The Role of Laccase from Zygomycetous Fungus Mortierella elasson in Humic Acids Degradation

Alexander Lisov,Andrey Konstantinov,Anna Zavarzina,Alexey Leontievsky,Oksana Belova

doi:10.3390/agronomy11112169

Abstract

Biochemical stability of soil humus is an important factor affecting soil quality. Fungi are among the most efficient decomposers of humic matter due to presence of oxidative enzymes, including phenoloxidase laccase. Production of laccase by zygomycetes, a group of cellulolytic fungi widespread in soil, is poorly studied. The potential role of laccase from zygomycetes in humus turnover is unknown. Here, we show for the first time that laccase of zygomycetous fungus Mortierella elasson can effectively depolymerize humic acids in vitro. The fungus produced laccase extracellularly in a liquid culture medium. Unlike in case of laccases in ligninolytic basidiomycetes, attempts to increase enzyme activity using inductors, changes in the source of nitrogen and carbon failed to lead to any increase in laccase production. Laccase was purified using ion exchange chromatography and gel filtration. The molecular weight of the laccase was 51.75 kDa. The laccase catalyzed the oxidation of ABTS and K4[Fe(CN)6], phenolic compounds, but not tyrosine. The laccase activity was inhibited by NaN3 and NaF. The pH optimum of the laccase activity was 3.0 for ABTS and 5.0 for 2,6-dimethoxy phenol. The enzyme had moderate thermal stability and was rapidly inactivated at 70 °C. Purified laccase depolymerized humic acids from retisol, compost and peat more effectively than culture liquid containing laccase. The results of the study extend our knowledge of the role of laccases from different producers in the transformation of natural organic matter.

Highlights

Soils store about 1500 Gt of organic C in one meter layer [1]
We show for the first time that laccase from a representative of zygomycete fungi can depolymerize HA both in the liquid fungal culture and in vitro
To optimize the production of laccase by M. elasson on Medium 1 we investigated the effect of some substances, often used as inducers of laccase in fungi, on the production of the enzyme

Summary

Introduction

Humic acids (HA—alkali-soluble, acid-insoluble components of humus) serve as the characteristic and quantitatively significant operational fraction of humus comprising 10–40% of Corg in humus horizons [4]. This fraction is largely represented by polar hydrophylic compounds bearing carboxylic and phenolic moieties. In mineral soil horizons the majority of these humic constituents are stabilized by interaction with soil mineral matrix. Destabilization of these polar compounds under soil management practices can lead to substantial carbon losses. It is important to understand biochemical controls for stability of these abundant humus components in order to better predict the dynamics of soil organic matter (SOM) under natural- and anthropogenically-induced disturbances

Methods

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Discussion

Conclusion