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Bacterial Terrarium
by Fred Stein

With just mud, paper and an egg, you can grow colonies of multi-hued microbes!

glowing bacterium

winogradsky columnIn the 1880's, a Russian microbiologist named Sergei Winogradsky discovered that water mud poured into a tall bottle and placed in the sun turned many different colors. He found that by adding a few simple things, such as sheese or paper, he could control which colors appeared. Here's a recipe for making your own Winogradsky column.

Materials:

  • a clear tennis-ball container or plastic soda bottle with the top cut off
  • containter top or plastic wrap and a rubber band
  • mud from a bay shore of the edge of a shallow pond (the smellier the better)
  • water
  • 1/4 newspaper page shredded
  • 1 raw egg (without shell)
  • bowl
  • spoon
  • lamp with a 40-watt incandescent bulb

To do and to notice:

Jar for Winogradsky column Remove any rocks or sticks from the mud, and put the mud in the bowl. Mix it with water until it's the consistency of heavy cream.

Remove the label from you container. Put in the shredded paper and egg, add an inch of mud, and mix well. Then, fill the container with mud up to an inch from the top and cover it. Wash your hands after you complete the setup.

Put the container about a foot from a 40-watt bulb that you can leave on all the time. Every few days, briefly remove the container's top to vent off the gases (if too much gas is allowed to build up, it can blow the top off). If the mud at the top is drying out, add a little water. In a few weeks, you'll see auras of brilliantly colored bacteria. Keep watching, and you'll see the colors develop and change during the next several months.

Many kinds of bacteria live in mud. Some are decomposers that get nutrients by breaking down organic materials things like the egg and paper. During the process of decomposition, all the oxygen near the bottom of the container is used up.

Other bacteria are photosynthetic. Blue-green bacteria near the top of the mud column use light, carbon from carbon dioxide, and hydrogen from water to make carbohydrates and give off oxygen just like plants. The carbon dioxide they need is released when the decomposers break down the paper and egg.

Jar for Winogradsky columnWhat's going on?

Red, orange and green bacteria that grow near the bottom of the column are less tolerant or completely intolerant of oxygen. They're photosynthetic, but they get their hydrogen from hydrogen sulfide, the gas that smells like rotten eggs. In your container, hydrogen sulfide is released when the decomposers break down the protein in the egg.

Your mud contained some of each of these kinds of bacteria when you collected it. You provided the bacteria with conditions that are just right for them, and their populations exploded. The colorful patches are colonies of billions of bacteria.

So what?

For the first billion years that life existed on the planet, the atmosphere contained little oxygen. Bacteria like those at the bottom of your mud column thrived. It wasn't until about two billion years ago that the blue-green bacteria filled the atmosphere with its photosynthetic waste product oxygen. This effectively banished other kinds of bacteria to places where there is no oxygen to mud and sediments and (more recently) to the insides of plants and animals.

But these oxygen-phobic bacteria still have crucial roles to play. Some take part in the global sulfur cycle. Others convert nitrogen into forms needed by plants. And neither cows nor termites would enjoy their cellulose-based diets without the helpful bacteria in their guts that digest the cellulose for them.


© 2001 Exploratorium