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Microbiology of Decomposition

Decomposition can be thought of as a parade of many very tiny creatures. How decomposition proceeds in your column depends on which bacteria and fungi inhabit it, what ingredients you have put inside, and environmental factors such as light, temperature and moisture.

The first decomposing organisms that go to work attack the most available food molecules, such as sugars, carbohydrates and proteins. As they grow, these first bacteria and fungi also change the environment. For example, they produce heat, change the pH and consume oxygen. You will see these changes in your column as plant parts become dark and slimy.

As they change their own environment, these organisms can create conditions that favor competing microbes. The biological definition of succession is the replacement of one type of organism by another, often caused by environmental changes wrought by the first organism.

In your Decomposition Column, for example, one type of bacteria might flourish, changing the pH and raising the temperature of the column in the process. These new conditions may be favorable for a more heat tolerant type of bacteria, which will take over the original bacteria.

A Decomposition Column will show you the dynamic process of decay: strange white fuzz may appear and cover your column for a few days before suddenly disappearing to be replaced by a dark fuzz that climbs up one side. You might see something orange and slimy moving slowly along a rotting twig. You may also observe nonmicrobial life such as fruit flies, mites and millipedes.

Bacteria, fungi, algae, protozoa and other organisms that live on dead or decaying matter are collectively known as saprophytes.

Saprophytes often secrete enzymes onto material they want to eat. Enzymes are biochemicals, responsible for all kinds of chemical reactions including the breakdown of matter into digestible parts for the decomposers. A crumbling log lying on the forest floor, for example, shows the work of enzymes made by saprophytes.

Bacteria are the most numerous of the decomposers. Good soil may have 100 to 1,000 million bacteria per gram. You may see bacterial colonies as round spots, ranging from white, to cream, to brown in color.

There are many types of bacteria. You might identify one type by its odor. These bacteria, called actinomycetes, live in the soil and are responsible for that fresh, earthy smell that accompanies newly plowed soil, or a long awaited summer rain.

Fungi might appear in your column as a fuzzy blanket of mold covering some delectable rotting thing. Mold fungi form mazes of tiny threads called mycelium. If you look closely, you may see tiny dots along the threads. These dots are fruiting bodies, which release fungal spores. A particularly common mold, Rhizopus, has a cottony appearance with black dots, and often shows up on bread, fruits and other food.

Slime molds are organisms that move, feeding on microorganisms such as bacteria. They are often brightly colored and have the appearance and consistency of pudding. Slime molds often move toward light, leaving snail-like tracks behind, and producing numerous tiny fruiting bodies, some resembling tiny mushrooms.

Algae might show up in your column as a green tinge on the soil surface or on a moist twig. You have probably seen algae, like Spirogyra, growing on the banks of a river, a lake, or perhaps the sides of a fish tank, or as seaweed in the ocean.

Protozoans are another organism with a role in the decomposition drama. These single-celled organisms, such as amoebas, vary widely in size, shape and the manner in which they move. You might see protozoa swimming if you mix a little water with some decomposing material and examine it under a microscope. Although much of the action takes place on a microscopic scale, decomposition is an exciting process even to the naked eye. By studying your Decomposition Column you can get a sense of the great diversity and activity of microbial life. Bacteria, fungi, algae and protozoa may be small but they are responsible for a great deal of change.

National Science Foundation   Bottle Biology, an instructional materials development program, was funded by a grant from the National Science Foundation administered by the University of Wisconsin-Madison.   Wisconsin Fast Plants