Slime mold or mould is a broad term describing protists that use spores to reproduce. Slime molds were formerly classified as fungi, but are no longer considered part of this kingdom.[1]
Their common name refers to part of some of these organisms' life cycles where they can appear as gelatinous "slime". This is mostly seen with the myxomycetes, which are the only macroscopic slime molds.
Slime molds have been found all over the world and feed on microorganisms that live in any type of dead plant material. For this reason, these organisms are usually found in soil, lawns, and on the forest floor, commonly on deciduous logs. However, in tropical areas they are also common on inflorescences, fruits and in aerial situations (e.g., in the canopy of trees) and also grow in air conditioners, especially when the drain is blocked. In urban areas, they are found on mulch or even in the leaf mold in gutters. One of the most commonly encountered slime molds is the yellow Physarum polycephalum, found both in nature in forests in temperate zones, as well as in classrooms and laboratories.
Most slime molds are smaller than a few centimeters, but some species may reach sizes of up to several square meters and masses of up to 30 grams.[2] Many have striking colours such as yellow, brown and white.
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Slime molds, as a group, 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 several supergroups, none of which are included in the kingdom Fungi.
Slime molds can generally be divided into two main groups.
In more strict terms, slime molds comprise the amoebozoan group of the mycetozoans. Mycetozoa, which includes the defunct phylum Myxomycota (now Myxogastria), include the following three groups:
Even at this level of classification there are conflicts to be resolved. Recent molecular evidence shows that the first two groups are likely to be monophyletic and the protostelids however to be polyphyletic. For this reason, scientists are currently trying to understand the relationships among these three groups.
The most commonly encountered are the Myxogastria. 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 web-work of yellow threads, up to a few feet in size. Fuligo forms yellow crusts in mulch.
The Dictyosteliida, cellular slime molds, are distantly related to the plasmodial slime molds and have a very different lifestyle. 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 Protostelids have characters intermediate between the previous two groups, but they are much smaller, the fruiting bodies only forming one to a few spores.
Non-amoebozoan slime moulds include:
| Grouping | Genera | Morphology |
|---|---|---|
| Amoebozoa > Conosa > Mycetozoa |
Class Myxogastria: Cribraria, Lycogala, Tubifera, Echinostelium, Fuligo, Lepidoderma, Physarum, Comatricha, Stemonitis, Arcyria, Trichia |
Syncytial or plasmodial slime molds |
|
Class Dictyostelia: Dictyostelium, Polysphondylium, Acytostelium |
Cellular slime molds | |
|
Class Protostelia: Planoprotostelium, Protostelium |
Intermediate between myxomycetes and dictyostelids, but they are much smaller, the fruiting bodies only forming one to a few spores. | |
| Rhizaria > Cercozoa > Endomyxa | Class Phytomyxea: Lignieria, Membranosorus, Octomyxa, Phagomyxa, Plasmodiophora, Polymyxa, Sorodiscus, Sorosphaera, Spongospora, Tetramyxa, Woronina | Parasitic protists which can cause cabbage club root disease and powdery scab tuber disease. They form coenocytes but are internal parasites of plants. |
| Excavata > Percolozoa > Heterolobosea | Order Acrasida: Acrasis | Cellular slime molds which have a similar life style to dictyostelids, but their amoebae behave differently, having eruptive pseudopodia. |
| Chromalveolate > Heterokontophyta > Labyrinthulomycetes | Order Labyrinthulida: Labyrinthulids, Labyrinthula, Thraustochytrids, Aplanochytrium, Labyrinthuloides, Japonochytrium, Schizochytrium, Thraustochytrium, Ulkenia, Diplophryids, Diplophrys | Slime nets which are marine and form labyrinthine networks of tubes in which amoeba without pseudopods can travel. |
| Fonticulida | Fonticula | Cellular slime mold which forms a fruiting body in a volcano shape. |
Slime molds 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. These 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.[6]
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 micro-organism.[7] 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; they superficially look like fungi or molds but are not related to the true fungi. These sporangia will then release spores which hatch into amoebae to begin the life cycle again.[6]
In Myxomycetes, the plasmoidal portion of the life cycle only occurs after syngamy, which is the fusion of cytoplasm and nuclei of myxoamoebae or swarm cells. Therefore, all of the nuclei are diploid at this stage and mitosis occurs simultaneously throughout the organism. Myxomycete plasmodia are multinucleate masses of protoplasm that move by cytoplasmic streaming. In order for the plasmodium to move, cytoplasm must be diverted towards the leading edge from the lagging end. This process results in the plasmodium advancing in fan-like fronts. As it moves, plasmodium also gains nutrients through the phagocytosis of bacteria and small pieces of organic matter.
The Myxomycete plasmodium also has the ability to subdivide and establish separate plasmodia. Conversely, separate plasmodia that are genetically similar and compatible can fuse together to create a larger plasmodium. In the event that conditions become dry, the plasmodium will form a sclerotium, essentially a dry and dormant state. In the event that conditions become moist again the sclerotium absorbs water and an active plasmodium is restored. When the food supply wanes, the Myxomycete plasmodium will enter the next stage of its life cycle forming haploid spores, often in a well-defined sporangium or other spore-bearing structure.
Slime molds show interesting behaviour. When a slime mold mass or mound is physically separated, the cells find their way back to re-unite. Studies on Physarum have even shown an ability to learn and predict periodic unfavorable conditions in laboratory experiments (Saigusa et al 2008)[8] [9] Professor John Tyler Bonner, who has spent a lifetime studying slime molds argues that Slime molds are "no more than a bag of amoebae encased in a thin slime sheath, yet they manage to have various behaviours that are equal to those of animals who possess muscles and nerves with ganglia -- that is, simple brains."
Atsushi Tero of Hokkaido University grew the slime mold Physarum polycephalum in a flat wet dish. Around its initial position representing Tokyo, he placed oat flakes corresponding to the locations of other major cities in the Greater Tokyo Area. As Physarum avoids bright light, light was used to simulate mountains, water and other obstacles. The mold first densely filled the space with plasmodia, then thinned the network to focus on efficiently-connected branches. The network strikingly resembled Tokyo's rail system.[10]
| Wikisource has the text of The New Student's Reference Work article Myxomycetes. |
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