• Science & Nature

Hunting Slime Molds

They're not animals and they're not plants, and biologists want to know a lot more about them.

• By Adele Conover
• Smithsonian magazine, March 2001

We are traveling in Great Smoky Mountains National Park. Forest ecologist Steve Stephenson bends over a decaying stump and parts a curtain of moss so I can see a tiny stand of what looks like shimmering, miniature dark-blue soccer balls atop toothpick stalks. Earlier in their lives these "critters," as their taxonomists call them, slithered around as oatmeal-like globs—plasmodia—hunting bacteria with carnivore self-confidence. They are not animals, however. And despite their present puffball-like appearance, the navy-blue balls are not fungi, nor do their stalks attest to the sedentary life of a plant. These are slime molds, or myxomycetes (myxos), of the kingdom Protoctista, the least understood of the five kingdoms of life, the others being animals, plants, fungi and bacteria (Smithsonian, July 1991). Slime molds are like nothing else on earth. In their plasmodium stage, they show a quality that could be called intelligence: chopped up and dropped into a labyrinth, they will put themselves back together and start to move, avoiding dead ends and heading unerringly for the prize—more food. Not surprisingly, they fascinate some biologists and amateur naturalists.

I am here astraddle the North Carolina-Tennessee border on a spectacular Indian summer day with a team of myxo taxonomists participating in the All Taxa Biodiversity Inventory (ATBI), a 15-year effort to identify and catalog as many of the organisms occurring in the 808-square-mile park as possible. The myxo section is coordinated by Stephenson, who is also a biology professor at Fairmont State College in West Virginia. His fellow taxonomists on this expedition include British researcher David Mitchell; Ted Stampfer, a researcher from New Mexico and from Stephenson’s West Virginia lab; visiting German scientist Martin Schnittler; and Stephenson’s research associate, Randy Darrah.

 The Smoky Mountains ATBI has been dubbed the biological equivalent of sending a man to the moon. In December 1997 dozens of taxonomists from some 25 universities and research institutions met in Gatlinburg, Tennessee, to launch the survey, set up a nonprofit overseer—Discover Life in America—and start raising funds. The National Park Service is providing housing, maps, vehicles and other support for the project. Scientists expect to uncover 100,000 species in the park, many new to science.

The taxonomy of myxos depends on what the fruiting bodies look like: size, shape, color and their "complexion" (smooth or warty), as well as microscopic characteristics detected back at a lab. They are found in rotting logs, stumps, bark and similar microhabitats on all continents. A myxo begins life as a microscopic spore. After it’s shaken out like a salt grain from its "parent" fruiting body, it germinates to produce a cell that in turn joins another of its fellows to form a zygote. The zygote devours bacteria found in decaying wood and elsewhere, increases its size by nuclear division, then masses into a blob called a plasmodium. The plasmodium—which can be clear, a preppy khaki, hot pink, or a flashy yellow, orange or red—acts like a giant amoeba, gorging on its prey of bacteria, spores and even other myxos until it runs out of food, whereupon it hikes off at about 1/25 of an inch per hour to a suitable location to sprout the fruiting bodies. The ideal spot is high enough to catch a passing breeze and dry enough to avoid fungi. Then the whole program repeats itself.

We clamber up the steep path to Clingmans Dome, at 6,643 feet the highest point in the park. Dozens of tourists march along with us, and for the most part, our band would pass without drawing undue attention. That is, until every so often a myxo team member brandishes his loupe, a magnifying glass slung from a cord about his neck, pulls out a small knife and plunges off the path into the bush.

Stephenson charges down a slope intent on a rotting log. Ted Stampfer scrabbles up to pore over a bush. Randy Darrah marches up to a still-standing tree and presses his loupe close to the bark. Englishman David Mitchell is examining a series of haggard-looking bushes. "Ah," he eventually says, smiling. Once again he has located the rare Licea sambucina, previously only seen in Europe and discovered here by Mitchell last year. It looks like a lilliputian orange. As this is such an uncommon find, every researcher goes for a bit of the bark. "It’s like getting a piece of the Berlin Wall," notes Schnittler, who ought to know, having grown up on the eastern side of the wall.

Looking for myxos requires a practiced eye. Not until I have passed off a pinkish splotch as discarded bubble gum a number of times does someone point out it’s a traveling slime mold plasmodium. "I don’t know how many I have just passed by," laments Stephenson. Nevertheless, in some 25 years he has discovered at least a dozen new species.

We view tiny pinkish red "fruits," little golden balls, spotted leopardy-looking fruiting bodies and still others that look like minuscule hot dogs on a stick. On the first day of our trip we spot a yellow plasmodium sprawling on a log. To resolve a bet, we revisit the log the following day. Digital pictures taken by Darrah reveal that it has, in fact, moved. Under the skirts of leafy liverworts, Mitchell finds the smallest blueberry look-alike of the trip, Barbeyella minutissima. Along a fallen tree Stephenson points to a bump on a log. As we peer through our loupes, we watch helplessly as a pack of fuzzy insect larvae feast on the hapless fruiting bodies of Hemitrichia calyculata. This is the ice-cream-cone myxo, so called because in its fruiting stage it resembles a coffee ice-cream cone. "To the larvae, this is a nice thick milkshake," comments Stephenson. Among the other enemies of slime molds are certain specialized beetles whose mandibles are modified into little scoops, all the better to shove creamy slime mold into their tiny gullets. For slime molds, the best defense against the elements and their enemies is a retreat. In really bad times, slime molds hunker down as hard little cystlike forms until better days arrive.

