Slime Mold

by Benn Bluestein-Veyra

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Black Slime Mold (PS) (14)

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Purple slime mold (15) (DA)

Classification/Diagnostic Characteristics

Domain: Eukarya
Kingdom: Protista
Phylum: Myxomycota/Acrasiomycota
Class: Myxomycetes (example)
Order: Physarales (example)
Family: Physaraceae (example)
Genus: Physarum (example)
Species: Polycephalum (example)

Description:
Slime molds include more than 700 different known species that fall under two encompassing phyla, Myxomycota and Acrasiomycota. Myxomycota are plasmodial slime molds, which contain multinucleate masses of cytoplasm formed by the combination of multiple amoeboid cells. Acrasiomycota, on the other hand, are cellular slime molds, which live for the majority of their lives as individual, single-celled amoeboid protists that combine into a macrocyst, or a great swarm of the individual cells, and undergoes meosis and mitosis to release haploid individuals upon the detection of a chemical signal. A similar phyla called Labyrinthulomycota, or "slime nets", are similar to slime molds but are more closely related to the Chromista rather than the main slime mold groups.

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Slime Mold phylogeny in relation to ancestral eukaryote (above)

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Slime Mold phylogeny with domain separation (above)


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Plasmodial Slime Mold (above)

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Cellular Slime Mold (above)

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Labyrinthulomycota slime net (above)



Relationship to Humans

Slime molds generally do not have any adverse effects on humans per se, other than having an unappealing physical appearance that can result in the loss of appetite. However, the Fuligo Septica plasmodial slime mold, or "dog barf" slime mold, possesses a degree of pathogenicity that can cause asthma or rhinitis, a stuffy nose.

In 2000, three scientists discovered that a plasmodial slime mold, Physarum polycephalum, could find the shortest path between two food sources in a maze. The maze had four routes with pieces of food places at the two exits. In about half a day, the slime mold changed shape until it was on the shortest path between the two food sources. The scientists concluded that slime mold can change shape to forage more efficiently, which could mean that it has a form of unicellular intelligence. (1)(JLau) (See Nutrient Acquisition for more details)

The Tokyo railway system was built to become the most efficient transport system between Tokyo and its surrounding cities. Scientists built a physical model of Tokyo's landscape using oat flakes and set the visible slime mold Physarum polycephalum in the central station location. The mold grew in lines and expanded from the center location to cover the largest areas in the shortest distances, in a strikingly similar pattern to the Tokyo railway system. In the future, scientists hope to utilizing the slime mold and its abilities mentioned above to map out template for building railways and similar structures and projects in the future. [9](AY)

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Slime mold growing in the corner of a rundown public restroom (13) (BH)
Slime mold growing in the corner of a rundown public restroom (13) (BH)



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Physarum polycephalum (11)(WSS)
Fuligo Septica (above)

Habitat and Niche

Generally, slime molds grow best in moist areas(13) (BH) that contain vegetation. Many find homes under rotting logs and fallen leaves, while others can survive in places that vary from aquatic habitats to the sides of buildings. As decomposers, the highest populations are found around dead organic material to decompose the dead organisms into organic compounds. Slime molds are a vital part of nutrient cycles in ecosystems across the world since the organic compounds that they produce are released into the soil to provide nutrients for plants.
Slim molds live under rotting logs and damp leaves. Some of the other habitats that slime molds will inhabit include water such as tropical fish tanks. They also live on lawns, in wood chips, garden beds, and sometimes even the sides of buildings. (HSC)

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Wolf's Milk slime mold growing on a log (above in yellow)


Predator Avoidance

Though certain species may try to consume the slime molds, the organism produces many spores to compensate for the cells that are lost due to organisms feeding upon the organisms. For example, when a fly lays its eggs upon the slime mold, spores continue to be released and the organism continues to survive although the hatchling maggots are consuming the slime. It is extremely hard to exterminate a motile slime mold colony since the plasmodial strands continue to grow and rejoin the separated sections no matter how many parts are severed or crushed.
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Nutrient Acquisition


Plasmodial slime molds engulf food particles through endocytosis as they slowly move. This type of slime mold primarily consumes bacteria, yeasts, spores of fungi, decaying animals and plants, as well as other small organisms.
Cellular slime molds act like a single "slug"-like organism called myxamoebas engulf bacteria and other nutrient particles. The individual cells consume the food through endocytosis and use the obtained energy for growth and reproduction.

A certain slime mold called Physarum polycephalum has evolved a highly efficient method of obtaining food. First, the slime mold grows in a tree-like fashion, extending its tendrils in nearly every direction. After a while, tendrils that don't locate food shrink back into the slime mold, while tendrils that do find food remain, and tendrils that take a roundabout route to locate food also disappear. The slime left over by the disappeared tendrils tells the slime mold to not grow in that direction again. Therefore, after some time, the slime mold locates all the food available, and the tendrils that still exist connect food pieces with the shortest path possible. An interesting experient with a slime mold on the map of the US and food pieces at major cities resulted in the tendrils of the slime mold accurately tracing out the major US highways. [1] (FZ)
Slime mold feeding on dead wood chips. (LC)
Slime mold feeding on dead wood chips. (LC)

