Sixty-five years ago this month, an iconic horror movie made a star out of Steve McQueen and showed disappointingly little of its titular character.
That movie, The Blob, involves a goo that crashes down to Earth, and McQueen as a tough teen who tries to warn others of the encroaching sentient slime. (And it features plenty of outdated gender problems, like teen girls who don’t speak unless they’re talking about a puppy or their baby brother, because women are only useful for their maternal instincts.)
The Blob, whose origin of species is never explored, may seem quixotic but actually resembles an earthly slime mold, which also eats by capturing snacks in a process called phagocytosis — but on the microscopic scale. But even if we don’t have any real extraterrestrial plasms on Earth, The Blob bears some resemblance to one of the stranger creatures on our planet. In a way, it is a slime mold movie. After all, slime mold may be the closest we ever get to a real Blob.
Reel Science is an Inverse series that reveals the real (and fake) science behind your favorite movies and TV.
The basics of slime mold
The “mold” part makes slime mold sound like a type of fungus, but Marie Trest, a botany professor at the University of Wisconsin–Madison, says they’re neither plant nor animal nor fungi, but protists. Protists are uni- and multicellular organisms from several independent groups that share an ancestor, along with animals and fungi. Slime mold falls into the phylum Myxomycota and the protist supergroup amoebozoa.
Slime mold needs to consume organic nutrients, Trest tells Inverse. “This is where we get that blob connotation,” she says. “They will engulf things, in this case, bacteria and small organic molecules and spores.” From single, mobile cells, slime molds form a structure called a plasmodium, which is one giant cell with thousands of nuclei at the helm. According to the National Park Service, some recorded slime mold masses have branched out at over 30 square meters.
Slime molds keep their portions small, sticking to organisms on the micron level. In the lab, Trest says, researchers may place oat flakes in a slime mold’s environment for it to snack on. The slime mold doesn’t subsume the entire oat, but rather the sugar molecules it holds. And just because a slime mold comes into contact with something, that doesn’t mean it will feast. It might slide over a blade of grass, harvesting spores and bacteria, but will leave the grass itself intact. In other words, humans need not fear the slime mold. Bacteria, however, should.
“If you're a bacteria breaking down an old mushroom and slime mold comes along, you should definitely be screaming and running through your life,” she tells Inverse.
Where the slime mold roam
The plasmodium is a multipurpose body part. It absorbs food, but also gets the slime mold around. The plasmodium pulsates, sending waves outward to push itself forward. To move, they use actin-myosin, which are proteins found in animal muscle tissue. They constrict and relax to push out and back in gentle waves, creating that oozing motion associated with The Blob.
“Even when it's growing in one direction, it's constantly pulsing out and back with a net movement in one direction,” Trest says. She adds that researchers believe these waves are a way of internal communication. “As it makes these pulsing waves, some of the signaling molecules say, ‘Hey, there's food over here.’ So it can communicate across this large cell where it should go to maximize its food.”
Slime mold famously can solve mazes — or at least, they appear to solve mazes. Like humans, first the slime mold explores its surroundings with its pulsating plasmodium, but then switches to food mode and finds the most efficient route, showing some signs of remembering. This isn’t necessarily a sign of intelligent life because they lack central nervous systems, but researchers have investigated this organism’s computing ability. The movie Blob moves like a goopy cheetah compared to real slime mold, though both are able to sense and move toward food. What’s more, slime mold tends to branch out while maintaining a central hub, while The Blob moves as one large mass.
But how do they know where they’ve been? Slime mold exudes an extracellular slime trail that Trest says could be a way for the organism to know where it's been, sort of like a trail of disgusting breadcrumbs. If it doesn’t find anything, its plasmodium will contract and explore the other direction.
“You can actually see the particles inside the plasmodium going in one direction for 30 to 60 seconds and then back,” Trest says. “It will do it very regularly, engulfing things as it goes.”
Don’t fear the slime mold
Slime molds hang out in temperate forests on decaying wood. They’re not eating the wood, but rather the fungus and bacteria decomposing it. But Trest says they grow all over the place: grasslands, wood mulch, gardens, even refrigerators. Trest says she’s used her refrigerator drawer as a slime mold substrate “to grow a really large one for class for fun.”
She’s heard of a story of a student who left their slime mold in a shared dorm refrigerator over winter break, and it “escaped the plate” holding it. “It just covered the inside of the refrigerator, and other roommates were really upset,” she says. Still, better to resent a careless roommate than be dinner for a blob. While slime molds can take up the space of their surroundings, they’re usually about five centimeters across in the wild, rather than meters tall and wide.
For anyone curious about how it feels to be engulfed by The Blob, Trest has hands-on experience with slime mold. “They feel slimy,” she says. She likens slime mold to children’s toy slime, adding that they’re mucousy and cool in temperature. Perhaps the biggest difference between our slime mold and The Blob is that if you feel like sticking your hand into some slime mold, there’s nothing to be afraid of. No gloppy masses will eat you today.
