Microbial depolymerization of Kraft lignin for production of Vanillic acid by indigenous ligninolytic strains

Abstract

Lignin is a potential renewable source for the production of many value-added bio-products through bacterial depolymerization. In this study, Kraft lignin (KL) was depolymerized by indigenous potential ligninolytic strains to obtain the optimum amount of Vanillic acid. Initially, fifteen bacterial strains were isolated from decaying wood mixed soil to assess their lignin degradation efficiency. Five potential lignin-degrading microbes (RW-B₂, RW-B₄, RW-B₉, RW-B₁₁ and RW-B₁₅) were identified by nutrient enrichment technique. RW-B₁₅ was observed as a dominant strain sharing a 99 % resemblance to Bacillus megaterium sp. Optimum temperature and bacterial inoculum concentration were noted as 35 °C and 1 %, respectively. Mixed microbial culture of five potential ligninolytic strains showed better KL degradation than pure culture due to different metabolic capacities, enzymatic mechanisms and synergetic impact among the association members. The KL degradation study was conducted with different KL concentrations (500–2000 mg/l) at optimum temperature using 1 % inoculums. The production of Vanillic acid increased from 60.1 to 199.9 mg/l with increasing concentration of KL from 500 to 2000 mg/l. The maximum production of Vanillic acid was obtained on 3rd day of degradation and it decreased thereafter which is probably due to an increase in microbial toxicity and enzymatic activity. However, the COD level continuously decreased for all KL concentrations with degradation time till the last day. The pH of the KL solution slightly decreased on 3rd day due to the acidic nature of Vanillic acid. GC-MS analysis confirmed the presence of Vanillic acid and showed the presence of other monomeric compounds formed after KL degradation.