Slime Molds

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Slime molds

Slime molds are organisms in two taxonomic groups, the cellular slime molds (Phylum Acrasiomycota) and the plasmodial slime molds (Phylum Myxomycota). Organisms in both groups are eukaryotic (meaning that their cells have nuclei) and are fungus-like in appearance during part of their life cycle. For this reason, they were traditionally included in mycology textbooks. However, modern biologists consider both groups to be only distantly related to the fungi . The two groups of slime molds are considered separately below.

Species in the cellular slime mold group are microscopic during most stages of their life cycle, when they exist as haploid (having one copy of each chromosome in the nucleus ), single-celled amoebas. The amoebas typically feed on bacteria by engulfing them, in a process known as phagocytosis , and they reproduce by mitosis and fission. Sexual reproduction occurs but is uncommon. Most of what we know about this group is from study of the species Dictyostelium discoideum. When there is a shortage of food, the individual haploid amoebas of a cellular slime mold aggregate into a mass of cells called a pseudoplasmodium. A pseudoplasmodium typically contains many thousands of individual cells. In contrast to the plasmodial slime molds, the individual cells in a pseudoplasmodium maintain their own plasma membranes during aggregation. The migrating amoebas often form beautiful aggregation patterns, which change form over time.

After a pseudoplasmodium has formed, the amoebas continue to aggregate until they form a mound on the ground surface. Then, the mound elongates into a "slug." The slug is typically less than 0.04 in (1 mm) in length and migrates in response to heat, light, and other environmental stimuli.

The slug then develops into a sporocarp, a fruiting body with cells specialized for different functions. A sporocarp typically contains about 100,000 cells. The sporocarp of Dictyostelium is about 0.08 in (2 mm) tall and has cells in a base, stalk, and ball-like cap. The cells in the cap develop into asexual reproductive spores, which germinate to form new amoebas. The different species of cellular slime molds are distinguished by sporocarp morphology.

Dictyostelium discoideum has been favored by many biologists as a model organism for studies of development, biochemistry , and genetics. Aspects of its development are analogous to that of higher organisms, in that a mass of undifferentiated cells develops into a multicellular organism, with different cells specialized for different functions. The development of Dictyostelium is much easier to study in the laboratory than is the development of higher organisms.

A food shortage induces aggregation in Dictyostelium. In aggregation, individual amoebas near the center of a group of amoebas secrete pulses of cAMP (cyclic adenosine-3'5'-monophosphate). The cAMP binds to special receptors on the plasma membranes of nearby amoebas, causing the cells to move toward the cAMP source for about a minute. Then, these amoebas stop moving and in turn secrete cAMP, to induce other more distant amoebas to move toward the developing aggregation. This process continues until a large, undifferentiated mass of cells, the pseudoplasmodium, is formed.

Interestingly, cAMP is also found in higher organisms, including humans. In Dictyostelium and these higher organisms, cAMP activates various biochemical pathways and is synthesized in response to hormones, neurotransmitters, and other stimuli.

The plasmodial slime molds are relatively common in temperate regions and can be found living on decaying plant matter. There are about 400 different species. Depending on the species, the color of the amorphous cell mass, the plasmodium , can be red, yellow, brown, orange, green, or other colors. The color of the plasmodium and the morphology of the reproductive body, the sporocarp, are used to identify the different species.

The plasmodial slime molds are superficially similar to the cellular slime molds. Both have a haploid amoeba phase in when cells feed by phagocytosis, followed by a phase with a large amorphous cell mass, and then a reproductive phase with a stalked fruiting body.

However, the plasmodial slime molds are distinguished from the cellular slime molds by several unique features of their life cycle. First, the germinating spores produce flagellated as well as unflagellated cells. Second, two separate haploid cells fuse to produce a zygote with a diploid nucleus. Third, the zygote develops into a plasmodium, which typically contains many thousands of diploid nuclei, all surrounded by a continuous plasma membrane.

The cytoplasm of the plasmodium moves about within the cell, a process known as cytoplasmic streaming. This is readily visible with a microscope . The function of cytoplasmic streaming is presumably to move nutrients about within the giant cell.

