Fluorescence spectroscopy: a tool to characterize humic substances in soil colonized by microorganisms?

The ability of eight soil microfungal species, Alternaria alternata, Clonostachys rosea f. rosea, Exophiala cf. salmonis, Fusarium cf. coeruleum, Fusarium redolens, Paecilomyces lilacinus, Penicillium canescens and Phoma sp., and two known basidiomycete humic acid (HA) degraders, Trametes versicolor and Phanerochaete chrysosporium, to modify fluorescence properties of fulvic acids (FA) and/or HAs was determined. Effects of minerals and/or glucose on the modifications were examined. FA purified on polyvinyl-polypyrrolidone (PVPP) chromatography column was used. Purification of FA on PVPP column removed the low-molar-mass FA-structural components and excess of extractant (NaOH) used during FA preparation. Excitation spectra of FA entering the purification, purified FA and the removal solution indicate that organic compounds rich in carboxylic groups dominate in the removal solution and higher content of phenolic groups is a characteristic of purified FA. Many microfungal species shifted the emission maximum (measured at 470 and 468 nm of excitation wavelength) of FA, and also HA to longer wavelengths. The opposite effect (shift of the HA emission maximum to shorter wavelengths) of microfungi was observed for HA complemented by glucose. Depending on the presence of glucose in the medium, most microfungi changed also the shape of the emission spectra of HA and FA and the excitation spectra of FA. HA excitation spectrum measured at 590 nm of emission wavelength was significantly affected by the presence of glucose. Mineral ions caused a minor shift in the position of excitation maximum (measured at 590 nm of emission wavelength) toward longer wavelengths.