How does a single-celled organism, one of the simplest life forms on Earth, manage to satisfy its nutritional needs? It is by studying social amoebae, elementary organisms that are distantly related to fungi and plants, that Audrey Dussutour, CNRS researcher at the Centre de Recherche sur la Cognition Animale and her Australian collaborators have, for the first time, demonstrated the nutritional preferences of such systems.
Despite lacking a centralizing organ, such amoebae are capable of regulating their nutrient supply. When faced with diverse nutritional situations, they adapt so as always to select an optimal ratio of nutrients.
The results are published in the journal Proceedings of the National Academy of Sciences on 9 February 2010.
In most animals, nutrition involves components specialized in the regulation of nutritional supply and demand (in most animals, for example, the brain controls the needs of the body cells). However, certain organisms, such as fungi, have neither a specialized component nor a coordination center. How then do they maintain an optimal supply of the nutrients essential for their survival and reproduction?
Audrey Dussutour and her University of Sydney collaborators have taken a keen interest in one of the simplest life forms on Earth: single-celled organisms. Both numerous and varied, they make up a large part of living organisms, including bacteria, yeasts, fungi, certain algae and some animals (or protozoa). The researchers focused their study on a certain type of amoeba, the most well-known of protozoa: the "social amoeba" or Physarum polycephalum. This vast single cell, with thousands of nuclei, is found naturally in undergrowth environments.
In a first experiment, the researchers offered the amoebae 35 different food sources, each source containing a specific ratio and concentration of nutrients. The results were unequivocal: the amoebae develop in an optimal manner on a diet composed of twice as much protein as sugar, whereas they do not survive on a sugar-rich diet. Furthermore, the amoebae extend farther on diluted nutrient sources, which increases the contact surface area and thus compensates for low nutrient concentration.
The researchers then subjected the amoebae to various choices of nutrients, each time comprising a protein-rich and a sugar-rich source in varying proportions (without offering them their "optimal" diet). The results demonstrated that the amoebae are capable of reconstituting the ideal diet required for their growth from these two sources. In fact, they move until they cover the nutrient sources so as to absorb twice as much protein as sugar. Their nutrient intake thus remains constant and unchanging, whatever the choice proposed. In a final experiment, eleven different food sources, once again containing variable quantities of protein and sugar, were offered to the amoebae. Most of the amoebae succeeded in selecting the food source containing twice as much protein as sugar.
Social amoebae are thus capable of solving complex nutritional challenges, quite a surprising feat for a very simple organism lacking a centralizing system. The researchers are now attempting to elucidate the mechanisms involved.
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