Comparison of decolorization by microorganisms of humic acids with different 13C NMR properties

Differences in decolorization of five humic acids, added to Czapek–Dox medium and inoculated with Coriolus consors, Coriolus hirsutus, Lenzites betulina, Streptomyces aureofaciens, or an unidentified actinomycete, were investigated. Percent decolorization after incubation for 21 d at 25 °C differed among humic acids and microbial strains. In the case of the three fungi, Cambisol humic acids, characterized as having low aromatic C and high carbohydrate C contents, were decolorized to a greater extent (12–51%) than were Histosol humic acids (3–29%) and Andosol humic acids (0–31%). Among humic acids decolorized by C. consors, those with higher percentage decolorization had high contents of carbohydrate C, low contents of aromatic C and a low degree of aromaticity. In contrast, S. aureofaciens decolorized Andosol humic acid 2 (86.5%) and both Cambisol humic acids (40.4–57.2%) but had little effect on Andosol 1 and Histosol. These results suggested that the resistance of humic acids to microbial decolorization is related to differences in their chemical structural and is microbial species dependent.