The year: 1973. The place: Dallas, Texas. The issue: the sudden appearance of large, 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 disappeared of its own accord. What was it? A slime mold.

If you ever notice scrambled egg-colored masses of goo in mulch that resemble 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 even beautiful.

What is a Slime Mold?

Slug and Lamproderma - Photograph by Randy G. Darrah (©Randy Darrah)

Scientists once thought slime molds were fungi (singular: fungus) since they reproduce by spores and often produce fruiting bodies that resemble small mushrooms. But slime molds also have a stage where they can move around, making them more like an animal, and some types spend most of their existence as single-celled organisms like amoebae. A 19th century mycologist first suggested "mycetozoan" (which is derived from the Greek words for "fungus" and "animal") as an appropriate name for slime molds.

Taxonomists today refer to slime molds as eumycetozoans. There are three different types of eumycetozoans-myxomycetes, or "true slime molds"; dictyostelids, or "cellular slime molds" and protostelids. The last two types are only visible through a microscope. All three types of slime molds are unicellular for part of their life cycle.

Slime Mold Feeding Stages

All slime molds typically have at least two life stages: one for feeding and the other for reproduction. In the myxomycetes, there are two feeding stages. The first of these is microscopic and thus somewhat similar to the comparable stage found in dictyostelids and protostelids. However, the second is much larger, sometimes exceeding several inches in extent. While in the feeding stage, the slime mold is amoeboid and thus able to move about, but only at speeds of few millimeters per hour. Slime molds are predators of fungi and bacteria and can be found in or on such substrates as dung, tree bark, decaying plant matter, soil and (in a few instances) 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 Mold Life Cycles

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 upon the group to which the slime mold in question belongs.

Myxomycetes

In myxomycetes, the second of the two food-gathering stages 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 although 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 (called a sclerotium) that is resistant to cold, drying, and physical injury.

Dictyostelids

Dictyostelids, the cellular slime molds, move about as single, unattached amoebae for most of their lives until a chemical, produced by one of the amoebae, signals that the single life is over. Then, one by one, up to 100,000 amoebae in an area find each other and fuse into a single multicellular body called a pseudoplasmodium which resembles a plasmodium but within which 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. What is in it for the other cells? Right now, we have no idea. Cellular slime molds are best known from the top couple of centimeters of the soil/humus layer. Only about 100 species are known worldwide, but 10 new species recovered from samples collected in the Great Smoky Mountains National Park will be described in a forthcoming paper.

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

Protostelids are a very small group of slime molds known only in laboratory 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 on the basis of the overall structure of their fruiting body. They are typically found by culturing pieces of live and standing dead plants and though different species myxomycetes are often associated with certain species of trees, most species of protostelids can be found on a wide variety of plants.

Slime Molds in Scientific Research

Steve Stephenson examining a specimen - photograph by Randy G. Darrah (©Randy Darrah)

Remember our friend the dog-vomit slime mold? Recent research in Finland, North Korea and Russia shows that this slime mold, Fuligo septica, is able to accumulate large amounts of heavy metals with no apparent negative impact. This ability seems to be unique to this species, but how or why it does this, or why such high zinc concentrations are not toxic to Fuligo septica, are questions requiring future 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 some 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

Physarum viride - a new species discovered in Great Smoky Mountains National Park, illustration by Nancy Lowe

In 1998, researchers in Great Smoky Mountains National Park began an effort to identify all forms of life within 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 the Great Smoky Mountains National Park, and the majority of these records were based upon specimens collected more than a half century ago. Since 1998, more than 120 species have

been added to this total. The most surprising finds are four species of myxomycetes not previously known from North America and several 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 and based at the University of Arkansas, to learn more about slime molds. One project, called PEET (Partnerships for Enhancing Expertise in Taxonomy) 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 Inventory) 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 these plants. All you have to do is look and that's what this project is all about.

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

Research and Links

• The Eumycetozoan Project is the official website set up for the two National Science Foundation slime mold projects that are related to our Hands on the Land study.
• MyoxWeb has great historical information and photos.
• The Elegance of Slime Molds is a fun article about slime molds.
• ATBI in the Smokies has some photos and information about myxomycetes in the park.
• Science Daily wrote an article about the Dictyostelid slime mold being used in Alzheimer's research.
• This scientific research paper is about a particular myxomycete that seems to accumulate heavy metals at very high concentrations with no adverse effects.
• Want to see what Dog Vomit slime mold looks like? Visit this fun website by mycologist Tom Volk.

Classroom Information

• Glossary: ameoboid, dictyostelids, eukaryotic, mycologist, myxomycetes, plasmodium, protoplasm, protostelids, spores, systematist, taxonomy

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