Mysterious slime molds rare in Southeast

Posted: Sunday, September 23, 2007

Despite their unattractive name, slime molds can be beautiful, and they are certainly among the most peculiar organisms in the world. More than 17 species have been recorded for northerly parts of Alaska, in both forest and tundra, but the total for our state is apparently unknown. Several kinds occur in Southeast, but they are seen sufficiently rarely that finding one is like a successful treasure hunt for trail-walking naturalists. For a long time, scientists couldn't decide if they were animals or fungi or something else altogether, because they have unique combinations of characteristics.

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The basic life cycle of the most common and widely distributed types of slime molds starts with single cells that live independently in the environment, usually in soil or rotting wood. Like animals, these cells can move, and they make a living by eating bacteria, yeasts, fungal spores, and bits of organic debris. If the environment is very wet, these cells may have whip-like flagella to help them move, but if the environment is drier, they ooze around like amoebae. Each of these forms can transform to the other, depending on conditions.

At some point, usually as the environment deteriorates, two separate cells come together and fuse (sexual reproduction), and the fused entity starts dividing; in some cases, however, one cell just starts dividing (a type of nonsexual reproduction). In either case, cell division continues until eventually one large mass of protoplasm with many nuclei is formed, with just one cell wall around the whole thing. This 'plasmodium' can move (like an animal), and feeds voraciously on bacteria and other tiny things. The signal to start forming a plasmodium may come from a variety of environmental conditions, but most species require light for this transformation.

Many plasmodia are small, perhaps an inch or less across, capable of squeezing through tiny openings in rotten logs. But occasionally gigantic ones are formed, covering several square yards. Most of them move slowly, but movement rates as much as 2 centimeters per minute have been recorded. One giant, several yards across and creeping over a suburban lawn, terrified the human inhabitants of a Texas town, some of whom thought it must have come from outer space. Once it was properly identified, excitement died down, but the next phase is biologically exciting in its own way. And this phase of the life cycle is the one we are most likely to see.

The plasmodium is the spore-producing phase of the life cycle. As food becomes scarce, the surface of the plasmodium gives rise to numerous small fruiting bodies that contain spores (like a fungus or fern). When they mature, the spores exit and disperse with a breeze or on animal feet. Eventually they germinate, giving rise to single cells, thus completing the cycle. The plasmodium disintegrates.

Aesthetically, at least some of the slime molds in the spore-producing phase are very colorful and attractive. One famous artist, Hieronymus Bosch, included many species in his fanciful paintings. I found my very own first slime mold last summer. It was a lovely translucent yellow with a white border, and it was draped over a clump of sedges in a muskeg. More recently, I found a white, fuzzy one growing on a decaying log right next to the trail. Other slime mold in the reproductive state may resemble insect eggs, mushroom caps, mini oranges, black pearls, or frilly lace. The colors range from white to black, with all the colors of the rainbow in between. Some of these curious and strangely elegant beings are photo-illustrated in Bob Armstrong and Marge Hermans' book, Southeast Alaska's Natural World, available in local bookstores.

If environmental conditions become very bad, either the single cells or the plasmodium can enter a tough, resistant phase. In this phase, prolonged harsh conditions can be endured, for decades, if necessary. A single gram of soil may contain hundreds of resting-phase cells.

Slime molds are ecologically important in a small way. They help control the populations of bacteria and yeasts in the soil. A few species of beetles and flies are associated especially with slime molds, usually eating the spores. Slime molds are attacked by certain fungi and by nematodes (round worms). Some of them can live on a diversity of substrates, but others are more specific: Some associate with certain mosses, others with the bark of living trees, some with leaf litter of conifer trees but others with litter from deciduous trees. One kind of slime mold is characteristic of melting snowbanks, and one of these species occurs in Southeast.

These lowly but sophisticated organisms have proved extremely useful to developmental biologists interested in how animal or plant cells communicate with each other within an organism, how certain immune-system cells move to attack foreign invaders, and what effect mutations have on how cells differentiate into various other kinds of cells. But many aspects of their evolutionary biology are still a mystery.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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