Slime Molds: Oozing Between Boundaries

BY TIA BÖTTGER

Slime molds, myxomycetes, are not molds. They’re not any type of fungi in the kingdom mycota. They aren’t even slime all of the time. They fall into the biological category of protists, where we lump creatures that are not quite a plant, animal, or fungus (1). Their cells seem poised between a single-cellular and unicellular lifestyle, readily able to convert between the two. They have no central nervous system or anything that resembles one, and yet many are capable of sophisticated behaviors which force us to question our definition of intelligence (2). Slime molds ooze in and out of any definition we try to fit them into, inviting us to expand our perception. What can we learn when we are opened up to possibilities outside of the expectations created by human-constructed categorization?

Slime molds look as bizarre and otherworldly as you might expect. They’re vibrant, unctuous, and alive. You’ll have to look closely to find them, as they are smaller than mushrooms, but they are found in similar environments. Slime molds live on organic matter like decaying logs, leaves, or compost. They’ll appear after a couple days of rain, or in moist environments. Supposedly you can grow them at home, taking some wood from the forest and keeping it damp (3). But they’ve been found everywhere– from deserts to the edge of arctic snow melts, even on the body of the helmeted iguana.

Slime molds have two main life stages, with four in total (4). They begin as soil-dwelling amoeba, usually around 5 to 10 micrometers large (5). In this stage, they will predate upon bacteria, pursuing a solitary, free-moving life. When deprived of a food source however, cells will undergo a radical change to seek comfort in community. The slime mold Dictyostelium discoideum sends out chemical signals, which causes thousands of separate amoeba organisms to coalesce into a “slug” form (5). They ultimately develop into a fruiting body: a pearly orb supported by cells in a structured stalk, with those near the top producing spores that will each become individual amoeba upon germination. The cells forming the stalk die, in a be- havior benefitting the entire structure despite having lived a mostly individual existence.

The most studied slime molds are plasmodial slime molds, which fit the picture of gooey slime. These similarly begin as amoeba-like cells, but grow together into plasmodia, containing many nuclei without cell membranes between them (4). Plasmodia will feed by engulfing other microorganisms, moving as a coordinated network at speeds up to 1.35 mm per second, the fastest rate recorded for any microorganism (6). This network is responsive, and has shown the ability to solve mazes, and even learn and predict unfavorable conditions. In a famous experiment, the slime mold Physarum polycephalum created the most efficient route between oat flakes scaled to match major cities near Tokyo, and bright light, used to simulate mountains, recreating Tokyo’s rail system (7). Their network efficiency is being used in labyrinth-like experiments designing urban transpor- tation systems and evacuation routes– they were even put to the test to find their way out of IKEA. Slime molds are also being used to solve mathematical problems in unconventional biocomputing, and inform simulations of the mysterious dark matter holding our cosmos together (8).

Slime molds navigate time as well as space. Researchers from Hokkaido University exposed slime mold to unfavorable conditions every 30 minutes, dropping the temperature and decreasing the humidity to create a dry environment. The plasmodium began to crawl more slowly, saving its energy in response. But even after the researchers stopped changing the environmental conditions, the slime mold consistently continued to slow down every 30 minutes, anticipating the change (9). In another study, it learned to ignore noxious substances, and remembered that behavior up to a year later (2). As a reminder, slime molds have nothing that resembles a brain.

In the words of John Bonner, who has been hailed the patriarch of the slime mold community, slime molds are “no more than a bag of amoebae encased in a thin slime sheath” (10). Merlin Sheldrake, author of Entangled Life, writes: "These studies raise a storm of questions. Are network-based life-forms like fungi or slime molds capable of a form of cognition? Can we think of their behavior as intelligent? If other organisms’ intelligence didn’t look like ours, then how might it appear? Would we even notice it?" (1).

The collective behavior of slime mold is astonishing, not just for their ability for cognition, but in the interactions between slime mold cells themselves. When plasmodial slime mold is physically separated, the cells find their way back and re-unite. The slime leaves behind a trail, as a kind of spatial memory, signaling to other cells where it has gone so that the conglomeration of cells can explore more ground, collectively rather than individually acquiring more nutrients (11). If a plasmodial slime mold encounters another plasmodium, it can also unite and fuse together to create a larger form. We have identified that slime molds have some 720 different sexes, another reminder that life beyond a binary is natural (3).

Slime molds invite curiosity and awe. They are found in places we associate with decay and dying, appearing all at once, unexpectedly, in a display of oozing yellow before disappearing again. They remind us of systems beyond ourselves, orienting us to learn from the complexity of a being just as easily ignored and overlooked. They live between slime and cell, collective and individual, dissolving our boundaries and opening our minds.