Use of absorption spectra and their second-order derivative to quantify degradation of lignohumate by filamentous fungi

Soil fungi are actively involved in the processes of humic substances synthesis, transformation and mineralization due to production of extracellular nonspecific oxidative enzymes. The work was aimed to evaluate using spectral methods transformation dynamics for the humic product (HP) from lignosulfonate (HPligno) by filamentous soil fungal cultures Alternaria alternata and Trichoderma harzianum. Experiments showed that direct spectroscopic study of HPligno introduced into the nutrient medium and its transformation during fungal growth is challenging due to strong absorption of light by nutrient medium, development of absorbing fungal metabolites, partial utilization and destruction of HP by fungi and therefore due to the need to register tiny changes in overlapping bands. To accomplish that task we proposed a novel algorithm for processing the absorption spectra, which has not previously been used to study fungal cultures. We calculated the second-order derivative in respect to wavelength for absorption spectra measured during fungal growth and found characteristic "patterns" for introduced HP: a maximum at 270-285 and a minimum within 290-300 nm. The spectral index determined from amplitudes in the second-order derivative spectrum reflects the relative content of HP in the nutrient medium in presence of other absorbing components. We resume that two fungal strains utilized HPligno in the 0,02 and 0,1% concentrations better at 30 g/L sucrose than at 3 g/L in the medium. Thus the second-order differentiated absorption spectra helped to quantify degradation of the HPligno during fungal growth.