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Slime Molds

What is that Thing?

The year – 1973; the place – Dallas, Texas; the issue – a large, outgrowth of oozing, spreading yellowish blobs climbing telephone poles and crossing lawns. Fear about alien life forms spread across the city. Firemen were called in to assist but efforts to destroy it with a high-pressure hose only made it grow more. It took several days, and many articles in the national media, before a mycologist recognized it for what it was and shortly thereafter it decomposed of its own accord. What was it? A slime mold.

If you ever notice scrambled egg-colored masses of goo in mulch that resembles something left by a sick dog, the culprit is a slime mold with the common name dog-vomit slime mold. Typically, slime molds are not gross to look at and don’t inspire fear, in fact, they are often quite colorful and beautiful to view.

What is a Slime Mold?

Scientists once thought slime molds were a fungus since they reproduce by spores and often resemble mushrooms. But slime molds also have a stage where they can move around, making them more like an animal. A 19 th century mycologist described slime molds using the Greek words for “fungus” and “animal” naming them mycetozoans.

Taxonomists today refer to slime molds as eukaryotic. There are three different types of slime molds or eumycetozoans – myxomycetes, or “true slime molds”; dictyostelids, or “cellular slime molds” and protostelids. The last two types are only visible through a microscope.

All eumycetozoans typically have at least two life stages: one for feeding and the other for reproduction. While in the feeding stage, the slime mold is amoeboid, or single-celled, and it is able to move around at speeds up to 1 millimeter per hour (with an exceptional few moving as fast as 2 centimeters per minute). Slime molds are predators of fungi and bacteria cells and can be found on dung, tree bark, decaying plant matter, in soil and in aquatic habitats. As slime molds flow over and engulf their food, they ingest it. If an item turns out to be inedible, they eject it.

Slime molds apparently use chemical signals given off by food sources to sense which way to move. Researchers in Japan recently proved that Physarum polycephalum will consistently work out the shortest path between two piles of nutrients in a maze.

Slime molds begin life as microscopic spores. When conditions are right, the spore opens and releases a single-celled organism called an amoeba. What happens next depends on which of the groups the slime molds belong to.

In myxomycetes, the food-gathering stage is termed a plasmodium. The plasmodium is acellular, meaning that it has multiple nuclei (sometimes in excess of millions) but no membranes to differentiate individual cells though there is one common membrane holding the “gob of protoplasm” together. The plasmodium is able to move by protoplasmic streaming or “flowing” across or through a substrate. The plasmodium is able to move through extremely small pores and some have never been seen outside of their substrate. When conditions are unsuitable, many myxomycetes can form a resting structure that is resistant to cold, drying, and physical injury.

Dictyostelids, the cellular slime molds, move about as single, unattached amoebas for most of their lives until a chemical signals that the single life is over. Then, one by one, up to 100,000 amoebas in an area find each other and fuse into a single multicellular body called a pseudoplasmodium which resembles a plasmodium but each individual cell remains as a separate unit. This multicellular body then forms a fruiting body. Interestingly, though many amoeboid cells are needed to join together to form the fruiting body, not all of them will get to produce spores and thus reproduce. Cellular slime molds are best known from the top couple of centimeters of the soil humus layer. Only about 100 species are known worldwide.

In the final stage of both groups–triggered by adverse conditions such as lack of food, too much heat, flooding, or wrong pH–fruiting bodies develop and release the spores of the next generation.

Protostelids are a very small group known only in culture. Only about 35 species are known worldwide but probably twice that number have been observed but not described. Like the other types of slime molds, individual amoebae collect together to form a fruiting body. The typical protostelid produces just a single spore per fruiting body, though some have been observed to produce up to 8. They are microscopic, unicellular organisms that are identified based on their fruiting body.

Slime Molds in Scientific Research

The lab of UGA cell biologist, Dr. Marcus Fechheimer, uses the dictyostelid slime mold Dictyostelium discoideum as a model organism for studying how cells work. It has many biochemical similarities to human cells, especially sensory cells. During the course of his work, he discovered that this species can be made to produce Hirano bodies, which are cellular structures commonly found in the cells of people suffering from Alzheimer’s disease and other forms of dementia. No one knows how these structures may be related to dementia but scientists had never previously been able to study them except in cells taken from dead humans. The new lines of research posed by this discovery could lead to a better understanding a perhaps treatment for Alzheimer’s and other neurodegenerative diseases.

Slime molds are studied in laboratories as models for cell differentiation for cancer research. They have also served as models for the study of cell cycles (because of their rapid nuclear division), cell growth and differentiation, and cell movements (because of their cytoplasmic streaming).

The Global Slime Mold Inventory

In 1998, researchers in Great Smoky Mountains National Park began an effort to identify all forms of life in the park’s boundaries. This huge inventory effort is called the All Taxa Biodiversity Inventory or ATBI. Prior to the ATBI, 92 different species of myxomycetes had been reported from Great Smoky Mountains National Park, and the majority of these records were based upon specimens collected more than a half century ago. In the first three years, more than 75 species have been added to this total. The most surprising finds are four species of myxomycetes not previously known from North America and two others that appear to be new to science. However, it is anticipated that there are many more species to be found. In fact, based upon the results obtained thus far, the Great Smoky Mountains National Park appears to be one of the world's "hot spots" for myxomycetes, with as many species present in the Park as anywhere else on earth.

These finds led researchers to want explore further. Currently there are two big research projects underway, both funded by the National Science Foundation, to learn more about slime molds. One project, called PEET (Partnerships for Enhancing Expertise in Taxomony) will produce a revision of the classification of slime molds. This will help scientists who study slime molds understand how they are related to other organisms. But in order to do this, more slime mold experts are needed since there are very few people worldwide who are experts. This project will help train young people to become systematists. The other large project, called PBI (Planetary Biodiversity Inventories) will study the worldwide distribution of slime molds. One thing the researchers suspect is that if there is plant life growing in an area, there are probably slime molds associated with that life. All you have to do is look and that’s what this project is all about.

This writeup was compiled from information provided by Dr. Steve Stephenson (myxomycetes), Dr. John Landolt (dictyostelids), Dr. Fred Spiegel (protostelids), Paul Super (National Park Service) and an article written by Susan Kaneko Binkley. Thank you all for your great work.

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