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Slime mold

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Slime mold
StemonitisPlasmodium.jpg
SCIENTIFIC CLASSIFICATION
Mycetozoa- plasmodial slime molds
Acrasiomycetes- cellular slime molds
Labyrinthulomycota - slime nets

Alternate Classification of Labyrinthulomycetes:

Plasmodiophorids- obligate parasites
Mycetozoa Haeckel.JPG
Haeckel's Kunstformen der Natur

Slime molds can refer to several groups of different and controversial classifications. They have certain characteristics similar to those of fungi, plants, and animals. Their reproduction includes the production of spores similar to that of fungi and some plants, but, like animals, slime molds move (although very slowly) and ingest their food [1]. These amazing organisms can be found almost anywhere, and the reproductive or fruiting phase of their life cycle, when the delicately stalked and often beautifully colored sporangia form, is the one most often observed. Another amazing feature possessed by plasmodial slime molds is that they are the largest unicellular organisms. Physarum polycephalum, for example, can grow to a size of 20 cm in diameter, but it is still a single cell [2]. Slime molds may be seen in forests and gardens all over the world and are found wherever decaying organic matter is present, such as on rotting wood and leaves. Generally, slime molds are divided into two groups: plasmodial or true slime molds and cellular slime molds.

Contents

Anatomy

This slime mold plasmodium is growing on a leaf.

There are four identified groupings of slime molds: the Myxomycota, the Acrasiomycetes, the Labyrinthulomycetes, and the Plasmodiophorids. Of the four groups, Myxomycota is the largest with about 1000 species [3]. Acrasiomycetes is the next largest group, with about 50 species [4]. These two groups comprise the plasmodial or true slime molds and cellular slime molds. Plasmodial slime molds include two subgroups of Mycetozoa: Myxomycetes and Protostelids. The third group of Mycetozoa (the Dictosteliids) and Acrasiomycetes make up the cellular slime molds.

Both plasmodial and cellular slime molds have a motile phase when growth and ingestion of food occurs and an immotile reproductive phase, and they differ mainly in their motile phase. Plasmodial slime molds begin as gamete cells that are either flagellated or amoeboid that fuse together and form a zygote [5]. The zygote's nucleus divides, but no cell walls form, resulting in a single-celled, multinucleate plasmodium that grows as the organism feeds and the nuclei continue to divide [6]. The plasmodium moves in amoeboid fashion using cytoplasmic streaming in order to find favorable conditions and food, and may move several feet in a single day [7]. The plasmodium continues to feed as long as conditions are good, but when food runs short or its habitat becomes too dry, the plasmodium changes into a fruiting body, the next phase in its life cycle. It hardens and produces stalked sporangia that contain spores, often after moving to a drier or better-lit location [8]. The spores are released and will develop into gametes to begin the life cycle again [9].

When slime molds experience very harsh conditions such as extremely cold or dry weather, they may harden into a sclerotium that can live for several years and then return to a plasmodium when conditions are favorable [10].

Cellular slime molds differ from plasmodial slime molds in that they are single-celled amoebas with only one nucleus in their motile phase. They move about like the plasmodial slime molds, ingesting food until the supply runs out. When there is no more food, all the slime mold amoebas in an area congregate and form a pseudoplasmodium, which is their fruiting body. The pseudoplasmodium becomes a stalked sporangium that releases spores which will germinate and restart the life cycle [11].

The two other groups of slime molds (Labyrinthulomycetes and Plasmodiophorids) are very small and some do not consider them to be slime molds at all. The Labyrinthulomycetes, or slime nets, are (usually) marine protists, sometimes parasitic, that grow as microscopic "nets" of branching filaments on algae, other marine grasses and plants, and on or in marine animals [12] [13]. The parasitic forms are responsible for some diseases of eelgrass and marine animals, but Labyrinthulomycetes also play a role in decomposition of dead organic matter in their habitats [14]. The Plasmodiophorids are obligate parasites that grow as naked protoplasms in plants, algae, fungi, and other small organisms and are responsible for certain diseases in plants such as cabbage clubroot disease [15].

