Millisecond spiking units in dispersed mycelial liquid culture MEA recordings absent in dehydrated and fungicidal assays

The extracellular electrophysiology of dispersed mycelial liquid cultures has not previously been investigated in terms of wide bandwidth frequency components and presence of putative spiking units. Here, we identify stereotypical discrete unit mycelial spikes in 150-3000 Hz filtered extracellular electrophysiological recordings of liquid dispersed mycelial cultures. Hard gold coated and custom designed microelectrode arrays with individual electrode radius of 100 {micro}m were used to identify spikes with frequency components between 150-3000 Hz. The dispersed mycelial cultures were estimated to comprise of a total of 177 spiking units across triplicates (T1= 83, T2=44, T3=50). The triplicates had a combined mean trough-to-peak time of 1.58 {+/-} 0.14 ms. Dehydration and H2O2 based fungicidal assays showed reduction of the spiking units to zero. Differences in power spectral density (PSD) and Fourier transform analysis of the respective traces suggest that physical structures in the dehydration assay were preserved but that the interface between the mycelium and the electrodes was removed. Biofilm formation was not, therefore, required to detect extracellular spikes using the methods described here. The fungicidal assays removed the discrete unit electrophysiological spike resulting in zero spiking units detected, confirming the physiological origin of the extracellular signals. Extracellular electrophysiology can be studied directly in dispersed liquid cultures of mycelium using suitable microelectrode interfaces without complex media or synthetic interventions such as conductive polymers or nanoparticles.