Influence of clay minerals on the formation of humic substances by Epicoccum nigrum and Stachybotrys chartarum

The influence of montmorillonite, kaolinite and finely ground quartz on the formation of humic acid-type polymers by Epicoccum nigrum and Stachybotrys chartarum was studied. The fungi were grown in shake and in deep (4 cm) or shallow (1.5 cm) stationary cultures. In general, clay shortened the time required for the formation of dark colored substances and increased the amounts of humic acid-type polymers in the culture solutions or extracted from the cells with NaOH. In some tests, the time of maximum humic polymer accumulation was much earlier in the presence of clay but total amounts formed in check cultures eventually equalled that of the cultures with clay additions. The ratios of the total humic acid to the total cell substance or to the glucose consumed were also generally increased by montmorillonite. Kaolinite and quartz exerted a similar but less pronounced effect. In deep stationary cultures of S. chartarum, total growth and humic acid formation was sometimes depressed by the higher concentrations of montmorillonite but in shallow cultures biomass and humic acid formation were increased. In cultures with an initial pH of 6.0, humic acid polymers were formed in the cells before they appeared in the solutions. During autolysis, some of the cellular polymers were either released into the medium or became more readily extractable with NaOH. The clays did not appreciably alter the chemical properties of the humic polymers, namely, C and N contents, exchange capacity, COOH groups, total acidity, or phenols released upon sodium amalgam reduction. In the presence of clays, phenols were formed more quickly in the culture solutions, but the kinds and relative amounts did not appear to be altered. Clays did not significantly affect oxygen consumption during autoxidation of phenol mixtures. The observations indicate that, by affecting growth and metabolism, the clays indirectly influence phenolic polymer formation.