Abstract
Lignite is difficult to degrade, thus stimulation of the autochthonous lignite microflora and introduction of additional microorganisms are required for lignite decomposition. Here, a packed bed reactor, filled with lignite samples from the Konin region (central Poland) was supplied continuously with M9 medium, supplemented with molasses (a by-product from the sugar industry), for 124 days to stimulate the autochthonous lignite microflora. Acidic fermentation of molasses was observed in the bioreactor. The simultaneous decomposition of lignite occurred under this acidic molasses fermentation condition. Our results show decay of free (non-bound) organic compounds during anaerobic lignite biodegradation. The concentrations of n-alkanes, n-alkanols, n-alkanoic acids, diterpenoids, triterpenoids and steroids present in non-biodegraded samples decreased significantly (some compounds to zero) during biodegradation. Interestingly, other compound classes like phenols, ketones and certain organic compounds increased. We interpret this phenomenon as a gradual decomposition of polymers, lignin and cellulose, present in the lignite. These changes resulted from microbial activity since they were not observed in pure solutions of short-chain fatty acids. The 16SrRNA profiling of the microbial community selected in the bioreactor revealed that the dominant bacteria belonged to the Firmicutes, Actinobacteria, Proteobacteria and Bacteroidetes, furthermore representatives of 16 other phyla were also found. All the known taxa of lignocellulolytic bacteria were represented in the microbial community. Synergistic relations between bacteria fermenting molasses and bacteria degrading lignite are assumed. The results confirm lignin degradation in acidic medium by bacteria under anaerobic conditions.
Highlights
Lignite is a biogenic sedimentary rock formed during diagenesis and coalification of terrestrial organic matter
These processes resulted from microbial activity since they were not observed in pure solutions of short-chain fatty acids
Lignite is a rich source of various microorganisms
Summary
Lignite ( known as brown coal) is a biogenic sedimentary rock formed during diagenesis and coalification of terrestrial organic matter. The primary organic constituents such as cellulose and the more recalcitrant lignin, humins or humic acids are commonly present in lignite (De Leeuw and Largeau, 1993; Fabbri et al, 2008; Fabbri et al, 2009; Grasset et al, 2010). Lignin is a mixture of polymeric compounds containing a number of aromatic rings based on phenylpropane units (Naseem et al, 2016). There are numerous biodegradable functional groups that facilitate the attack and cleavage of aromatic rings (Fabiańska and Kurkiewicz, 2013). The occurrence of methane of a biogenic origin in coal deposits worldwide (so called coal-bed methane) indicates that under anaerobic conditions coal can be degraded to methane by microbial activity (Batstone and Jensen, 2011; Ritter et al, 2015; Szafranek-Nakonieczna et al, 2018)
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