How can the passage of nitrogen through fungal networks be measured? Existing techniques are limited, but in this issue Tlalka et al. (pp. 173–184) describe a new method for the continuous imaging of the radioactivity distribution in a living labelled sample – or, as here, a whole mycelium.
The authors were able to image the translocation of a 14C-labelled amino acid analogue, α-aminoisobutyric acid (AIB) in a foraging mycelium of the wood-decaying fungus Phanaerochaete velutina. N conservation mechanisms in wood-decaying fungi are seen as important for their success in nature since these fungi normally obtain their resources for growth from a substrate with very limited N content. AIB is taken up by fungi and transported as other amino acids, but is not incorporated into proteins (Watkinson, 1984; Lilly et al., 1990). Labelled AIB is thus an important tool for dynamically investigating the reallocation of N according to the local needs of the fungal mycelium. The principle of the method described consists of allowing a fungal mycelium to extend from an inoculum over a plastic intensifying screen (scintillating screen) – normally used for intensifying the recordings on X-ray films in autoradiography of gels – and then recording the light output after addition of radioactive isotopes with a photon counting camera. With the new method it was possible to make continous recordings and produce time-lap films of the events (see http://www.blackwell-science.com/products/journals/suppmat/NPH/NPH288/NPH288sm.htm).
The two main findings were that the translocation of labelled AIB was faster than diffusion and that there was a pulsatile component in the tranlocation. Translocation more rapid than diffusion has been shown before (Olsson & Gray, 1998), but the observation of a pulsation in the translocation is completely new. The pulsatile component has a period of 11–12 h and there is no obvious explanation. It could hardly have been detected without the continuous recordings. Similar experiments should now be preformed for many other fungi to see if such pulsations are a common characteristic (although the authors do report that they have investigated Serpula lacrymans, but found no pulsation). A whole range of interesting experiments with P. velutina can be envisaged to elucidate the cause of the pulsations, and these might give important insights into the mechanism(s) and regulation of nutrient reallocations in fungi.
The method is a clever combination of standard intesifying screens and photon counting systems combined with image analysis. There is no need for a dedicated and expensive radioactivity β-scanning system. The main improvement to existing methods using β-scanners (Olsson & Gray, 1998; Lindahl et al., 1999) is in spatial resolution, but it also makes it possible to record continuously over a long period. The general method, to use a scintillating screen and a photon counting camera to record the radioactivity in the sample, can potentially be used for any thin biological sample and for all kinds of β-emitting radioactive isotopes which are at least as penetrating as 14C. The only requirement is that the sample is thin and that the light absorption from the biomass is negligible.