Routing Physarum “Signals” with Chemicals

Abstract

The chemotaxis behaviour of the plasmodial stage of the true slime mould Physarum polycephalum was assessed when given a binary choice between two volatile organic chemicals (VOCs) placed in its environment. All possible binary combinations were tested between 19 separate VOCs selected due to their prevalence and biological activity in common plant and insect species. The slime mould exhibited positive chemotaxis towards a number of VOCs with the following order of preference: farnesene $> \beta$-myrcene $>$ tridecane $>$ limonene $>$ p-cymene $>$ 3-octanone $> \beta$-pinene $>$ m-cresol $>$ benzylacetate $>$ cis-3-hexenylacetate. For the remaining compounds no positive phototaxis was observed in any of the experiments, and for most compounds there was an inhibitory effect on the growth of the slime mould. By assessing this lack of growth or failure to propagate it was possible to produce a list of compounds ranked in terms of their inhibitory effect: nonanal $>$ benzaldehyde $>$ methyl benzoate $>$ linalool $>$ methyl-p-benzoquinone $>$ eugenol $>$ benzyl alcohol $>$ geraniol $>$ 2-phenylethanol. This analysis shows a distinct preference of the slime mould for non-oxygenated terpene and terpene like compounds (farnesene, $\beta$-myrcene, limonene, p-cymene and $\beta$-pinene). In contrast terpene based alcohols such as geraniol and linalool were found to have a strong inhibitory effect on the slime mould. Both the aldehydes utilised in this study had the strongest inhibitory effect on the slime mould of all the 19 VOCs tested. Interestingly 3-octanone which has a strong association with a “fungal odour” was the only compound with an oxygenated functionality where Physarum Polycephalum exhibits distinct positive chemotaxis. We utilise the knowledge on chemotactic assays to route Physarum “signals at a series of junctions. By applying chemical inputs at a simple T-junction we were able to reproducibly control the path taken by the plasmodium of Physarum. Where the chemoattractant farnesene was used at one input a routed signal could be reproducibly generated i.e. Physarum moves towards the source of chemoattractant. Where the chemoattractant was applied at both inputs the signal was reproducibly split i.e. at the junction the plasmodium splits and moves towards both sources of chemoattractant. If a chemorepellent was used then the signal was reproducibly suppressed i.e. Physarum did not reach either output and was confined to the input channel. This was regardless of whether a chemoattractant was used in combination with the chemorepellent showing a hierarchy of inhibition over attraction. If no chemical input was used in the simple circuit then a random signal was generated, whereby Physarum would move towards one output at the junction, but the direction was randomly selected. We extended this study to a more complex series of T-junctions to explore further the potential of routing Physarum. Although many of the “circuits were completed effectively, any errors from the implementation of the simple T-junction were magnified. There were also issues with cascading effects through multiple junctions. This work highlights the potential for exploiting chemotaxis to achieve complex and reliable routing of Physarum signals. This may be useful in implementing computing algorithms, design of autonomous robots and directed material synthesis. In additional experiments we showed that the application of chemoattractant compounds at specific locations on a homogeneous substrate could be used to reliably control the spatial configuration of Physarum.