: A Natural History of Subterranean Life. Cambridge, MA: Perseus Publishing, 2002.
Presents a glimpse of a mysterious underground world and the scientists who study it.

Article

Eliot, John. "Deadly Haven: Mexico's Poisonous Cave." National Geographic, May 2001, pages 70-85.
Explores Villa Luz, home to microbial colonies called snottites and other forms of life that live on sulfur.

Web Sites

NOVA Online—Mysterious Life of Caves
www.pbs.org/nova/caves/
Provides program-related articles, interviews, interactive activities, and resources.

Biospeleology
www.utexas.edu/depts/tnhc/.www/biospeleology/
Explores the world of cave biology.

Investigating Bacteria with the Winogradsky Column
www.woodrow.org/teachers/bi/2000/Winogradsky_Column/ winogradsky_column.html
Provides in-depth information about building a Winogradsky column and identifying the microorganisms within.

The "Microbial Townhouse" activity aligns with the following National Science Education Standards:

Grades 5-8

	Science Standard A: Science as Inquiry

Understanding scientific inquiry

• Scientific explanations emphasize evidence, have logically consistent arguments, and use scientific principles, models, and theories. The scientific community accepts and uses such explanations until displaced by better scientific ones. When such displacement occurs, science advances.

• Science advances through legitimate skepticism. Asking questions and querying other scientists' explanations is part of scientific inquiry. Scientists evaluate the explanations proposed by other scientists by examining evidence, comparing evidence, identifying faulty reasoning, pointing out statements that go beyond the evidence, and suggesting alternative explanations for the same observations.

Grades 9-12

	Science Standard G: History and Nature of Science

Nature of scientific knowledge

• Science distinguishes itself from other ways of knowing and from other bodies of knowledge through the use of empirical standards, logical arguments, and skepticism, as scientists strive for the best possible explanations about the natural world.

• Because all scientific ideas depend on experimental and observational confirmation, all scientific knowledge is, in principle, subject to change as new evidence becomes available.