Lignin is a complex and heterogenous aromatic biopolymer that can be found in plant cell walls as one of the components of lignocellulose. Large quantities of lignin are daily generated as a by-product in paper and biofuel industries. To date, lignin is still treated as waste and is mostly burned as a low-quality fuel, even though it could potentially be a great source for producing value-added chemicals and fuels as it is an abundant source of renewable aromatic carbon. As a result of the energy crisis, the interest in lignin and its valorization has grown considerably in recent years, mainly focusing on more environmentally friendly lignin degradation by microorganisms. The majority of research has so far been concentrating on fungal lignin degradation. However, fungi unfortunately seem to be unsuitable for large-scale application because of the complexity of their enzymes. Ligninolytic enzymes are very rare in bacteria, but those that possess this physiological trait present a new, better option than using fungi to valorize lignin.
Our research focuses on bacterial lignin degradation, with the objective to study the ligninolytic activity of 5 laccase-producing bacterial strains, laccase being an important lignin-modifying enzyme. Our research question was: ''Did biodegradation occur in any of the samples? In case the answer is affirmative, which products were generated by the degradation?'' Based on the existing literature we defined 20 phenolic compounds, which could potentially appear as products of bacterial lignin degradation. 13 of those compounds were then included in two analytical methods that we developed for targeted screening of lignin degradation products. The analytical methods were based on determination by reversed-phase liquid chromatography coupled to tandem mass spectrometry. Prior to the LC-MS/MS analysis, we purified the samples by solid-phase extraction using extraction plates on a positive pressure extraction system. The analytical method was then tested on real samples, containing lignin treated with either of the laccase-producing bacterial strains, presuming that detection of any of the potential degradation products in a sample would prove biodegradation. The absence of those potential degradation products does, however, not mean that biodegradation did not take place. It merely suggests the absence of the specific degradation pathways, found in the literature. Since the target analysis did not confirm the presence of the expected analytes, we also performed nontargeted analysis using information-dependent acquisition-mediated LC-MS/MS screening. Acquired chromatograms showed significant differences between lignin samples and blanks, confirming its degradation. We presented our research at the 12th Jožef Stefan international postgraduate school students' conference.
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