Slime mold

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

• Myxogastria/Myxomycetes: plasmodial or coenocytic slime molds.
• Protostelia : smaller plasmodial slime molds.
• Dictyosteliida : cellular slime molds.
• Acrasidae : similar life style to Dictyostelids, but of uncertain taxonomy.
• Plasmodiophorids : cabbage club root disease.
• Labyrinthulomycetes : slime nets.

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 become 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.

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 Migration of the plasmodium is accomplished when more protoplasm streams to advancing areas and protoplasm is withdrawn from rear areas.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

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History

The Slime mold Lycogala epidendrum, then called Simonus Fungi cito crescentes, was first described in the year 1654. In 1753 Linnaeus described five separate species, laying the foundation for the nomenclature of slime molds. Slime molds were originally considered fungi by mycologists and amoebae by zoologists, respectively classified as Myxomycota (slime fungi) or Mycetozoa (fungus animals). Both names are still used among different groups of specialists. The first person to connect the different stages of the creature was Elias Fries; even though he still considered them to be species of fungi.

Life cycle

The plasmodial slime molds begin as amoeboid cells each with a single haploid nucleus. These may begin feeding on bacteria and multiply. Under very moist conditions they may convert into biflagellate swarmers. If conditions become poor the unicellular amoebae can form walled off dormant resistant cells called cysts. Pairs of amoebas or swarmers can fuse and become zygotes if they are of different mating types.

Most organisms reproduce by cell fission when their cells reach a certain size, that is, the nucleus divides by mitosis and the cell divides with it to result in two smaller daughter cells each with their own nucleus. This is what haploid slime mold amoebae do. However the diploid zygotes act differently from most other organism. Instead of cell divisions, myxomycete zygotes just grow bigger; the nucleus divides by mitosis, but the cell does not divide with it, resulting in a new larger cell with two nuclei. This is called a coenocyte (or plasmodium). As the coenocyte grows, the nuclei keep dividing. In some species the plasmodium will be centimeters in size with thousands of nuclei.

This plasmodium spreads out like an amoeba and migrates slowly at about 1 cm. per hour. It continues to grow and feed on various microorganisms in the rotting vegetable matter the slime molds live on. When environmental conditions change or food runs out, the slime mold changes behavior. It migrates to the outer surface of a rotting log if that's where it is living or it migrates up grass blades, etc. It then transforms into fruiting bodies or sporangia. In different species these take on many forms from amorphous blobs to delicate lacy structures.

First, the nuclei undergo meiosis and become haploid. Then, cell membranes finally form around each of these nuclei, one to a cell, to form normal cells again. Other structures like stalks and hydroscopic fibers which help to release the spores. Finally the cells transform into spores, the structure dries out and the spores can be released into the air to travel to new substrates. These spores are haploid like the original amoebae from which the slime mold started.

These spores can then germinate. They will either form new amoebae to begin the cycle over again, or they may form biflagellate swarming cells if free water is present. These cells (still haploid) can serve as gametes and initiate the diploid plasmodial stage of the life cycle.

Under dry conditions, the plasmodia can also form resting structures called sclerotia, which then begin growing again when moist conditions come back.

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 bright colors such as yellow, brown, and white.

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 these 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 amoeba do not form huge coenocytes and remain individual. They live in similar habitats and also 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 similar life style 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 labyrynthine networks of tubes in which amoebae without pseudopods can travel.

Evolutionary origins

It now appears that the protostelids gave rise to both the myxogastrids and dictyostelids.

Utility in research

  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 which surround mitosis since all the nuclei in a medium sized plasmodium divide in syncrony. 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 an easy evaluation of complete cell fusion 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.

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. More recently, in the popular RPG NetHack, "slime mold" is the default name of a sought-after and delicious 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, that is 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 led 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 where he speaks of slime molds, saying "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, 4^th 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.
• 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).

• Slime Molds
• Slime Mould Solves Maze Puzzle from abc.net.au
• Hunting Slime Molds from Smithsonian Magazine
• Robot Piloted by a Slime Mold. Slashdot (2006). Retrieved on February 15, 2006.
• dictyBase is an online informatics resource for Dictyostelium, a cellular slime mould.
• nomen.eumycetozoa.com is an online nomenclatural information system of slime moulds (Myxomycetes, Dictyostelids and Protostelids) of the world.
• good photo gallery