The good ooze on slime mould

Despite what the name suggests, the slimy blobs known as slime moulds aren’t actually moulds.

Originally miscategorised as a fungi, slime mould is an informal name for more than 900 species of unrelated eukaryotic organisms that can live freely as single cells. Physarum polycephalum is just one of these – bright yellow and widely used in scientific research, it’s been the subject of two different SCINEMA International Science Film Festival entries over the years.

Let’s explore seven cool things you may not have known about it.

1. The “blob” can grow rather large

Physarum polycephalum is also commonly known as the “blob” after the 1958 cult science fiction horror film of the same name.

In the film, a corrosive amoeboid entity crashes to Earth inside a meteorite, engulfs everything in its path and eventually grows to become larger than a building. But while a slime that size is purely fantastical, in reality it can still grow to cover a staggering 10m².

This is pretty cool when you consider that just one cell membrane, with cytoplasm and multiple nuclei (a structure called a syncytium) floating around inside it, is keeping it all together.

2. It’s an expert in optimised transportation

Slime mould is well known for its remarkable ability to find the shortest, most efficient route between two food sources (solving the shortest path problem) and optimising the transport of food between multiple remote parts of its filamentous network.

By placing food sources – usually oak flakes – at different points to represent the geographical locations of major cities, two different research teams have shown that the P. polycephalum network corresponds strikingly to the structure of transport systems on Earth.

Researchers did this in 2010, positioning the flakes like the cities surrounding Tokyo, and found that the resulting network developed similarly to the structure of the railroad network connecting them. In 2011 the same was done with Mexico City and the Mexican federal highway network.

To learn more about this, watch Smart Slime?, winner of the SCINEMA 2019Award for Scientific Merit

3. It senses patterns of strain

It’s not just the presence of food that influences how the blob reacts to its environment. It’s been shown to make long-range decisions based on mechanosensation – the ability to detect outside mechanical forces and convert them into intracellular signals.

Researchers found that blobs prefer to grow in the direction of three glass disks sitting next to each other, instead of a single disk or three stacked on top of each other. The disks were putting different mechanical strains on the agar gel on which the slime mould was growing – and therefore on itself – showing that the slime mould wasn’t simply reacting to weight.

4. Slime mould helps us understand the universe

Slime mould’s amazing networking capabilities aren’t just useful for contributing to future developments in bio-inspired transport planning – the blob has also helped us shed light on our local universe.

In 2020, researchers built a 3D map of the cosmic web – the filamentous scaffolding of dark matter and low-density gas upon which galaxies are created. They did this by designing a computer algorithm inspired by slime mould network behaviour, consequently discovering where to look to find the web of gas that connects 37,000 galaxies.

5. Slime mould has a good “memory”

Despite having no brain or nervous system, slime mould is able to “remember” where it found food for future reference. German scientists discovered that its internal structure undergoes changes to encode the location of nutrient sources by growing and shrinking the diameter of interlaced internal tubes.

Nutrient sources trigger the release of a softening agent that gets transported by the flow within this tubular network. It softens and increases the diameter of tubes to increase the flow towards the nutrient source at the expense of ones further away, creating a kind of tube hierarchy that then shapes the direction of future migration.

6. It learns by habituation and can share “memories”

Those aren’t the only kind of slime mould “memories”, with the blob also learning by a process known as habituation, where a response to a stimulus decreases after repeated or prolonged exposure to it. Slime moulds have been trained to ignore the presence of chemicals, such as salt, caffeine and quinine (a bitter compound), that they usually do not like at all.

They can also share these “memories” with other blobs! Taking a naive slime mould – one that has never before encountered one of these compounds – and fusing it with a habituated slime means that it also becomes habituated.

Interestingly, if you take that habituated slime mould and enter it into a dehydrated dormant state known as sclerotia, you can rehydrate and wake it back up a year later and it will still be habituated.

To learn more about this process, watch The Slime Minder, SCINEMA’s Best Short Film in 2021.

7. Slimes in space on the ISS

Joining the hundreds of ongoing investigations aboard the laboratory on the ISS, several pieces of sclerotia embarked on a journey to orbit Earth on a refuelling freighter in August 2021.

Rehydrated in September, the blob allows students to replicate the studies being conducted in orbit, with thousands of specimens, cut from the same strain, distributed to 4500 schools in France. The students can see how the slime mould’s behaviour is affected by microgravity: comparing its speed, shape, and growth while it eats or explores its environment.

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