Sequence based analysis of microbial communities in anaerobic digestion reveal key organisms and rate limiting hydrolysis pathways in the methanisation of whisky distillation waste

Anaerobic digestion is the breakdown of an organic material into a methane rich biogas through the action of a complex microbial community. This research primarily uses sequence based technologies such as gene amplicon sequencing, qPCR, metagenomics and metatranscriptomics to investigate population composition, dynamics and functionality in these communities when fed a liquid waste product from malt whisky distillation termed pot ale.
Initial batch tests complemented with gene amplicon sequencing explored the potential of used wastewater dewatering polymer, a cationic polyacrylamide, and a community inoculum. Communities produced from this material showed production of biogas and methane and increased abundance in carbohydrate degrading organisms such as Bacterodia. Poor methane yields could be attributed to the inhibitory effects of the acrylamide containing inoculum so subsequent digestion of pot ale used an alternative inoculum. To quantitatively determine how the microbial community was changing, a microbial spike-in method was developed. This spike-in method involved characterisation of an organism not typically found in anaerobic digestion, Sulfolobus solfataricus, and showed to normalise DNA extraction procedures in addition to population quantification.
Further pot ale anaerobic digestion was performed in a custom-built lab scale system over a period of six months. This revealed that methane production from pot ale occurs in two distinct stages, the first fast and the second slow. DNA samples were collected over this time and metagenomic sequencing was used to reconstruct key metabolic pathways which revealed a functionally diverse, and functionally robust community. Genes specifically related to the two stage degradation of pot ale digestion were investigated using metatranscriptomics which revealed initial methane production was caused by hydrolysis of malto-oligosaccharides by Clostridia followed by the second, rate limiting, hydrolysis of beta-glucans by Bacterodia. By identifying this rate limiting enzymatic hydrolysis step, this opens the door to feedstock specific enzyme supplementation to increase digestion efficiency.