Urban planning, slime mold style

by Katie Bowell, Curator of Cultural Interpretation

This post is part of our series celebrating the International Year of Biodiversity.

Physarum polycephalum (photo from http://www.genome.gov)

Slime molds are fungus-like organisms that almost defy description. Once thought to be fungi, slime molds often look like fungus, are colored like plants, can move like animals, and are none of the above. Today they fall under the broad category of Eukarya, which doesn’t help much since that category includes a lot, even the scientists who are doing the categorizing!

Simply described, slime molds are organisms that produce large, single-celled multinucleate (multiple nuclei) bodies called plasmodia. Most slime molds reproduce by spores (like fungi) that germinate and produce microscopic, amoeba-like organisms that flow on thin films of water. When several of these amoeba-like organisms meet, they fuse together and the plasmodium begins to grow. Plasmodia are the feeding and growing stages of slime mold life, and some plasmodia can reach sizes of over 2 feet in diameter while others can move as fast as 2 cm/hour (several feet in one day). No matter how big they get or fast they move, they still remain a single cell.

Slime molds are often found feeding on microorganisms in soil, logs, trees, and even telephone poles (in 1973, one such incident in Dallas, Texas, had some residents convinced that an alien life form was invading the city).They’re sometimes slimy, often use spores to reproduce, and come with such attractive names as “Bubblegum,” “Spaghetti,” “Wolf’s milk,” and “Dog vomit.” If all of that wasn’t cool enough, it’s also been demonstrated that slime molds can remember and researchers have used them as robots’ brains. And if all that isn’t enough to astound your (hopefully non-slime mold controlled) mind, they’re skilled at urban planning, too!

Interested in testing the slime mold Physarum polycephalum’s response to a complicated pattern, researchers in Japan allowed it to grow on a damp surface that they populated with oat flakes that mimicked the location of cities around Tokyo. Since P. polycephalum avoids light, the experiment used light to simulate mountains, lakes, and other landscape obstacles. P. polycephalum was placed in the center of the map (Tokyo), and researchers monitored where the plasmodia went. Because P. polycephalum sends its plasmodia out from a central location and develops optimal paths to food (in this case, the bacteria on the oat flakes), it creates efficient pathways, strengthening branches that work and removing ones that don’t. Over the next 26 hours, the slime mold sent out plasmodia and, by the end of the experiment, the organism had a series of branches that looked remarkably like the real Tokyo rail system connecting those communities. What took people years to design took the slime mold hours. Click here to watch P. polycephalum in action, and pay particular attention to the way that branches are built and discarded based on whether or not the slime mold found food.

Even though the stories of a secret subway system in Fort Collins are only tall tales, if the city ever decides that we should have one I say lets offer these slime molds the design contract.

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