چكيده لاتين
Lignin, a heteropolymer in the lignocellulose structure, plays a key role in the strength and stability of plants. This heteropolymer is often considered a "nuisance" in industries like pulp and paper mills because various physical and chemical methods are used to separate cellulose and hemicellulose from it. Unfortunately, the separated lignin is usually not properly disposed of and is therefore improperly released into the environment. Using biological methods for lignin degradation offers significant advantages. For example, these methods are not only cost-effective but can also break down lignin into simpler compounds with higher added value. Consequently, the resulting compounds can be used as raw materials in other industries, preventing the improper release of lignin into the environment. The objective of this research was to search for and isolate bacteria with lignin-degrading enzymes. Furthermore, experiments were conducted on the bacterium Deinococcus radiodurans to evaluate its potential in this area. Three bacteria, including Acinetobacter olivorans (SFD1), Enterobacter cloacae (SFD2), and Escherichia coli (SFD3), were isolated and registered with the accession numbers PP767398, PP951854, and PQ368848 on the NCBI website, respectively. The activity levels of manganese peroxidase (MnP), lignin peroxidase (LiP), and laccase (Lac) enzymes, which are involved in the lignin degradation process, were measured in these three isolates by the oxidation of substrates such as 2,6-dimethoxyphenol, veratryl alcohol, and 3-ethylbenzothiazoline-6-sulfonic acid (ABTS), respectively. The results showed that the highest activities for lignin peroxidase, manganese peroxidase, and laccase were recorded for Deinococcus radiodurans (615 units), Acinetobacter olivorans (130 units), and Enterobacter cloacae (4120 units), respectively. Additionally, the gene encoding the multicopper oxidase (CueO) enzyme, which is similar to laccase, was identified in the Escherichia coli isolate using a set of designed primers, which justifies the ability of this isolated strain to degrade lignin. The released low-molecular-weight compounds were evaluated and identified using Gas Chromatography-Mass Spectrometry (GC-MS) analysis. Fourier-Transform Infrared Spectroscopy (FT-IR) analysis also showed changes in the chemical structure and the production of alcohol, phenolic (OH-), and aromatic (C=C) compounds by the isolates. The obtained results indicated that the isolated bacteria and the Deinococcus radiodurans strain efficiently degrade lignin, with the highest degradation (66.4%) observed for the Enterobacter cloacae isolate. Notably, this research for the first time isolated and introduced a naturally-occurring Escherichia coli strain with lignin-degrading ability (44%). Given the importance of Escherichia coli as a host strain in industrial bioprocesses, this isolate holds high potential for use in biorefineries for lignin valorization and for the removal of lignin from industrial effluents.
