Catabolic Fate of Streptomyces viridosporus T7A-Produced, Acid-Precipitable Polymeric Lignin upon Incubation with Ligninolytic Streptomyces Species and Phanerochaete chrysosporium†

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Degradation of ground and hot-water-extracted corn stover (Zea mays) lignocellulose by Streptomyces viridosporus T7A generates a water-soluble lignin degradation intermediate termed acid-precipitable polymeric lignin (APPL). The further catabolism of T7A-APPL by S. viridosporus T7A, S. badius 252, and S. setonii 75Vi2 was followed for 3 weeks in aerated shake flask cultures at 37°C in a yeast extract-glucose medium containing 0.05% (wt/vol) T7A-APPL. APPL catabolism by Phanerochaete chrysosporium was followed in stationary cultures in a low-nitrogen medium containing 1% (wt/vol) glucose and 0.05% (wt/vol) T7A-APPL. Metabolism of the APPL was followed by turbidometric assay (600 nm) and by direct measurement of APPL recoverable from the medium. Accumulation and disappearance of soluble low-molecular-weight products of APPL catabolism were followed by gas-liquid chromatography and by high-pressure liquid chromatography, utilizing a diode array detector. Identified and quantified compounds present in culture media included p-coumaric acid, ferulic acid, p-hydroxybenzoic acid, p-hydroxybenzaldehyde, protocatechuic acid, vanillic acid, and vanillin. The further catabolism of these APPL-derived aromatic compounds varied with the culture examined, and only S. setonii and P. chrysosporium completely degraded all of them. Some new intermediates of APPL metabolism also appeared in culture media, but the patterns were culture specific. Additional evidence from high-pressure liquid chromatography analyses indicated that one strain, S. badius, converted a water-soluble fraction evident by high-pressure liquid chromatography (7 to 10 min retention time range) into new products appearing at shorter retention times. Mineralization of a [14C-lignin]APPL was also followed. The percent 14C recovered as 14CO2, 14C-APPL, 14C-labeled water-soluble products, and cell mass-associated radioactivity, were determined for each microorganism after 1 and 3 weeks of incubation in bubbler tube cultures at 37°C. P. chrysosporium evolved the most 14CO2 (10%), and S. viridosporus gave the greatest decrease in recoverable 14C-APPL (23%). The results show that S. badius was not able to significantly degrade the APPL, while the other microorganisms demonstrated various APPL-degrading abilities. The significance of these findings relative to the fate of APPLs in nature was discussed.

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