Biodegradation of alkaline lignin by Bacillus ligniniphilus L1

Abstract

Background: Lignin is the most abundant aromatic biopolymer in the biosphere and it comprises up to 30% of plant biomass. Although lignin is the most recalcitrant component of the plant cell wall, still there are microorganisms able to decompose it or degrade it. Fungi are recognized as the most widely used microbes for lignin degradation. However, bacteria have also been known to be able to utilize lignin as a carbon or energy source. Bacillus ligniniphilus L1 was selected in this study due to its capability to utilize alkaline lignin as a single carbon or energy source and its excellent ability to survive in extreme environments.

Results: To investigate the aromatic metabolites of strain L1 decomposing alkaline lignin, GC-MS analysis was performed and fifteen single phenol ring aromatic compounds were identified. The dominant absorption peak included phenylacetic acid, 4-hydroxy-benzoicacid, and vanillic acid with the highest proportion of metabolites resulting in 42%. Comparison proteomic analysis was carried out for further study showed that approximately 1447 kinds of proteins were produced, 141 of which were at least twofold up-regulated with alkaline lignin as the single carbon source. The up-regulated proteins contents different categories in the biological functions of protein including lignin degradation, ABC transport system, environmental response factors, protein synthesis, assembly, etc.

Conclusions: GC-MS analysis showed that alkaline lignin degradation of strain L1 produced 15 kinds of aromatic compounds. Comparison proteomic data and metabolic analysis showed that to ensure the degradation of lignin and growth of strain L1, multiple aspects of cells metabolism including transporter, environmental response factors, and protein synthesis were enhanced. Based on genome and proteomic analysis, at least four kinds of lignin degradation pathway might be present in strain L1, including a Gentisate pathway, the benzoic acid pathway and the β-ketoadipate pathway. The study provides an important basis for lignin degradation by bacteria.

Keywords: Alkaline lignin; Bacillus ligniniphilus L1; GC–MS; Proteomics.

Fig. 1

Growth of strain L1 during 7 days incubation with lignin or glucose as carbon source. Symbols: closed circles glucose and lignin as carbon source incubated at 50 °C; closed squares lignin as single carbon source incubated at 50 °C; closed diamond glucose and lignin as carbon source incubated at 30 °C; closed triangles lignin as single carbon source incubated at 30 °C; open circles the concentration of glucose during incubation of strain L1 in culture with glucose and lignin as carbon source

Fig. 2

Lignin degradation and decolorization rate during 7 day’s incubation. Symbols: open circle lignin concentration; open squares uninoculated sample’s lignin concentration; closed circle lignin decolorization proportion; closed squares uninoculated sample’s lignin decolorization proportion

Fig. 3

Scanning electron micrograph of lignin. a Untreated lignin, b lignin treated by 7 days incubation in MM63 medium with lignin as single carbon source without strain, c lignin treated by 7 days degradation with strain L1 in MM63 medium with lignin as single carbon source

Fig. 4

The variation of absorption peaks area of part aromatic compounds identified by GC–MS during 7 days’ incubation with lignin as single carbon source with uninoculated sample as control. Numbers represent the aromatic compounds were same with Table 1, and C2, C7, C8, C9, C10, and C11 represent the compounds peaks of control sample

Fig. 5

Comparison of absorption peaks of aromatic compounds identified by GC–MS during culturing process. a The variation of absorption peaks at first and fifth days’ incubation. b The proportion of absorption peaks area of aromatic compounds in the fifth days’ incubation. c The proportion of absorption peaks area of aromatic compounds in the control samples. Numbers represent the aromatic compounds were same with Table 1

Fig. 6

Putative lignin degradation pathways of strain L1. Numbers represent the aromatic compounds were same with Table 1