In the microbial community of decaying wood, yeasts are important for the recycling of nutrients. Nevertheless, information on their biodiversity in this niche in the Northern hemisphere is limited. Wood-colonising yeasts encounter identical and similar growth-inhibitory compounds as those in spent sulphite liquor (SSL), an energy-rich, acid hydrolysate and waste product from the paper industry, which may render them well-suited for cultivation in SSL. In the present study, yeasts were isolated from decaying wood on the Faroe Islands and identified based on sequence homology of the ITS and D1/D2 regions. Among the yeasts isolated, Candida argentea, Cystofilobasidium infirmominiatum, Naganishia albidosimilis, Naganishia onofrii, Holtermanniella takashimae and Goffeauzyma gastrica were new to decaying wood in cold and temperate climates. C. argentea and Rhodotorula are rarely-isolated species, with no previous documentation from cold and maritime climates. The isolates were further tested for growth in a medium with increasing concentrations of softwood SSL. Most grew in the presence of 10% SSL. Isolates of Debaryomyces sp., C. argentea and Rhodotorula sp. were the most tolerant. Representatives of Debaryomyces and Rhodotorula have previously been found in decaying wood. In contrast, the least tolerant isolates belonged to species that are rarely reported from decaying wood. The relative importance of individual inhibitors to yeast growth is discussed. To our knowledge, none of the present yeast species have previously been cultivated in SSL medium. Decaying wood can be a useful future source of yeasts for valorisation of various hydrolysates to industrial chemicals and biofuels.

Lignocellulosic biomass is a challenging ecological niche for microorganisms. Spent sulphite liquor (SSL), which derives from acid hydrolysis of lignocellulosic biomass, is an even greater challenge, due to the presence of toxic phenolic compounds, specific monosaccharides, lignosulphonates and inhibitors, such as HMF, furfural, formic acid and acetic acid. One of these inhibitors is vanillin, a lignin monomeric derivative. Could yeasts that originate from wood tolerate vanillin and grow in the presence of SSL? A basidiomycetous yeast, Cystobasidium laryngis strain FMYD002, grew in vanillin-supplemented media, and biotransformed vanillin into vanillyl alcohol. It is part of a collection of yeasts isolated from decaying wood on the Faroe Islands. The aim of the present study was to determine the vanillin biodegradation profiles and the ability to grow in the presence of SSL.

These yeasts were identified by ITS1–5.8S–ITS2 and D1/D2 sequence homology. The relationship between wood-habitat and vanillin tolerance by cultivating the yeasts in the presence of 1 mM vanillin. The vanillin biodegradation profiles were determined by LC-MS, using the standards: vanillin, vanillyl alcohol and vanillic acid. The growth in different concentrations of SSL was evaluated.

Strains of Cystobasidium laryngis, Cystofilobasidium infirmominiatum, Goffeauzyma gastrica, Goffeauzyma sp., Naganishia sp., Holtermanniella sp., Rhodotorula sp., Nadsonia starkeyi-henricii, Debaryomyces hansenii, Debaryomyces sp., Candida sake and Candida argentea were identified. Most strains were able to grow in vanillin-supplemented medium. The predominant biodegradation product was vanillyl alcohol followed by vanillic acid. Several other biodegradation products were detected. Most strains were able to grow in the presence in SSL. Species of Candida and Debaryomyces were most tolerant, whereas species of Nadsonia, Holtermanniella and Naganishia grew poorly.

Many of the yeast species described herein are associated with wood or cold environments. Ability to grow in the presence of vanillin did not completely correlate with tolerance to SSL. However, the strains that grew at the highest concentration of SSL also grew well in the presence of vanillin, from which they rapidly producedl arge amounts of vanillyl alcohol, and many other biodegradation products. Conversely, the isolates with poor or no growth in vanillin had extremely low or no tolerance to SSL. Thus, high tolerance to vanillin appeared to be a prerequisite for growth in SSL-based medium. Different yeasts have tolerance to different inhibitors present in SSL. A comprehensive analysis of growth and biodegradation of vanillin produced five groups, containing specific yeast genera.

Vanillin is an aromatic aldehyde found as a component of lignocellulosic material, and in the cured pods of orchidaceae plants. Like other phenolic substances, vanillin has antimicrobial activity and can be extracted from lignin either by a thermo-chemical process or through microbial degradation. Vanillin, can serve as a model monomer in biodegradation studies of lignin. In the present study, a yeast isolated from decaying wood on the Faroe Islands, was identified as Cystobasidium laryngis strain FMYD002, based on internal transcribed spacer sequence analysis. It demonstrated the ability to convert vanillin to vanillyl alcohol, as detected by ultra-high performance liquid chromatography–quadrupole-time-of-flight. Structural analysis of vanillyl alcohol was carried out by using gas chromatography–mass spectrometry and 1H NMR spectroscopy, and further verified by synthesis. The reduction of vanillin to vanillyl alcohol has been documented for only a few species of fungi. However, to our knowledge, this biotransformation has not yet been reported for basidiomycetous yeast species, nor for any representative of the subphylum Pucciniomycotina. The biotransformation capability of the present strain might prove useful in the industrial utilisation of lignocellulosic residues.

Cystobasidium species are mostly found in temperate or cold environments. They have been isolated from a wide range of habitats: plants, soils, rocks, aquatic environments and indoor dust. The genus Cystobasidium mainly consists of species of red yeasts in the Rhodotorula minuta clade. These basidiomycetous yeast species are commonly found in temperate to cold regions. In the present study, two strains of Cystobasidium sp. were isolated from decaying wood of housing on the Faroe Islands, where the average yearly temperature ranges from 2°C to 13°C. The sequences of the two strains had two identical gaps within the ITS1ހ5.8SހITS2 region and a second gap within the D1/D2 LSU unit, when aligned to those of C. laryngis CBS 2221, their closest match. The isolates were designated as Cystobasidium sp. Both isolates converted vanillin into vanillyl alcohol in the presence of oxygen. The biotransformation of vanillin into vanillyl alcohol has been documented for only a few species of fungi, but to our knowledge, it has not previously been reported for any basidiomycetous yeast species. Rhodotorula rubra, a distantly related basidiomycetous yeast converts vanillin into vanillic acid. In the present study, the two isolates of Cystobasidium sp. did not produce any trace of vanillic acid, as determined by LC-MS, 1HހNMR and GC. Oxidizing vanillin into vanillic acid should be preferred by the fungi, since it results in more chemical energy, as compared to reducing it to vanillyl alcohol. The fungus may choose this pathway to escape the toxicity of both vanillin and vanillic acid. Vanillin has antimicrobial activity, and vanillic acid is more toxic than vanillyl alcohol. Vanillin is a constituent of the lignin molecule. Cystobasidium species are commonly found in the phyllosphere. Their ability to utilize plant chemicals should render them successful competitors on plants and wood.