Slime mold

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Slime molds
Aethalium of a slime mold (Fuligo septica)
Aethalium of a slime mold (Fuligo septica)
Scientific classification
Domain: Eukaryota
Kingdom: Amoebozoa
Phylum: Mycetozoa
Typical orders

Protostelia

Protosteliida

Myxogastria

Liceida
Echinosteliida
Trichiida
Stemonitida
Physarida

Dictyostelia

Dictyosteliida

Slime Mold (or Mycetozoa) is a broad term often referring to roughly six groups of Eukaryotes. The taxonomy is still in flux. Originally, slime molds were considered Fungi, but now they have been split into various groups:

The Myxogastria, Protosteli, and Dictyosteliida make up the group Mycetozoa. The mycetozoan groups all fit into the unikont supergroup Amoebozoa, whereas the others fit into various bikont groups. Slime molds feed on microorganisms in decaying vegetable matter. 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 leaf mold which collects in gutters.

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 Migration of the plasmodium is accomplished when more protoplasm streams to advancing areas and protoplasm is withdrawn from rear areas.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).

Contents

[edit] Types of slime mold

Mycetozoa from Ernst Haeckel's 1904 Kunstformen der Natur (Artforms of Nature)
Mycetozoa from Ernst Haeckel's 1904 Kunstformen der Natur (Artforms of Nature)

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 bright colors such as yellow, brown, and white.

Slime molds can generally be divided into two main groups. A cellular 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, plasmodial slime molds (Dictyosteliida) 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 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.

[edit] Utility in research

Slime mold on lawn, USA. Trail of movement can be seen.
Slime mold on lawn, USA. Trail of movement can be seen.

Dictyostelids are used as examples of cellular communication and differentiation, and may provide insights into how multicellular organisms develop.

Slime molds like Physarum polycephalum are useful for studying cytoplasmic streaming. They have also been used to study the biochemical events that surround mitosis, since all the nuclei in a medium-sized plasmodium divide in synchrony. It has been observed that they can find their way through mazes by spreading out and choosing the shortest path, an interesting example of information processing without a nervous system. Myxomycete plasmodia have also been used to study the genetics of asexual cell fusion. The giant size of the plasmodial cells allows for easy evaluation of complete or partial cell fusion.

In 2006, researchers at the University of Southampton and the University of Kobe reported that they had built a six-legged robot whose movement was remotely controlled by a Physarum slime mold. The mold directed the robot into a dark corner most similar to its natural habitat.

Slime molds are sometimes studied in advanced mathematics courses. Slime mold aggregation is a natural process that can be approximated with partial differential equations.

[edit] Slime molds in culture

Slime mold from Olympic National Park, USA (Possibly Physarum)
Slime mold from Olympic National Park, USA (Possibly Physarum)

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. [1] 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."

[edit] 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).

[edit] External links

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