How Slime Molds Our World

It’s one thing for Dr. Fleming to have discovered pencillin by accident. It’s another to admit to everyone – most people would just say, “I meant to do that.” Fleming was great at serendipity; he discovered human lysozyme when some snot dripped from his nose when he had a cold onto a bacteria filled agar plate and they died.

Have you ever had the fortunate experience of looking for one thing and finding something better? Your find wasn’t what your were looking for, or even meaning to find, but there it is. Now you’ve got something you can really use. And you didn’t even know it was out there. I think they call it serendipity.

Serendipity plays a crucial role in doing science, and there are myriad examples of how scientists have discovered one thing while looking for something else. Penicillin is a good example. Alexander Fleming had an awful time keeping extraneous organisms from growing on his bacterial agar plates. It was his prepared mind that recognized that the mold (penicillium) growing on one of them had cleared the bacteria away in a wide circle.

Today we have an example of serendipity in story-telling. I set out to continue our story of undulipodia(flagella and cilia) use in the different types of organisms. We had used the undulipodia to talk about how hard it is to classify protists, first animal-like protists, then plant-like organisms, and today the fungus-ish protists.

However, in terms of undulipodia, the fungal-like protists are pretty much all the same. They all use flagella to propel their gamete cells. It’s interesting that some fungal cells are motile, but it’s not the most interesting thing in their story. In looking at the three phyla of fungus-like protists, I found that the biology of each is amazing – see if you agree.

Phylum Acraisomycota– As unappealing as it may sound, this phylum consists of the cellular slime molds. These protists spend much of their life as individual cells, moving around via amoeboid motion through the soil, looking for decaying organic material and bacteria to eat. But when hunger, they become ranchers. Yee-ha!

When food is scarce, the amoebas of D. discoidium will join together into a grex, or sometimes called a slug. They remain individual cells, unlike acellular slime molds that join together as a single huge cell. This video is sped up considerably.

When food is scarce, acraisomycota cells join together, both emotionally and physically. Dictyostelium discoideum is a cellular slime mold that has been used in research labs for years and is a well-studied example of the slime mold development.

When about 100,000 cells join together to form a grex or a slug, they all move as one to try and find food, and to produce reproductive bodies so that their progeny will be protected, waiting as spores for better conditions. Without the benefit of speech or sign language, the single cells will start to take on different jobs, including building stalks that stick up into the air with fruiting bodies on top.

Inside the fruiting bodies are the spores, the progeny cells inside protective cellulose spore coats. They are environmentally resistant in this form and can wait until the conditions are right to become amoeboid cells of the next generation.

After forming a slug, fruiting bodies will be formed on top of D. discoidium stalks. Remember that the individual cells are organisms, but they act like a multicellular organism.

Now it gets interesting. Some researchers noticed (serendipity?) that some of the fruiting body spores contained bacteria, but only some of them. Were they a contaminant? When the scientists killed off the bacteria nothing bad happened, but they noticed that the spores of the progeny that had contained bacteria also sported some bacteria in their newly formed spores.

Their 2011 paper showed that some of the dictyostelium cells were not eating all the bacteria available; they were letting them reproduce and then storing them in their spores. When those spores were blown or carried to new locations and germinated, the bacteria would grow as well, becoming food for the cells. In turn, their progeny would be gathered into the next generation of spores.

Protist ranchers are amazing enough, but there’s more. The ranchers hire cowhands to protect their herd. The spores contain not only the food bacteria, but some other bacteria as well. These other bacteria (of the family Burkholderia) secrete chemicals that keep the non-ranching clones of dictyostelium from rustling the bacterial cattle. Not only are some of the protists ranchers, they protect what they raise using armed guards! It only took us several hundred million years to catch up to them.

Phylum Myxomycota - These are the acellular slime molds. The individuals cells don’t join together just when food is scarce, they spend most of their lives all packed together.

This is P. polycephalum, or dog vomit mold. I bet you have seen it before and walked FAR around it. When grown in high moisture and food, it is slimy, when it dries out, it forms a crust over the top to protect the huge plasmodium below.

With acellular slime molds, the individuals actually merge together as one cell, not one multicellular organism. This plasmodium is a single cell with thousands of nuclei, reaching sizes of 0.3 meters (1 ft. or more), so these are called the plasmodial slime molds.

A good, if not so pleasantly named, example of the myxomycota is the dog vomit mold (Physarum polycephalum). It’s named that because it often looks like that. But the skills of this slimy mass help it to overcome the poor name and the yellow gooeyness (gooiness?). Acellular slime molds are math geniuses – and they are going to help you avoid your math homework by playing on the internet.

It all started when a 2000 paper showed that dog vomit mold can find its way through a maze. With food at the opposite end of a labyrinth, P. polycephalum will consistently find the shortest path to the food.  The mold grows toward food in such a way as to be most efficient.

Here, efficient means using the least amount resources and the quickest route, ie. the shortest path. It creates a redundant system as well – more than one way to get to the target in case the primary path is disrupted by a misplaced footstep or a carelessly discarded whoopie pie wrapper (I must be hungry).

Finding fast, short, efficient, and redundant paths involves high level math - very high level math. P. polycephalum doesn’t have a calculator, or even a brain, but we're learning a lot from this mathlete.

In an amazing 2010 study, pieces of food were placed on an agar plate in the relative locations of Japanese towns around Tokyo. A small amount of P. polycephalum was placed where Tokyo would be, and it was allowed to grow toward the food. The result – the mold recapitulated the Tokyo rail system map!!! Five dollars worth of agar and 48 hours achieved the same design result as years of time and hundreds of millions of Japanese yen – maybe just a bit embarrassing?

The networks formed by dog vomit mold become well defined as they are reinforced. The fronts are looking for food, and many are resorbed to reveal only the most efficient pathways once food isfound (watch the left side).

All this math is important because many of the things you care about use this slime mold derived math to build virtual ….stuff. For example, the internet gets you pages of information by bouncing the information around networks. The more efficient that bouncing is, the faster your page loads.

Graphics programs use slime mold math to build geometric shapes which become smooth and realistic surfaces and moving objects in your video game. Dog vomit mold can even be used to model the movements of characters within the games. Who knew that Assassin’s Creed involved so much math?

Phylum Oomycota - These are the water molds or downy mildews. We have talked about them before in terms of their presence in your bathtub and shower, but they have more stories to tell. For instance, they were responsible for the number of Irish Catholic priests in America.

One of the most amazing things about this phylum is that you pronounce both of the first two O’s – say “Oh! Oh! Mycota” real fast. The name means “egg fungi,” and as with the two other phyla, they used to be mistaken for fungi. However, these molds grow in long filaments, not as slime molds.

This what happens to potatoes contaminated with the P. infestans parasitic oomycete. The potatoes rot in the ground, so when you finish growing them all summer, you have nothing to harvest. It isn’t the plague, but it still killed a million people.

The oomycota were originally mistaken for fungi because many feed on decaying material, but some other species are parasitic. They cause damage to crops and fisheries. One species attacks potatoes.