In nature, plasmodial slime molds grow well in wet and humid environments, and under such conditions the plasmodium of some species can be quite large. After a particularly wet spring in Texas in 1973, several residents of a Dallas suburb reported a large, moving, slimy mass, which they termed "the Blob." One reporter in the local press speculated that the Blob was a mutant bacterium, able to take over the earth. Fortunately, a local mycologist soberly identified the Blob as Fuligo septica, a species of plasmodial slime mold.

Another plasmodial slime mold, Physarum polycephalum, is easily grown in the laboratory and is often used by biologists as a model organism for studies of cytoplasmic streaming, biochemistry, and cytology. The plasmodium of this species moves in response to various stimuli, including ultraviolet and blue light. The proteins actin and myosin are involved in this movement. Interestingly, actin and myosin also control the movement of muscles in higher organisms, including humans.

See also Mycology

Slime Molds

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Slime Molds

There are two major unrelated phyla of slime molds. The Myxomycota are the true (plasmoidal) slime molds, and the Dictyosteliomycota are the cellular slime molds. Both were formerly classified as fungi but are now considered protists. Slime molds are often found on old, well-rotted logs because there they can find the moisture and bacteria required for survival. Their small, delicate fruiting bodies tend to be fungal in appearance. Most of the fruiting bodies are only a millimeter or two in height, and therefore often difficult to notice.

Myxomycota

A myxomycete exists in nature as a plasmodium, a multinucleate blob of protoplasm up to several centimeters in diameter, without cell walls and only a cell membrane to keep everything in. It resembles a large amoeba and feeds much the same way, by engulfing its food (mostly bacteria) with pseudopodia ("false feet"), in a process called phagocytosis . Thus the slime mold ingests its food and then digests it. (In contrast, true fungi have cell walls and digest their food externally before ingesting it.) When the plasmodium runs out of food, or environmental conditions become harsh, fruiting bodies form. These fruiting bodies produce dormant, resistive spores. These later germinate to form uninucleate myxamoebae or flagellated swarm cells. These later fuse and then divide mitotically to form a plasmodium, completing the life cycle. Myxomycetes are important scavengers in dark, damp parts of the ecosystem . Occasionally, during rainy periods, large plasmodia (up to a few meters in diameter) crawl out of the woods and into people's lawns and gardens. These plasmodia were the inspiration for the science fiction movie The Blob and are eaten in parts of Mexico.

Dictyosteliomycota

The Dictyosteliomycota are also known as the social amoebae. Their life cycle is considered among the most bizarre among microorganisms. It begins with free-living amoeboid cells (not to be confused with the Amoebae); there is no true plasmodium. As long as there is enough food (usually bacteria) the amoebae thrive. However, when food runs out, the amoebae send out chemical signals to surrounding amoebae. Next, they stream toward a central point and form a sluglike multicellular pseudoplasmodium, which can then migrate like a single organism. When conditions are right, the pseudoplasmodium stops migrating and forms a multicellular fruiting body. Some of the cells become spores that disseminate, while the rest form stalk cells whose only function is to raise the spores up into the air to be more easily caught in air currents.

The Dictyosteliomycota pose an interesting challenge for evolutionary theory, since some of the cells (in the stalk) actually seem to sacrifice their own reproductive potential so that others (the spores) can be transported to a new location where there is more food and they can grow again. This altruistic sacrifice would seem to be counter to the reproductive interests of the cells that became the stalk (because they never reproduce) and genes for stalk-forming behavior would therefore be selected against. It may be maintained if the spore cells are closely related to the stalk cells (and thus both have the stalk-forming genes) or if the allocation of cells to spore versus stalk is random, so that genes for stalk formation are preserved over time. However, evidence suggests that the position of the cells in the slug and thus in the fruiting body is determined by the timing of their coming into the aggregation stream, rather than by genetics.

see also Endocytosis; Fungi; Protista; Sociobiology

Tom Volk

Bibliography

Farr, M. L. How to Know the True Slime Molds. Dubuque, IA: William C. Brown Publishers, 1981.

