Physarum polycephalum

Physarum polycephalum on a strain.

The Physarum polycephalum, also called slime mold, is a species of myxomycetes belonging to the family Physaraceae and the reign of Amoebozoans. (like the amoeba). This curious organism is composed of a single giant cell. Although it does not have a brain or nervous system, this organism is still capable of learning. It is neither an animal, nor a plant, nor a fungus. It has been living in the undergrowth for more than a billion years. In its natural state, it feeds on bacteria and mould (fungi). In the laboratory, the scientists give them oats flakes, but they actually feed on the bacteria on the oats. There are more than 1000 different species of physarum polycephalum.

Reproduction and life cycle

Reproduction

P. polycephalum, like us, results from the fusion of two sexual cells. They are not called eggs or sperms because p. polycephalum has 720 different sexes. Once in a moist living environment, these sex cells (called spores like mushrooms) go in search of a cell of the opposite sex. With more than 720 possibilities, the task is rather easy. When two cells of the opposite sex (called gametes) meet, they fuse together to become a single cell (egg). But then, unlike us, the cell will not divide, only its nucleus will do so. The slime mold will therefore grow with the divisions of its nuclei until it reaches record sizes.

Life cycle

Physarum polycephalum life cycle.

The main phase of the p. polycephalum is the plasmodium. This plasmode is made up of networks of veins which ensure the distribution of the nutrients, and numerous nuclei. It is during this stage that the organism looks for food. P. polycephalum surrounds its food and secretes enzymes to digest it. This process is called phagocytosis. If environmental conditions lead to dehydration of the p. polycephalum during feeding or migration, a sclerotia will form. The sclerotia is made up of highly reinforced tissue that serves as a dormant stage, ensuring the protection of physarum polycephalum for many years. Once favourable conditions return, p. polycephalum reappears to continue its search for food.

The blob in its environment

Blob's Pseudopods

To move, p. polycephalum uses excrescences called pseudopods. The movement of the slime mold is related to the movement of its cytoplasm called cytoplasmic current. This movement is characterized by a change of direction from front to back of the cytoplasmic flow, with a time interval of about two minutes. Within the slime mold, the driving force is generated by the Muscle contraction followed by the relaxation of membranous probably composed of an actin protein. The layer of filaments creates a pressure through which the cytoplasm flows inside p. polycephalum. Physarum polycephalum, unlike living beings able to learn does not use an internal memory like the man for example but an external memory. He uses for this his mucus that he leaves wherever he goes. As a result, each time he comes in contact with his mucus, he will know that he has already passed and will not come back. Physarum polycephalum has no eyes and therefore do not see its congeners but thanks to the mucus or rather the molecules chemicals present in its mucus, a slime mold can detect the presence of another slime mold. The largest slime mold observed in the laboratory was 10 m² (square meter). The largest observed in the wild measured him 13 hectares.

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