Sunday, July 04, 2021

The wonderful world of slime mold

I have been reading Life's Edge: The Search for What It Means to Be Alive by Carl Zimmer, and one chapter is devoted to slime mold, something I have never had cause to read about before. There are hundreds of species of slime molds, and they can be found in pretty much any forest, where they look like (if they can be seen at all) a kind of fine golden web spreading over decaying logs or fungi. They have no brain as such, and a very basic biology, but they do seem to exhibit some hallmarks of life: they use their own store of fuel, enzymes and the logic encoded in their genes to look for food and grow. While our cells divide and generate new cells each with its own nucleus, a growing slime mold generates new nuclei, but does not split its cells - it grows into a single gigantic cells with lots of nuclei, which is pretty cool, I'd say. Perhaps "not life as we know it, Jim", as Dr. McCoy might gave said, but life nevertheless.

A simple experiment shows how slime mold works: if it "smells" a food source, it extends tentacles out towards it, retracting any tentacles that were sent out towards any false food leads. If it encounters a barrier but can still "taste" sugars and other molecules that it considers food somewhere nearby, it will go around the barrier and continue the hunt for food. In fact, it can find its way through quite complex mazes in search of food and, even more amazingly it will eventually (and quite quickly) find the shortest and most efficient way through such a maze, and even the shortest path to several food sources at once. And all this without a brain!

It goes further. It turns out that slime mold ideally needs a food source that is two parts protein to one part carbohydrate. If presented with two different food sources - say, one with nine parts protein to one part carbohydrate, and the other with one part protein to three parts carbohydrate - it will actually "analyze" the foods in some way and "calculate" how much of each it needs to take to yield its ideal blend of food constituents. Again, all without a brain!

Slime molds even have their own rather bizarre version of sex. They make spores, which are carried away on the wind. If a spore lands in a promising spot, it tears open and cells called amoebae are released, each of which has just half a set of chromosomes, composed of different proteins, marking them as one of hundreds of different mating types. These amoebae crawl across the forest floor, and if they meet another amoeba of a different mating type, they merge to form the slime mold equivalent of an embryo. This embryo cell gradually grows tentacles, makes copies of its chromosomes, and a new generation of slime mold takes off. Sexy, eh?

It may not be what we normally think of as life or intelligence, but slime mold responds to a changing environment in a way that helps keep it alive. And, at base, that's what it's all about, right?

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