German researchers discover how one-celled organism makes memories without a brain

Scientists have identified how the giant unicellular slime mold, Physarum polycephalum, uses its own body network to encode the position of a food source for future use.

German researchers Mirna Kramar and Karen Alim presented their work in the Proceedings of the National Academy of Sciences (PNAS), Feb. 22. 

Combining theoretical modeling and experiments with Physarum, the researchers show how the slime mold can quickly change its tubular body structure to find an optimal diet, "find the shortest path between nutrients in a maze," connect food sources in an optimal network, and "solve the two-armed bandit problem," the paper states.

To create a memory without a nervous system, the slime mold changes the arrangement of the cytoplasm in its tubes in a series of wave-like contractions. Some body tubes expand and others shrink when a nutrient source is encountered, thus permanently imprinting the information in its architecture. 

The researchers show how this information is stored in its thicker tubes and then used to orientate the organism for successful future foraging. The process of dilating and fixing the new tube size takes only 15 minutes, the researchers found.

"Given the simplicity of this living network, the ability of Physarum to form memories is intriguing. It is remarkable that the organism relies on such a simple mechanism and yet controls it in such a fine-tuned manner,” co-author Alim stated in a Technical University of Munich press release. 

Alim is head of the Biological Physics and Morphogenesis group at the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany, and professor for the Theory of Biological Networks at the Technical University of Munich.

Physarum has puzzled scientists for years, the news release notes, with its intricate networks of tubes and its ability as a single cell to stretch for meters. The paper notes that it has often been termed an "intelligent unicellular eukaryote" for its "capability to mount decisions that solve complex problems."

The researchers describe how Physarum's contact with a nutrient source leads to the release of a softening agent that flows within the tubular network and enlarges the diameter of the receiving tube. They suggest that a chemical agent, not yet identified, acts on "the mechanical properties of the tubes."

Flow is the key mechanism. 

"Ingeniously these flows are used to enhance transport toward a nutrient source by growing specifically those tubes that are quickest to be reached from the stimulus site by flow-based transport," the paper states. "Having thick tubes as memories of previous stimuli positioned close to the stimulus allows the new stimuli to spread more quickly and reorganize mass transport more efficiently." 

Lead author Kramar said in the university news release, “For the softening chemical that is now transported, the thick tubes in the network act as highways in traffic networks, enabling quick transport across the whole organism. Previous encounters imprinted in the network architecture thus weigh into the decision about the future direction of migration.”

Kramar is at the Max Planck Institute for Dynamics and Self-Organization. 

The paper concludes, "The concept that tube diameter hierarchy serves as memory not only elucidates the remarkable problem-solving capabilities of P. polycephalum, but also demonstrates its ability to mimic phenomena known from higher organisms. ... Demonstrating the ability of the network to exhibit a phenomenon reminiscent of associative memory may very well be of relevance for the plethora of living flow networks and contribute to biologically inspired design for biomimetic materials and soft robots."