During a time of year when most people look to the sky, tracking northern lights and smoky fires, many Inland Northwest residents will remember the summer of 2004 more for a rash of yellow blobs that bubbled up from the ground.
The phenomenon seemed to have been triggered during our cool wet June, then carried right through the dog days of July and August. In fact, the initial foaming, yellow stage that grew out of their grass or mulching bark made some neighbors think that bad dogs had been turned loose to vomit in their yard. When the puddles of protoplasm appeared to move, certain children of the 1950s recalled an early Steve McQueen vehicle called The Blob, in which a jelly-like mass from outer space oozed into a movie theater. Instead of swallowing up the family, however, these current spongy blobs — a variety of slime mold — quickly dried into rusty scabs that poofed out dust like puffball fungi.
Slime molds have long been the stuff of myth — there are fossil amber samples that trapped exactly the same stuff in ancient resins. The scientific community has been snickering at them for centuries, and until recently classified all slime molds as fungi. Now, however, the three main groups of slime molds have been filed under the heading of protozoa: single-celled animals such as the amoeba that slurp beneath high school microscopes.
That rude puddle in your garden mulch has a taxonomic title, Fuligo septica, and a quaint common name, "dog vomit slime mold." Its life cycle follows the general run of the family, which goes something like this: The foamy yellow mass, known as the plasmodium, is the vegetative body of the organism. The plasmodium creeps across its preferred habitat before drying into a dark, scabby mass. The dust that rises from its remains consists of millions of spores that float away on the wind. The outer case of each spore, or cyst, protects a speck of life within, and is constructed to remain viable over a long period of time.
The kind of cool, humid conditions that our rainy June supplied allow a Fuligo cyst to absorb water and split open, releasing an amoeba that's much more than just a plant. The amoeba travels, feeds by absorbing microscopic life, then undergoes a series of rapid and bizarre changes that restart the slime mold life cycle in spectacular fashion.
The depletion of a single amoeba's food source, technically known as starvation, somehow triggers the release of a signaling chemical. The chemical is called cAMP, and may be compared to the pheromone scents broadcast by insects to call in potential mates. Surrounding amoeba cells respond to the cAMP by moving toward it. Within hours, they join together to form streams, like little rivulets of water flowing into a puddle. Within eight to 12 hours, most often overnight, these streams come together to form an aggregation that in our Fuligo looks like a beautiful new car sponge, a pile of regurgitated stomach acids or a gelatinous material that your third-grader made but won't explain to you. Then again, maybe it's a Creature from Another Planet.
But biologists know that it's just a plasmodium, a single giant cell, with its many amoebas forming multiple nuclei; these slime mold aggregations, in fact, comprise the largest single cells known to science. Cytologists study them to learn how nutrients and information are transferred within our own cell walls.
The plasmodium cell is far from a static single world. As soon as the gathering begins, some of the amoebas begin to differentiate into what are called stalk cells. These move up to the top of the aggregate to form the creeping part of the plasmodium that, in some slime mold species, looks like (and in fact is called) a slug. The stalk cells of the slug eventually grow into fruiting bodies that in different species resemble flasks, globes and goblets of astonishing grace. Soon each vessel is filled with spores that are really encysted amoebas, poised to jet into a larger universe at the first stomp of a foot. Within 24 hours, the stuttering, impossible life cycle of the slime mold has flashed through its final stage, and the spores are waiting, with infinite patience, for the right moment to begin the process all over again.
Two last things to keep in mind about these creatures: First, they appear to be impossible to control, at least with the fungicides, pesticides and herbicides that work on more familiar garden nuisances. Since slime molds do not fall into any of these three categories, that shouldn't be surprising.
Second, no one has ever proved that slime molds are harmful in any way. Temperate broadleaf and tropical rain forests contain many species that look very different from our dog vomit variety — more than a thousand different kinds have been named so far, with apparently many more as yet unknown. (Right now, the southern Appalachians, Central America and Southeast Asia are particular hot spots of discovery.)
And in news that should also not be surprising, some cultures have long utilized slime molds as a food sources, with one New World species often described as having the color, consistency and taste of scrambled eggs. Some readers might want to have another look at what's growing out in their garden; others might want to check out a book published right here in the Northwest called Myxomycetes: A Handbook of Slime Molds, by S.L. Stephenson & H. Stempen (Portland, Oregon: Timber Press, 1994).
Publication date: 08/05/04