Skip to main content

A Biological Improbability

The newspaper headline read, "Scientists track slime mold in space." For a wild moment, I thought my early suspicions were confirmed: a critter as preposterous as a slime mold must come from outer space. But no. The Associated Press story beneath the headline explained that slime molds were among the experimental organisms sent up from Earth on a recent space shuttle flight. Drat.

Part of my difficulty in accepting slime molds as ordinary denizens of this world may come from the way in which I first encountered them. A university colleague showed slides of her field work in California, and among her photos was one depicting a gray blobby animal that looked like an odd slug.

"Banana slug?" I asked, knowing those were found in California.

"Nope," she said. "Slime mold. Biggest one I've ever seen."

Now, obviously the thing in the photograph was crawling, or at least oozing, over twigs and leaves. No mold I knew of could move under its own power, slimy or not. I thought she was joking, and said so.

"Slime molds are really amoebas," she said, dismissing my ignorance and moving on to the next slide. That was even more improbable. I'd met amoebas in biology classes. They're single-celled organisms, too small to be seen without a microscope. An amoeba as big as the thing in the picture belonged in the Boone and Crockett record book. I went to the library and looked up "slime mold."

After that, I began to think of creatures from outer space.

One of the dearest discussions of slime mold is in a 1973 book by Ernest Borek, The Sculpture of Life. Microbiologist Borek introduces a slime mold known as Dictyostelium discoideum, or Dd for short. Dd is a voracious amoeba-like organism that lives in dark, damp soil. Like all amoebas, it moves by pushing forth pseudopodia, tentacle-like extensions of its one-celled self.

It uses pseudopodia to eat, too, surrounding edible particles and dismembering them with enzymes. Bacteria are among its favorite foods. Given enough bacteria, one well-fed Dd will become two in about four hours.

So far, it all sounds very normal, pretty much like life in most portions of the microscopic world. And Dd are microscopic; according to Borek, it would take about 220 of them to fill the dot above this letter i.

Borek put a few Dd in a pasture from paradise---a Petri dish filled with nutrient-laden agar on which colonies of bacteria were growing apace. Then, after allowing time for a Dd population explosion, Borek became the serpent in this amoebic paradise. He filtered and treated the dish contents to kill the bacteria but save the Dd.

Then he put the filter pads containing the now-foodless Dd in a dark, cool place and checked them every few hours. After about two hours, there were small mounds, just visible to the naked eye, forming on the pads. Examined under a microscope, these features could be seen as piles of Dd, with streams of tens of thousands more heading in toward the mounds like ants returning to an anthill.

If the incubator containing the Dd is covered to exclude light almost completely, the mounds will grow until they look like slugs. Put a pinhole in one end of the cover and let a bit of light shine through, and all the slugs glide in that direction.

Uncover the incubator and let the slugs bask in daylight, and they begin to sprout little turret-like structures. Slowly the turrets extend into stalks, reaching about a quarter-inch high after 24 hours. Atop each stalk is a little pale sphere. The effect is just like the fruiting bodies on molds.

And these are fruiting bodies. The spheres are spores, containing tiny encapsulated amoebas in suspended animation. They can live a long time in those spores, wafting on air currents to new bacterial pastures where they can start the whole incredible cycle over again. Amoebas, sluglike animals, molds, all in one organism---doggone it, they should be from outer space !