Slime molds range in size from a few centimeters to a foot or more in diameter [16]. They come in a variety of colors, ranging from dark browns, blacks, and purples, to bright yellows, oranges, or reds, while others may be gray or white [17]. A few tropical species are even bioluminescent (glow in the dark) [18].

Reproduction

a plasmodium forming sporangia
Individual cellular slime mold amoebas aggregating to form a multicellular "slug" for reproduction

Plasmodial slime molds reproduce sexually. Reproduction occurs during the immotile phase. In plasmodial slime molds, the plasmodium moves to a dry, well-lit area and hardens. Reproduction begins as asexual with the production of spores in frutifications that take on one of four forms. The most common form is the sporangium, which is usually stalked and ends in a capsule containing the spores [19]. Other forms are the aethelium, a flat mass of fused sporangia; the pseudoaethalium, where the sporangia are very close together; and the plasmodicarp, which appears branched or veined [20]. The base of the fruiting body is the hypothallus [21]. In sporangia, the spores are in a capsule at the end of a stalk, but other fruiting bodies do not have stalks. The portion of the stalk that extends into the peridium (the capsule containing the spores) is the columella [22]. Inside the peridium, spores are connected to the capillitium, a tiny, thread-like network that runs throughout the peridium [23]. The spores are released from the peridium and scattered by the wind or other factors. When one reaches a damp, cool environment, the spore absorbs water, the cellulose wall breaks open, and a single, naked cell emerges [24]. This cell, a gamete, may have a flagellum depending on the level of moisture (cells released into aquatic environments will be flagellated, while those in drier habitats will not have a flagellum) [25]. The gametes feed for a short time, then merge to form a zygote. Flagellated gametes (called swarm cells) join with other flagellated gametes, and amoeboid gametes fuse with other amoeboid gametes [26]. The merging of gametes to form a zygote makes the reproduction of plasmodial slime molds sexual rather than asexual. The zygote continues to feed, and its nucleus divides as it grows into a plasmodium. However, no cell walls form, resulting in a large, single-celled organism that may have millions of nuclei [27].

Cellular slime molds live as individual cells that join together to form the reproductive structure. They can reproduce sexually or asexually [28]. In asexual reproduction, the individual cells release cAMP, a signaling molecule, when they are starving [29]. As other slime mold cells cross the cAMP tracks made by the starving amoebas, they also begin to produce cAMP and move to the highest concentration of cAMP, where all the other amoebas have gone [30]. Once they have aggregated, the amoebas fuse together into a "slug" called the pseudoplasmodium that moves around for a short time, eating and looking for a place to form the fruiting body [31]. Like the plasmodial slime mold, the pseudoplasmodium looks for a light, dry area to form the frutification, which is called a sorocarp and consists of a sorophore (the stalk) and a sorus (the cluster of spores) [32]. As many as 125,000 amoeboid cells may join together, each becoming a different part of the fruiting body [33]. Anterior cells become the base and stalk, while posterior cells become the spores, which will germinate and become the next generation of amoebas [34].

Cellular slime molds may also reproduce sexually. Two amoebas will fuse together and begin engulfing other amoebas, forming a macrocyst that divides through meiosis and mitosis to produce more amoebas [35].

Ecology

This slime mold, Tubifera ferruginosa, eats decaying matter on a rotting log and has moved to the top of the log to reproduce

Slime molds live in dark, moist habitats where there is abundance of food. They eat bacteria, protozoans, yeasts, fungi, decaying organic materials, and other microorganisms [36] [37]. They are most often found in forests and lawns, under rotting logs and leaves [38]. Cellular slime molds also live in moist soil or manure [39]. Both cellular and plasmodial slime molds move across their habitats using amoeboid movement, ingesting food by the process of phagocytosis, a form of endocytosis [40]. Slime molds may also use chemotaxis, following the chemical gradient given off by their food sources, to find food [41]. Chemotaxis is also used by cellular slime molds when they aggregate, following the chemical gradient of cAMP [42].

