The Role of Ligninolytic Enzymes Laccase and a Versatile Peroxidase of the White-Rot Fungus Lentinus tigrinus in Biotransformation of Soil Humic Matter: Comparative In Vivo Study

Soil organic matter (SOM) turnover by ligninolytic fungi is a large-scale process that controls organic carbon geochemistry in terrestrial ecosystems. However, the role of certain oxidative enzymes (e.g., laccase) in humus degradation remains unclear, as well as the molecular structure and recalcitrance of SOM components. In order to address these questions, the degradation of forest soil humic acid (HA) in the presence of laccase and a versatile peroxidase (VP) has been studied in the liquid culture of Lentinus tigrinus. Contrary to the evolving views on humus structure, we have found that alkali-extractable and acid-insoluble constituents of SOM (HA) contain true macromolecular components, stable in the presence of 0.1% sodium dodecyl sulfate but degradable/resynthesizable by oxidative enzymes acting on covalent linkages. The HA degradation in the presence of laccase (high N medium) occurs at slower initial rate than in the presence of VP (low N medium). However, each of the enzymes caused about 60% color loss and almost complete degradation of HA into smaller molecules (gel-filtration data) within 2weeks of cultivation. Depolymerization of HA in the culture liquid in the presence of laccase was accompanied by polymerization of degradation products on mycelium. Our results show that (1) humus macromolecules are not stable to oxidative enzymes once desorbed from the mineral phase, (2) laccase of Lentinus tigrinus is comparable by its degradation potential to VP, and (3) interfacial secondary synthesis reactions occur during humus decay in the presence of laccase. Our results highlight the important role of laccases in SOM sequestration in soils.