Cellular sensation (slime molds)

Introduction

Fig. 1. Physarum polycephalum growing in a petri dish.

Plasmodial slime molds are an excellent creature for investigating how cells detect and respond to their environment. These giant multi-nucleate amoebae form intricate networks based on streaming flows of cytoplasm[1]. Previous studies suggest that they have two distinct forms of "memory"[2][3]¹, can find find the shortest path between two food sources [4], and can find low-cost, yet fault-tolerant networks connecting multiple food sources[1]. They are very large (Fig. 1), and can be cultured with no special equipment², so they are convenient for low-budget cell biology.

This project focuses on what and how they respond to their environment. During attempts to improve imaging methods for the project on self organization in Physarum polycephalum, it became clear that immersion in water strongly affects the flow in the plasmodium. Adding water on top of the plasmodium immediately stopped flow in the vein for several minutes. When the flow started up again, it did not seem to have such a clear central channel, but was more disorganized. How do they sense the water?

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Preliminary observations

In the video below, the slime mold was in air until the 0 second mark, at which point a few drops of water were placed over the part of the plasmodium shown (most of the rest of the plasmodium was not immersed). The flow in the vein stopped within 2 seconds, and did not resume until ~200 s, and didn't fully resume until ~500 s. The contraction cycle of the vein seemed to stop at the same time and resumed (maybe more irregularly?) as the flow resumed.

The water used was Beaufort NC tap water. The video was made using an Olympus microscope camera mounted on a Leitz inverted microscope with a 4x objective lens. The dish was covered with plastic wrap except for a space to stick a pipette near the filmed portion. To make the cytoplasmic flow more visible, the slime mold had been fed oats ground with activated charcoal³ and cooked briefly in water. It took up the charcoal powder nicely. The plasmodium is surrounded by slime coat (visible prior to adding water) and was always in moist conditions (kept moist with a damp paper towel).

This was repeated 3 times with video in different portions of the plasmodial network (plus several times not caught on film). While the strength of the response and duration varied, in all cases adding water affected the flow through the plasmodium.

Some hypotheses:

There are a number of possibilities for the proximal signal that disrupts the contraction cycle and cytoplasmic flow. These include at least the following:

• The mechanical disturbance of adding the water
• Osmotic differences between the water and the slime, leading to swelling or shrinking of the vein
• Changes in membrane potential due to differences in the ion concentrations in the water vs the slime
• Changes in the concentration of some molecule dissolved in the slime around the plasmodium

Each of these should be fairly easily testable (e.g. by changing ion concentrations in the water to alter membrane potential; by changing osmolarity of the water; etc.)

Literature cited

1. Tero A, Takagi S, Saigusa T, Ito K, Bebber DP, Fricker MD, Yumiki K, Kobayashi R, Nakagaki T. 2010. Rules for biologically inspired adaptive network design. Science.327(5964):439-42. DOI:10.1126/science.1177894. http://www.ncbi.nlm.nih.gov/pubmed/20093467

2. Saigusa T, Tero A, Nakagaki T, Kuramoto Y. 2008. Amoebae Anticipate Periodic Events. Phys Rev Lett (USA).100(1):018101. http://link.aps.org/doi/10.1103/PhysRevLett.100.018101

3. Reid CR, Latty T, Dussutour A, Beekman M. 2012. Slime mold uses an externalized spatial “memory” to navigate in complex environments. Proceedings of the National Academy of Sciences.109(43):17490-4. DOI:10.1073/pnas.1215037109. http://www.pnas.org/content/109/43/17490.abstract

4. Nakagaki T, Yamada H, Toth A. 2000. Intelligence: Maze-solving by an amoeboid organism. Nature.407(6803):470-. DOI:http://www.nature.com/nature/journal/v407/n6803/suppinfo/407470a0_S1.html. http://dx.doi.org/10.1038/35035159

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biology cell physarum slime-mold

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Footnotes

1. It is not memory like ours, but past conditions can affect their current behavior in interesting ways. In one case (Saigusa et al 2008 [2]) the "memory" of the period of dry conditions seems to involve shifting the phase relations of intracellular oscillations that occur across varying frequencies. In another case (Reid et al 2012 [3]), "memory" involves laying down slime (outside the organism) that the amoeba does not like to crawl over, so it can tell where it's already been, much like ants leaving trails of pheromones.

2. Just oats, paper towel, a clean dish, and water

3. A bottle from 1966! Maybe not so activated?