Slime molds: Weird, odd and singular

A starry mid-summer night in 1957. Two teenagers, Steve Andrews and Jane Martin, are parked on a quiet rural hillside outside the small town of Phoenixville, Pennsylvania, not far from Valley Forge. And then. …

Tearing across the sky, a brilliant white streak, a meteor, crashes to earth no more than a mile away. First to reach the steaming space rock is an old fellow who, prodding it with a stick, is astonished when it cracks apart, spitting out a bluish gel that latches onto his hand. By the time the kids arrive on the scene, the poor man is rolling in agonies of pain with the growing, pulsating ooze now covering most of his arm.

Some few of us may recall from Paramount Picture’s 1958 cult classic “The Blob” that the next day or two don’t go so well for the good citizens of Phoenixville.

Perhaps the closest life forms to the Blob that Planet Earth has to offer would be our 900 or so species of slime molds. Though harmless inhabitants of decaying logs, mulch and turfgrass, the unappealingly named slime molds nonetheless share several peculiarities with the creepy intergalactic visitor.

In fact, when it comes to the weird, odd and singular, I’d argue the fictive alien’s got nothing on our real Earth species. To begin with, slime molds are not animals, plants or fungi; they are their own thing, and their two main groups — cellular and plasmodial slime molds — are about as different as chalk and cheese, as the saying goes.

Cellular slime molds spend most of their lives as microscopic single-celled protozoans looking very much like the amoebas you studied in 10th grade biology. But under certain conditions, these independent cells come together to form a multicellular organism that roams about like a slug sliming a sidewalk, a feat accomplished without the benefit of either a nervous system or muscles.

Plasmodial slime molds, however, consist of a single humongous cell containing thousands of nuclei and look all the world like a ladleful of Play-Doh colored tapioca slapped on a log. But this tapioca has the ability to slowly crawl — flow would be a more accurate description — along the log in search of food.

Like the Blob, both forms of slime mold feed by engulfing their prey, digesting it whole and using the absorbed nutrients to grow. However, unlike the Blob, which in the movie handily dispatches of a cowering waitress in a phone booth, slime mold diets are restricted to bacteria, fungi and bits of decaying organic matter.

Though common enough, cellular slime molds seldom are seen because even their multicellular “slug stage” is small and inconspicuous. But what they lack in size, they more than make up in their bizarre life history.

Happy enough as individual amoebas crawling about the humus on the forest floor so long as conditions remain moist and there’s adequate food, things get interesting when the environment becomes less benign. The amoebas emit a chemical signal causing them to migrate toward one another, ultimately assembling themselves into the mobile multicellular slug stage.

This unit now oozes its tiny way to a well-lit spot where it morphs into a mini-tower with a spore-containing capsule perched atop. On splitting open, the capsule releases hundreds of spores (asexual reproductive cells) to hopefully start the life cycle over in some more favorable locale. My words don’t do justice to this striking phenomenon. Take a minute to look at a time-lapse video of the process at www.youtube.com/watch?v=bkVhLJLG7ug.

More commonly encountered are the much larger plasmodial slime molds. Essentially giant bags of cytoplasm studded with legions of nuclei, these are the brightly colored masses often found on landscaping mulch, especially after a rainy period. Ranging in size from a few inches to a foot or two across, some forms appear flat, bumpy and wet while others have more of a puffed-up foamy look.

Like the Blob, plasmodial slime molds advance (slowly, though) toward potential food sources by extending and retracting lobes from the main body. Amazingly, several studies have shown they appear to have some ability to “learn” to tell the difference between harmful and repulsive (but not dangerous) chemicals blocking the path to a desirable patch of food. An interesting trick to perform without a nervous system, much less a brain.

Though many consider them unsightly invaders in the garden — a frequently encountered bright yellow plasmodial species is known as dog vomit slime mold — they are not harmful to plants, simply gliding over stems and leaves picking food off their surfaces. However, if so desired, they easily can be washed away with a garden hose or spaded over with a trowel.

Ken Baker is a scientist and a retired biology professor. If you have a natural history topic you’d like the author to consider for an upcoming column, email your idea to rweaver@advertiser-tribune.com.

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