Keller, Harold W., and Karl L. Braun. Myxomycetes of Ohio: Their Systematics, Biology and Use in Teaching. Columbus, OH: Ohio Biological Survey, 1999.

Raper, Kenneth B. The Dictyostelids. Princeton, NJ: Princeton University Press, 1984.

Stephenson, Steven, and Henry Stempen. Myxomycetes: A Handbook of Slime Molds. Portland, OR: Timber Press, 1994.

Volk, Thomas J. Tom Volk's Fungi. <http://www.wisc.edu/botany/fungi/volkmyco.html>.

Slime Molds

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Slime Molds

Resources

Slime molds are microscopic organisms that are eukaryotic; they have their genetic material contained within a membrane inside the cell. Once thought to be fungi, slime molds are now recognized to be very different from fungi. Indeed, slime molds are now classified as one of the five main divisions of life (the other four are fungi, bacteria, plants, and animals).

There are three main groups of slime molds. The first group is known as the plasmodial slime molds, or Myxomycetes. These slime molds can exist as cells that appear similar to amoeba, and which are able to move to find food. A common habitat for these cells is underneath rotting logs and damp leaves, where the cellulose that the cells use for food is abundant. These cells can move to an environment that is drier and has more light, where they then fuse together to form an enormous single cell that contains thousands of nuclei. This form, called a pseudoplasmodium, can ooze about seeking a region of acceptable warmth and brightness. Then, the aggregate settles to form a plasmodium. A plasmodium can be several inches in diameter and is often vividly colored.

Scientists use plasmodia to study a phenomenon called cell streaming, where the contents of a cell move about. The large size of a plasmodium and the fact that cell streaming is readily visible using a low-power magnification light microscope, makes this slime mold a good choice for a model system. Another plasmodial slime mold, Physarum polycephalum, moves in response to various stimuli including ultraviolet and blue light. The proteins actin and myosin are involved in this movement. Actin and myosin also control the movement of muscles in higher organisms, including humans.

The second group of slime molds are known as the cellular slime molds. These are typically single-celled. In response to a chemical signal, however, the cells can aggregate (gather) to form a great swarm of cells. This aggregation is of intense interest to scientists who study the physical and genetic development of cells.

The final group of slime molds are called the protostelids.

All three types of slime molds are capable of forming a structure called a sporangium. This structure is formed when conditions are unfavorable for the growth or survival of the slime mold. A sporangium is a cluster of spores on a stalk. Each spore is a bundle of genetic information. Dispersal of the spores by air currents can lead to the formation of new slime molds when the spores land and germinate.

Besides their complex life cycle and scientific interest as model system for study, slime molds have been noteworthy for other reasons. After a particularly wet spring in Texas in 1973, several residents of a Dallas suburb reported a large, moving, slimy mass, which they termed the Blob. Reporters in the local press speculated that the Blob was a mutant bacterium. Fears of an alien invasion also were raised. Ultimately, however, a local mycologist soberly identified the growth as Fuligo septica, a species of plasmodial slime mold.

Additionally, researchers were later able to formulate mathematical equations that explained the single cell to aggregate process of cellular slime molds. The slight modification of these equations formed the basis of the programs that are now used to control some of the behaviors of the figures in video games.

Resources

BOOKS

Kratz, Rene. Microbiology the East Way. New York: Barrons Educational Series, 2005.

Prescott, Lansing M., John P. Harley, and Donald A. Klein. Microbiology. New York: McGraw-Hill, 2004.

Tortora, Gerard J., Berdell R. Funke, and Christine L. Case. Microbiology: An Introduction. 9th ed. New York: Benjamin Cummings, 2006.

Slime Molds

views updated

Slime molds

Slime molds are microscopic organisms. As slime molds are eukaryotic organisms, they have their genetic material contained within a membrane inside the cell . Once thought to be fungi , slime molds are now recognized to be very different from fungi. Indeed, slime molds are now classified as one of the five main divisions of life (the other four are fungi, bacteria , plants, and animals).

