Context. The anaerobic digesters have potential to produce 14,000 TWh from currently available feedstocks, corresponding to 6-9 % of the world's energy consumption.
Gap. While the gas-producing microbiota are well described and understood, little is known about the hydrolysing microorganisms, although this step represents one of the biggest limitations during the anaerobic digestion.
Aim and Methods. We examined an anaerobic digester reactor fed with cellulose at the metabolic level using the approach of retrieving simultaneous information on the identity of the microorganisms (metagenomics assemblies) and the activity from the 13C-labelled proteins (Protein-SIP) after incubation with 13C-labelled cellulose.
Findings. The metagenome conceived 17,155 variants of carbohydrate-active enzymes, corresponding to 238 different enzymes, 6 of these were directly associated to cellulose degradation and were linked to 756 scaffolds in the metagenome. The enzymes cover cellulase, cellobiose phosphorylase, cellodextrin phosphorylase, cellulose 1,4-beta-cellobiosidase (non-reducing end), cellulose 1,4-beta-cellobiosidase (reducing end), and cellobiose dehydrogenase. These enzymes were affiliated with four different phyla: Actinobacteria, Euryachaeota, Firmicutes, and Proteobacteria.
A total of 2,075 peptides were identified, and overall 20 peptides were found to have assimilated labelled carbon from degradation of 13C-cellulose. These peptides were binned to the metagenome and revealed that the majority affiliated with organisms representing the phylum Firmicutes.
The 13C isotopic profiling during the start of incubation showed that CH4 and CO2 are the main products of cellulose degradation. After two days of incubation the isotopic ratio was 0.116, corresponding to the isotopic ratio fed to the reactor (10 %).
Utilization. The combined approach of with metagenomics and metaproteomics using 13C-labelled compounds has been successfully shown to provide new insight into the most active cellulose degrading communities in anaerobic digesters and provide a unique approach to couple function to identity.
