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Abstract: Nine Streptomyces strains were screened for their ability to solubilise and mineralise 14C-labelled lignin during growth in solid-state fermentation. Streptomyces viridosporus was confirmed as an active lignin-degrading organism along with a new isolate, Streptomyces sp. UAH 15, further classified as Streptomyces cyaneus CECT 3335. This organism was able to solubilise and mineralise the [14C]lignin fraction of lignocellulose (44.96 ± 1.77% and 3.41 ± 0.48% respectively) after 21 days of incubation. Cell-free filtrates from Streptomyces sp. grown in solid-state fermentation were capable of solubilising up to 20% of the [14C]lignin after 2 days incubation, with most of the product detected in the acid-soluble rather than in the water-soluble fraction. Identification of the extracellular enzymes produced during growth of S. cyaneus CECT 3335 revealed that extracellular peroxidase and phenol oxidase activities were present, with the activity of phenol oxidase being 100 times greater than peroxidase activity. The activity of these two enzymes was found to correlate with both solubilisation and mineralisation rates. This is the first report of phenol oxidase activity produced by a Streptomyces strain during growth in solid-state fermentation. A role for the enzyme in the solubilisation and mineralisation of lignocellulose by S. cyaneus is suggested.

Abstract: Two Streptomyces strains, UAH 30 and UAH 51, have been shown to decolourise a paper-mill effluent obtained after semichemical alkaline pulping of wheat straw. Fractionation of the effluent decolourised by strains UAH 30 and UAH 51 showed that 60% and 80% respectively of the alkali-lignin fraction have been removed from the effluent after 7 days of growth. 13C NMR cross polarization and magic angle spinning (CPMAS) spectra of the alkali-lignin remaining in the effluent after decolourisation revealed a decrease in the relative amount of aromatic lignin units compared to that obtained from the untreated effluent along with a reduction in the ratio of syringyl:guaiacyl units. Gas chromatography/mass spectrometry analysis of the low-molecular-mass compounds extracted from the decolourised effluent revealed the presence of new aromatic lignin-related compounds that were not present in the untreated control effluent. This was linked to a general depolymerization of larger lignin molecules occurring during decolourisation by the two Streptomyces strains. Identification of low-molecular-mass aromatic compounds extracted from the decolourised effluent revealed only the presence of p-hydroxyphenyl units in effluents decolourised by the strain UAH 30 while p-hydroxyphenyl, guaiacyl and syringyl units were detected in effluents decolourised by Streptomyces strain UAH 51. The study indicates that, while decolourisation is a common feature of the two Streptomyces strains, the mechanisms involved in the degradation of the lignin fractions may be different and strain-specific.