Slime molds are frequently found in moist areas of gardens, such as bark or mulch beds. They may occasionally grow over garden plants as well [43]. Although they do not directly harm living, healthy plants, slime molds do use them for structural support, and a large plasmodium may block sunlight from a plant and prevent photosynthesis from occurring [44]. A few slime molds do cause disease in plants, such as cabbage clubroot disease, which is caused by Plasmodiophora brassicae, a Plasmodiophorid (obligate parasite) [45]. Gardeners can get rid of slime molds by raking, mowing, or by spraying them forcefully with a garden hose (however, this may also cause the plasmodium to grow since they thrive in damp environments) [46]. If they are not damaging plants, slime molds may be allowed to disappear naturally (they will die off when the weather becomes drier) [47].

Slime molds are found in late summer and early fall [48] [49]. Most species of slime mold can be found all over the world, especially in forests and gardens [50]. The reproductive phase is the one most often observed because the slime molds move to a better lit and thus more visible location to form their frutifications.

Slime Molds Solve Mazes and Control Robots

Three Japanese scientits were awarded the Ig Nobel Prize for Cognitive Science in 2000 for the discovery that a plasmodial slime mold, Physarum polycephalum, was able to find the shortest path between two food sources in a maze [51]. They placed pieces of the slime mold into a 30-square-centimeter maze with an agar base (agar is a gelatinous substance made from seaweed) and waited until its pseudopodia had grown to fill the maze [52]. The maze had four routes through, and pieces of food were placed at each of the two exits [53]. Over a period of about half a day, the slime mold retracted the pseudopodia that did not lead to food, until it was only on the shortest path between the two food sources [54]. The three scientists, Toshiyuki Nakagaki, Hiroyasu Yamadam, and Ágota Tóth, concluded that the slime mold was able to change shape to forage more efficiently, possibly evidencing a form of unicellular intelligence [55]. The article that won them the prize is available here upon subscription.

Klaus-Peter Zauner from the University of Southampton, UK, along with associates from Kobe University in Japan, created a six-legged robot that is controlled by the same slime mold studied in the maze experiment, Physarum polycephalum. Zauner grew the slime mold in a six-pointed star shape and connected it to the circuits of the robot, with one point controlling each leg [56]. The slime mold, which spends most of its life in dark areas under logs and leaves, moves naturally away from light. When researchers shone a white light on one of the points, the slime mold would react by moving away and the corresponding leg of the robot would also move [57]. Shining light on points of the slime mold in an ordered way caused the robot to walk [58]. The use of biological cells in robots could lead to autonomous robots able to solve unexpected complex situations. Living organisms are able to solve problems that would trap a robot controlled by a computer program [59]. In addition, cells are able to repair and restructure themselves, so a machine controlled by cells would have some potential for self-repair [60].

In addition to these two experiments, scientists have been able to gain valuable information about how cells perform their life processes by studying slime molds. Since the plasmodial slime mold is one giant cell, it is much easier to see their processes taking place. They have been helpful to researchers studying mitosis, as well as cytoplasmic streaming.

Taxonomy

Because of their shared characteristics with animals, plants, and fungi, the classification of the various groups of slime molds is still subject to debate and revision. Although previously classified in the fungi and plant kingdoms, the four groups are currently classified under four different supergroups and their place in the taxonomic heirarchy is unclear [61]. Information in the taxonomy box has been obtained from the most current available sources and is not universally agreed upon by scientists. Classification of slime molds will continue to change as scientists study the relationships between the groups. Taxonomy on this page was taken from the following sources:

Myxomycetes Gallery

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References

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