There are three main groups of slime molds. The first group is known as the plasmodial slime molds, or Myxomycetes. The slime molds can exist as cells that appear similar to amoeba , and which are able to move to find food. A common habitat for these cells is underneath rotting logs and damp leaves, where the cellulose that the cells use for food is abundant. These cells can move to an environment that is drier and has more light, where they then fuse together to form an enormous single cell that contains thousands of nuclei. This form, called a pseudoplasmodium, can ooze about seeking a region of acceptable warmth and brightness. Then, the aggregate settles to form a plasmodium. A plasmodium can be several inches in diameter and is often vividly colored.

Scientists use plasmodia to study a phenomenon called cell streaming, where the contents of a cell move about. The large size of a plasmodium and the fact that cell streaming is readily visible using a low-power magnification light microscope , makes this slime mold a good choice for a model system. Another plasmodial slime mold, Physarum polycephalum, moves in response to various stimuli including ultraviolet and blue light. The proteins actin and myosin are involved in this movement. Actin and myosin also control the movement of muscles in higher organisms, including humans.

The second group of slime molds are known as the cellular slime molds. These are typically single-celled. In response to a chemical signal, however, the cells can aggregate to form a great swarm of cells. This aggregation is of intense interest to scientists who study the physical and genetic development of cells.

The final group of slime molds are called the protostelids.

All three types of slime molds are capable of forming a structure called a sporangium. This structure is formed when conditions are unfavorable for the growth or survival of the slime mold. A sporangium is a cluster of spores on a stalk. Each spore is a bundle of genetic information. Dispersal of the spores by air currents can lead to the formation of new slime molds when the spores land and germinate.

Besides their complex life cycle and scientific interest as model system for study, slime molds have been noteworthy for other reasons. After a particularly wet spring in Texas in 1973, several residents of a Dallas suburb reported a large, moving, slimy mass, which they termed "the Blob." Reporters in the local press speculated that the Blob was a mutant bacterium. Fears of an alien invasion also were raised. Ultimately, however, a local mycologist soberly identified the growth as Fuligo septica, a species of plasmodial slime mold.

Additionally, researchers were later able to formulate mathematical equations that explained the single cell to aggregate process of cellular slime molds. The slight modification of these equations formed the basis of the programs that are now used to control some of the behaviors of the figures in video games.

See also Microorganisms; Nucleus, cellular.


Resources

books

Alexopoulos, C.J, C.W. Mims, and M. Blackwell. Introductory Mycology. 4th ed. New York: John Wiley, 1996.

periodicals

Conover, A. "Hunting Slime Molds: They're Not Animals and They're Not Plants, and Biologists Want to Know a Lot More About Them." Smithsonian March 2001: 26–30.

slime moulds

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slime moulds Small simple organisms widely distributed in damp habitats on land. They exist either as free cells (myxamoebas) or as multinucleate aggregates of cells (see plasmodium), depending on the stage of the life cycle. In the plasmodial (or true) slime moulds the nuclei of the plasmodia are not separated by plasma membranes; in cellular slime moulds the myxamoebas retain their plasma membranes after aggregating to form a ‘pseudoplasmodium’. Cell walls are generally absent. Slime moulds show amoeboid movement and feed by ingesting small particles of food. They reproduce by means of spores. Slime moulds were formerly classified as fungi. They are now usually classified in two phyla, Myxomycota (plasmodial slime moulds) and Rhizopoda (cellular slime moulds plus other amoebas), of the kingdom Protoctista.

slime mould

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slime mould A type of eukaryotic micro-organism in which either a plasmodium or pseudoplasmodium is formed. Slime moulds are often included in mycological classification schemes, although probably they are not related to fungi; they are also included in zoological classification schemes, either as protozoa or as a separate phylum (Gymnomyxa, equivalent to the botanical Myxomycota).

slime mould

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slime mould Any of a small group of strange, basically single-celled organisms that are intermediate between the plant and animal kingdoms. During their complex life cycle, they pass through several stages. These include a flagellated swimming stage, an amoeba-like stage, a stage consisting of a slimy mass of protoplasm with many nuclei, and a flowering, sporangium stage.