Slime molds are microscopic organisms. As slime molds are eukaryotic organisms, they have their genetic material contained within a membrane inside the cell. Once thought to be fungi, slime molds are now recognized to be very different from fungi. Indeed, slime molds are now classified as one of the five main divisions of life (the other four are fungi, bacteria, plants, and animals).
There are three main groups of slime molds. The first group is known as the plasmodial slime molds, or Myxomycetes. The slime molds can exist as cells that appear similar to amoeba, and which are able to move to find food. A common habitat for these cells is underneath rotting logs and damp leaves, where the cellulose that the cells use for food is abundant. These cells can move to an environment that is drier and has more light, where they then fuse together to form an enormous single cell that contains thousands of nuclei. This form, called a pseudoplasmodium, can ooze about seeking a region of acceptable warmth and brightness. Then, the aggregate settles to form a plasmodium. A plasmodium can be several inches in diameter and is often vividly colored.
Scientists use plasmodia to study a phenomenon called cell streaming, where the contents of a cell move about. The large size of a plasmodium and the fact that cell streaming is readily visible using a low-power magnification light microscope, makes this slime mold a good choice for a model system. Another plasmodial slime mold, Physarum polycephalum, moves in response to various stimuli including ultraviolet and blue light. The proteins actin and myosin are involved in this movement. Actin and myosin also control the movement of muscles in higher organisms, including humans.
The second group of slime molds are known as the cellular slime molds. These are typically single-celled. In response to a chemical signal, however, the cells can aggregate to form a great swarm of cells. This aggregation is of intense interest to scientists who study the physical and genetic development of cells.
The final group of slime molds are called the protostelids.
All three types of slime molds are capable of forming a structure called a sporangium. This structure is formed when conditions are unfavorable for the growth or survival of the slime mold. A sporangium is a cluster of spores on a stalk. Each spore is a bundle of genetic information. Dispersal of the spores by air currents can lead to the formation of new slime molds when the spores land and germinate.
Besides their complex life cycle and scientific interest as model system for study, slime molds have been noteworthy for other reasons. After a particularly wet spring in Texas in 1973, several residents of a Dallas suburb reported a large, moving, slimy mass, which they termed "the Blob." Reporters in the local press speculated that the Blob was a mutant bacterium. Fears of an alien invasion also were raised. Ultimately, however, a local mycologist soberly identified the growth as Fuligo septica, a species of plasmodial slime mold.
Additionally, researchers were later able to formulate mathematical equations that explained the single cell to aggregate process of cellular slime molds. The slight modification of these equations formed the basis of the programs that are now used to control some of the behaviors of the figures in video games.
Alexopoulos, C.J, C.W. Mims, and M. Blackwell. Introductory Mycology. 4th ed. New York: John Wiley, 1996.
Conover, A. "Hunting Slime Molds: They're Not Animals and They're Not Plants, and Biologists Want to Know a Lot More About Them." Smithsonian March 2001: 26–30.