Definitions

true slime mold

Slime mold

Slime Mold is a broad term referring to amoeba-like organisms, hence the name slime, which feed on microorganisms in decaying vegetable matter, hence the name molds. They can be found in the soil, on lawns, and in the forest commonly on deciduous logs. They are also common on mulch or even in the leaf mold in gutters.

Taxonomy

Slime molds are polyphyletic. They were originally represented by the subkingdom Gymnomycota in the Fungi kingdom and included the defunct phyla Myxomycota, Acrasiomycota and Labyrinthulomycota.Today slime molds have been divided between four supergroups and paradoxically none of them include Fungi. These are:

The mycetozoan groups all fit into the unikont supergroup Amoebozoa, whereas the others fit into various bikont groups.

Life cycle

They begin life as amoeba-like cells. These unicellular amoebae are commonly haploid and multiply if they encounter their favorite food, bacteria. These amoebae can mate if they encounter the correct mating type and form zygotes which then grow into plasmodia which contain many nuclei without cell membranes between them, which can grow to be meters in size. One variety is often seen as a slimy yellow network in and on rotting logs. The amoebae and the plasmodia engulf microorganisms. The plasmodium grows into an interconnected network of protoplasmic strands (Ling, 1999).

Within each protoplasmic strand the cytoplasmic contents rapidly stream. If one strand is carefully watched for about 50 seconds the cytoplasm can be seen to slow, stop, and then reverse direction. The streaming protoplasm within a plasmodial strand can reach speeds of up to 1.35 mm per second which is the fastest rate recorded for any organism (Alexopoulos, 1962). Migration of the plasmodium is accomplished when more protoplasm streams to advancing areas and protoplasm is withdrawn from rear areas. When the food supply wanes, the plasmodium will migrate to the surface of its substrate and transform into rigid fruiting bodies. The fruiting bodies or sporangia are what we commonly see, superficially look like fungi or molds but they are not related to the true fungi. These sporangia will then release spores which hatch into amoebae to begin the life cycle again (Ling, 1999).

Types of slime mold

Most slime mold are smaller than a few centimeters, but the very largest reach areas of up to thirty square meters, making them the largest undivided cells known. Many have striking colors such as yellow, brown and white.

Slime molds can generally be divided into two main groups. A plasmodial slime mold involves numerous individual cells attached to each other, forming one large membrane. This "supercell" is essentially a bag of cytoplasm containing thousands of individual nuclei. By contrast, cellular slime molds spend most of their lives as individual unicellular protists, but when a chemical signal is secreted, they assemble into a cluster that acts as one organism.

A common slime mold which forms tiny brown tufts on rotting logs is Stemonitis. Another form which lives in rotting logs and is often used in research is Physarum polycephalum. In logs it has the appearance of a slimy webwork of yellow threads, up to a few feet in size. Fuligo forms yellow crusts in mulch.

The Protostelids' life cycle is very similar to the above descriptions, but they are much smaller, the fruiting bodies only forming one to a few spores.

The Dictyosteliida, cellular slime molds, are distantly related to the plasmodial slime molds and have a very different life style. Their amoebae do not form huge coenocytes, and remain individual. They live in similar habitats and feed on microorganisms. When food runs out and they are ready to form sporangia, they do something radically different. They release signal molecules into their environment, by which they find each other and create swarms. These amoeba then join up into a tiny multicellular slug-like coordinated creature, which crawls to an open lit place and grows into a fruiting body. Some of the amoebae become spores to begin the next generation, but some of the amoebae sacrifice themselves to become a dead stalk, lifting the spores up into the air.

The Acrasidae have a life style similar to Dictyostelids, but their amoebae behave differently and are of uncertain taxonomic position.

The Plasmodiophorids also form coenocytes but are internal parasites of plants (e.g., club root disease of cabbages).

Finally, the Labyrinthulomycetes are marine and form labyrinthine networks of tubes in which amoebae without pseudopods can travel.

Slime molds in culture

Although usually overlooked, slime molds have occasionally found their way into art and literature. Traditional Finnish lore describes how malicious witches used yellow Fuligo (there called "paranvoi," or butter of the familiar) to spoil milk. In many popular roguelikes, as a hold-over from the original Rogue, "slime mold" is the default name of a food item. Whether or not most actual slime molds are delicious, or even edible, is unclear, and some may be poisonous. However, mycologist Tom Volk reports that the plasmodium of Fuligo is eaten in Mexico. The graphic novel Nausicaä of the Valley of Wind features a highly dangerous mutated slime mold that engulfs entire cities. Philip K. Dick's novel Clans of the Alphane Moon contains a character called Lord Running Clam, a "Ganymedean Slime Mold" who talks and is very intelligent and has telepathic powers. In Jeffrey Darlington's comic General Protection Fault, one character's poor hygiene leads to the development of a sentient species of slime mold in his apartment that split the rent with him. In the DVD release of This is Spinal Tap there is an outtake of an interview with David St. Hubbins in which he speaks of slime molds: "They are both plant AND animal...it's like they can't make up their mind...and, you know, they think it's them...who've been running the earth all this time."

References

  • Sleigh, Michael. "Protozoa and Other Protists". Routledge, Chapman and Hall Inc. 1989
  • Alexopolous, C.J., Charles W. Mims, M. Blackwell et al., Introductory Mycology, 4th ed. (John Wiley and Sons, Hoboken NJ, 2004) ISBN 0-471-52229-5
  • Martin, G.W and C. J. Alexopoulos. 1969. "The Myxomycota" Iowa University Press.
  • Ling, H. 1968. "Light and Fruiting in Didymium iridis" Mycologia Vol. pp 966-970.
  • Ling, H. 1999. "Myxomycetes, Commonly Overlooked Plants" The Native Plant Society of NJ Newsletter, Fall p5.
  • Alexopolous, C.J. 1962, second edition. "Introductory Mycology" John Wiley and Sons, p. 78.
  • Lister,A. 1925. "A Monograph of the Mycetozoa" Johnson Reprint Corp. NY.
  • Raper, K.B. (1984) The Dictyostelids. Princeton University Press.
  • Karling, J.S. (1968) The Plasmodiophorales. Hafner Publishing Co.
  • Bill Bryson's "A Short History of Nearly Everything".
  • Nick Arnold's "Nasty Nature" (a volume in the "Horrible Science" series).

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