The yeast Arxula adeninivorans has been previously shown to secrete a large amount of an electro-active molecule. The molecule was produced by cells that had been cultivated in a rich medium, harvested, washed and then suspended in phosphate-buffered saline (PBS). The molecule was easily detectable after 60 min of incubation in PBS, and the cells continued to produce the molecule in these conditions for up to 3 days. The peak anodic potential of the oxidation peak was 0.42 V, and it was shown to be a solution species rather than a cell-attached species. We have optimised the production of the molecule, identified it by high-pressure liquid chromatography (HPLC) fractionation and high-resolution mass spectrometric analysis and determined the pathway involved in its synthesis. It has a mass/charge ratio that corresponds to uric acid, and this identification was supported by comparing UV spectra and cyclic voltammograms of the samples to those of uric acid. An A. adeninivorans xanthine oxidase gene disruption mutant failed to produce uric acid, which added further validity to this identification. It also demonstrated that the purine catabolism pathway is involved in its production. A transgenic A. adeninivorans strain with a switchable urate oxidase gene (AUOX) accumulated uric acid when the gene was switched off but did not when the gene was switched on. Cultivation of cells on amino acid and purine-free minimal media with an inorganic nitrogen source suggests that the cells synthesise purines from inorganic nitrogen and proceed to degrade them via the normal purine degradation pathway.

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The yeast Arxula adeninivorans has previously been shown to naturally secrete the redox molecule uric acid (UA). This property suggested that A. adeninivorans may be capable of functioning as the catalyst for a mediator-less yeast-based microbial fuel cell (MFC) if the level of UA it secretes could be increased. We investigated the effects of a number of parameters on the level of UA produced by A. adeninivorans. The concentration of UA accumulated in a dense cell suspension of A. adeninivorans after 20 h incubation was shown to be significantly lower in aerated suspensions compared with that in anaerobic conditions due to UA being rapidly oxidised by dissolved oxygen. The presence of carbon sources, glucose and glycerol, both caused a reduction in UA production compared with that in starvation conditions. The transgenic A. adeninivorans strain, G1221 (auox), showed higher UA production at 37 °C, but at 47 °C, the wild-type LS3 accumulated higher concentrations; however, elevated temperatures also resulted in very high cell mortality rates. An initial buffer pH of 8 caused a higher concentration of UA to accumulate, but high pH is detrimental to cell metabolism and the cells actively work to lower the pH of their environment. It appears that most parameters which increase the amount of UA produced by A. adeninivorans have concomitant disadvantages for cell metabolism, and as such, its potential as a self-mediating MFC catalyst seems doubtful.