- Slime moulds are catagorised in two distantly related families: “cellular” and “acellular”.
- Cellular slime moulds spend the majority of their life cycles as individual, single celled amoebas. Once they have exhausted the resources of their immediate environment the individual cells join together to form a slug which travels to fresh pastures before fruiting and dispersing spores.
- Acellular slime moulds have a “plasmodium” stage in their life cycles. Plasmodium is made up of millions of nuclei which share a single, gigantic cell without any membranes to separate them.
- Much of the research around slime moulds is focused on the acellular variety (usually Physarum polycephalum), as it is a much more unusual life form, with many unexpected properties. For the remainder of this research, the term slime mould is used to refer to acellular slime moulds.
- Although it lacks a brain or a central nervous system, polycephalum is capable of performing complex behaviours such as finding the shortest path through a maze, solving computationally difficult puzzles and making decisions based on multiple objectives.
- polycephalum is part of the Protista kingdom, which contains a variety of unicellular eukaryotes (an organism with a complex cell or cells) that do not fit into the other kingdoms.
- They are where the world of cellular biology collides with the macroscopic world.
How we perceive them
- Due to their unusual biological properties and ease of culture, the acellular slime mould Physarum polycephalum is a favourite amongst cell biologists.
- More recently, due to the way it constructs networks, it has become the become a model organism for a variety of disciplines including behavioural ecologists, town planners, computer scientists and artists.
How it grows, how it nurtures itself
- Life cycle diagram
- Plasmodia of acellular slime moulds form a fan-like sheet at the front, followed by a network of interconnected veins through which cytoplasm flows.
- Because it forms a network with several fronts, polycephalum can feed from multiple food sources simultaneously.
- Under nutritional stress and when exposed to light, vegetative growth stops and the slime mould fruits, releasing male and female sexual myxamoebas into the environment.
- The myxamoebas feed and divide, and when a male and female amoebas meet they fuse to form a binucleate cell, which develops into a plasmodium.
- In the natural environment slime moulds feed on bacterium, fungi and detritus.
- In labs they are cultured on moist filter paper, agar or rolled oats.
- Their optimal diet consists of 2 parts protein to 1 part carbohydrates.
- Plasmodium are self-repairing, and can regenerate when part of it is cut away.
- Slime moulds grown from different inoculations are repellant to each other, but can merge if the environmental factors force them to.
How it lives, how it interacts with its surroundings
- When a vein contacts a food source, biochemical oscillators give rise to propagating waves, redirecting cytoplasm to that vein. The vein becomes thicker at the expense of those which do not form a direct link between the two parts of the organism. This combination of positive and negative feedbacks allows the organism to connect food sources via the shortest path.
- Slime mould leave a thick mat of non-living, extracellular slime behind their path. It uses this as an external memory to avoid areas where it has been before. Using this, it is capable of solving the U-shaped trap problem, a test of autonomous navigational ability used in robotics.
- The slime mould is capable of using a hierarchy of rules, for example: it will crawl across extracellular slime if they detect the presence of food.
- There is evidence that slime moulds also have an intracellular memory which allows them to anticipate periodic events. A slime mould will rhythmically reduce their speed of movement if they become attuned to a light flashing on and off – even once the light remains permanently off.
- Slime moulds are capable of making intelligent decisions. When offered a choice between food sources of differing quality (concentration of oatmeal), it is capable of choosing the source with the highest concentration.
- Plasmodia trade off risk against food quality, a food needs to be 5 times higher in concentration before the slime mould will forage in the light.
- Speed of decision making affects the accuracy of those choices. Then stimulated to make a fast decision between two food sources of differing concentration by the presence of light, it was less likely to make the correct choice.
- Slime moulds also alter their search pattern depending on the quality of food sources they are already exploiting. When consuming a high quality food source the plasmodia will perform an area-restricted search. When feeding from a low quality source it will move away from the source before sweeping the area for alternative food sources.
- Plasmodia can distribute their biomass proportionally across multiple food sources of differing nutrient quality to receive an optimal diet.
What affects its health in positive and negative ways
- The distribution of a slime mould’s biomass across an environment displays the organism’s relative comfort level in each location. By observing this, researchers have been able to isolate which factors encourage and inhibit the growth of a slime mould.
- Attractive substances such as oats increase flow of cytoplasm to a location, repellant substances such as salt reduce it.
- Low light levels and high humidity increase flow, high light levels and low humidity reduce it.
How it eventually dies
- When the resources in the slime mould’s environment become exhausted, or it becomes too large to maintain itself the plasmodia diverts all its energy away from growth and into producing spores.
- The spores spread out, leaving the slime mould to decompose.
- Cytoplasm veins are conductive and can carry voltages high enough to power an LED without killing the slime mould.
- Nano particles can be picked up by the plasmodia and transported along it’s veins.
- Magnetic nanoparticles (barium hexaferrite crystalline nanoparticles) are bio-compatable with the slime mould. Magnetic fields can be used to deflect the path of magnetized slime moulds in order to control their path.
- Plasmodia have been found to have memristive properties.
- Because separate colonies of slime moulds repel one another, multiple inoculations on nutrient rich agar spread out and form voronoi diagrams where their boundaries meet. Voronois are materially effiecient – potentially load bearing structures.
- This semi-self-repelling nature also allows physical Boolean logic gates to be made.
- Physarum is photo-sensitive, and experimental data suggests it can tell the difference between red and blue light.
- When a plasmodia’s vein is heated up to 40oC its resistance increases 1000 times, making it a biological thermic switch. In about 10 minutes the slime mould reforms and continues unharmed.
- To produce biodiesel efficiently an organism requires a high concentration of lipids in their bodies, which is true of slime moulds. Because they produce biomass at a rate that exceeds algae, they could be an alternative biological factory for the fuel.
Its applications in the world
- polycephalum can be used to model network formation in a biological system.
- Network performance involves a trade-off between cost, transport efficiency and robustness.
- Inspired a mathematical model “The Physarum Solver” which is able to find the shortest path between many points in a real world network, such as the Tokyo subway.
- The slime mould’s ability to anticipate periodic events hints at the cellular origins of primitive intelligence.
- The slime mould’s networking behaviours have been used to explore and model creative thinking.
- People migrate towards sources of safe life and higher income. Physarum migrates into environmentally comfortable areas and towards sources of nutrients. Plasmodium have been used to model pathways of migration from Mexico into the USA.
- Slime moulds have been used as biological sensors, using their aversion to light to steer a small hexapod robot.
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