Reproduction and Development

All types of slime molds reproduce asexually. Plasmodial slime molds form fruiting structures, which rise from aggregated masses of plasmodium and possess spores. As these structures develop, the diploid cells of the plasmodium undergo meiosis and divide. The divided cells create haploid nuclei at the end of the structure and form a sporangia, in which the nuclei become surrounded by cellular walls to form spores, which are released once the fruiting structure dries. Once the spores germinate into swarm cells, a haploid, wall-less state, they can either function as gametes to reproduce or divide mitotically into more haploid copies of itself. The growth and development of the plasmodial slime mold involves expeditious mitotic division along with great increase in cytoplasmic volume, but there is no cytokinesis. This results in the plasmodium to be a coenocyte, in which many nuclei are contained within a plasma membrane.
Cellular slime molds reproduce through mitosis and fission and grow in swarms of independent cells. They also utilize fruiting structures for spore germination and the release of myxamoebas.
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Life cycle of plasmodial slime mold

(10)(NC)

Slime mold spores survive in the soil and thatch. During warm, wet weather the spores germinate and develop into a slimy mass that grows over the soil and nearby plant parts during wet weather. The pustules observed on turfgrass leaves are reproductive structures that contain numerous spores. (8) (MC)

Integument

The outer cytoplasm of the plasmodial slime molds is less fluid than the interior cytoplasm to provide structural rigidity. During tough environmental conditions, a sclerotium, or an irregular mass of hardened, cell-like components, is formed in a plasmodial slime mold to protect it during unfavorable conditions.

Movement

Plasmodial slime molds use streaming movement to spread when the outer plasmodium loses its rigidity and becomes more fluid, allowing the plasmodium to stretch. This streaming movement is powered by microfilaments and myxomyosin, a contractile protein that interacts with the microfilaments. Its movement correlates to the growth and development of the slime mold, and it can move and grow infinitely as long as sufficient resources are provided to maintain its homeostasis.
Cellular slime molds move in swarms until they find a substrate over which they can grow and reproduce.

Sensing The Environment

The growth potential for the slime mold under favorable conditions, such as optimal pH, temperature, unlimited food supply, moisture, and other factors, is virtually infinite. However, when one of these conditions becomes unfavorable there are two possible outcomes. The first survival option for the plasmodial slime mold is the formation of a sclerotium, when the plasmodium forms an irregular mass of hardened, cell-like components for protection. Once favorable conditions return, the slime mold quickly returns to its plasmodial state. The plasmodium can also produce fruiting structures, which rise from masses of plasmodium and possess rigid structures from the thick walls that form between the nuclei.
Cellular slime molds aggregate and form fruiting structures when conditions become unfavorable and the individual cells form a pseudoplasmodium, or a "slug". However, each individual cell remains unique and they continue to possess their original plasma membranes.

Gas Exchange

Since slime molds are aerobic organisms who use oxygen for respiration, they tend to aggregate in areas with higher oxygen. They use oxygen in the environment to produce ATP in their mitochondria for cellular functions and then release carbon dioxide into the atmosphere. Slime molds play a role in the carbon cycles by emitting carbon dioxide into the atmosphere.

Waste Removal

Slime molds release carbon dioxide through cellular respiration. The gas molecules are released through membrane diffusion.

Environmental Physiology

Slime molds grow in moist areas with high oxygen content. Since they feed off of decaying organic matter and bacteria, colonies are found in highest concentration around dead plants and rotting animal carcasses. They play a major role in the breakdown of organic matter along with the carbon cycle through the release of carbon dioxide in aerobic respiration.

Slime molds need plenty of water or at least some liquid, because if they do not have the proper amount they wil dry out and in turn begin to decay. Not having enough water can lead to death of slime mold. (16)(ES)

Internal Circulation

Slime molds are single-celled organisms, so they do not have the complex internal circulation of larger, multicellular organisms. Slime molds do use an analogy to circulation for motility through increased and decreased rigidity of the cytoplasm. While the inner cytoplasm of plasmodium is always fluid to promote normal cellular function, the outer cytoplasm varies in rigidity depending on how much the slime mold needs to move. With greater fluidity, the slime mold's cytoplasm can move and the organism can then engulf food particles or move to areas with more optimal conditions.

Chemical Control

Cellular slime molds have been shown to release both steroid hormones and peptide hormones that function similarly as the way they do in higher organisms. Some slime molds release a sexual hormone in a secreter-responder system, where one strain secretes it and the opposite one responds. These have been found to be species-specific, so that that only a slime mold of the same species will respond to the chemical signal. (12) (DM)

Review Questions

1. What is the significance behind the slime mold containing two cytoplasms: the ectoplasm and endoplasm? How do these two structures work together? (SJ)
2. What are the two possible growth outcomes when a slime mold encounters unfavorable conditions? What are some of the environmental variables that, when altered to become unfavorable, would produce these outcomes? (JLev)
3. In which ways can slime molds be harmful to human health? How severe are their effects?
4. Why is it so difficult to terminate a slime mold colony, even when parts of it are removed or severed? Describe how a slime mold colony would expand after part of it was destroyed. (AA)
5. Why are slime molds an important contributor to ecosystems? (KG)
6. How do fruiting structures start off the reproduction cycle? (SM)
7. How many known species are there of slime mold? (BS)
8. How can slime molds peptide and steroid hormones function similarly to ours when their nervous system isn't as complex as ours? (TM)

The Significance of Slime Molds (AWC)




References
Hillis, David M., et al. Principles of Life. Sunderland, MA: Sinauer Associates, Inc., 2012. Print.

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    http://www.scientificamerican.com/article.cfm?id=brainless-slime-molds

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