Apparently myxos have little impact on humans "for good or ill," notes Stephenson. For Mitchell, "their failure to have any discernible commercial value for humans" is why he studies them. Occasionally myxos get bad press in the gardening pages of newspapers, which advise treating creeping plasmodia with a gardener’s equivalent to riot control: powerful streams of water from the hose. But others have appreciated these beautiful forms of life. The medieval artist Hieronymus Bosch, known for his fantastic paintings of Heaven and Hell, was also a meticulous painter of natural history. In his painting Garden of Earthly Delights, one scientist found representations of at least 22 species of slime molds.

There are two kinds of slime molds: the acellular, of which there are today 1,000 known species; and the cellular, of which about 70 species have been identified. The marked difference between them is that the acellular slime molds have many nuclei but only one cell wall during the plasmodial stage, while the cellular ones are composed of individual cells. In any case, slime molds, said to be a billion or so years old, could be one of the first organisms formed by independent cells joining together. Some are famous as lab "animals," like the cellular slime mold Dictyostelium, which has been figuring in genetics and uncovering the origins of complex organisms like ourselves, in whom many once independent cells gave up their separate identities.

To reach the spore state, the cellular slime molds start out as single cells that communicate by exuding certain chemicals, among them acrasin. This compound is a call for fellow cells to aggregate into a tiny slug, rather than a plasmodium, that then crawls around until it proceeds to stalkdom. Princeton professor John Bonner, who has been researching cellular slime molds for decades, named "acrasin" for the witch in Edmund Spenser’s Faerie Queene, who lured men to her side and then transformed them into beasts.

 The acellular may conquer mazes, but Bonner maintains that cellular slime molds are hardly slouches. "My beasts are not stupid," he insists. And apparently they are just as hell-bent on getting their slugs to a feeding ground of "bacteria . . . whereupon they sprout up into the air to be snagged by passing beasts . . . [mites, worms, etc.]." To do this they "center" themselves by exuding ammonia to prevent competition between individuals and to warn each other of the presence of an adjacent substrate such as a wall. Astutely they also orient themselves to the warmest spot in the soil whether it’s night or day. "Remarkable feats for a bag of amoebas," notes Bonner.

We slog to the apex of Clingmans Dome and onto the Appalachian Trail. Climbing down the north slope some 600 feet, we are searching for Schnittler’s new myxo species, Lamproderma granulosum. He has described it as having "the false but iridescent colors of the hummingbird." We are high enough to look down on the black backs of crows beating to windward over puffy clouds.

Schnittler first discovered this myxo in northern Germany and just this past year uncovered another patch here in the Smokies, on a similar rock face, an unusual habitat for eastern North America. For two such tiny myxos of the same rare species to exist some 5,000 miles apart seemed to me to hint of an ancient lineage. Their ancestors may have lived together on the supercontinent Pangea, been carried apart by plate tectonics, and been dispersed still farther on high-altitude winds.

When we reach the spot where the rock face had been, however, we find a landslide has toppled boulders, uprooted trees, spilled tons of dirt and wiped out what had been the myxo’s microhabitat: a vertical slope, a seep of water and an overhang of moss.

Schnittler is not giving up. He and Darrah edge their way across a tree-strewn slope to different myxo pastures, and after rooting around woolly thickets of liverworts, they discover a close relative—Lamproderma columbinum—which, when magnified by the loupe, look like caviar set in a bed of ice.

Despite their beauty, these myxos are goners, as the "ice" is a fungus—a deadly enemy of myxos. Fungi insinuate their threads into myxo spores so that they can’t reproduce, says Stephenson. In spite of the local fungi, however, he asserts that the Great Smokies ecosystem is thought to have more myxo species than even the tropical rain forests. "There the myxos are limited because of fungi," he adds.

At least one slime mold species does not take the ascent of fungi lying down. Their revenge is to eat the young. The slime mold P. polycephalum is a major fungus spore predator. Although we never see P. polycephalum, in an illustration of their lumpy-looking gray fruiting bodies in Stephenson’s book Myxomycetes: A Handbook of Slime Molds, they appear almost thuggish—like a fist. Their plasmodia can also be impressive, perhaps resulting from their fungus-spore diet, notes Stephenson, who once saw one plasmodium that was nearly three yards wide. Slime molds are so different from us vertebrates that they might as well be aliens from other worlds. Yet they may help us understand our own bodies as great aggregations of once separate and independent organisms. If we think of ourselves as communes, we won’t be far off.

By Adele Conover