Epsilon Archive for Student Projects

Abstract

Renewable energy sources, such as biogas, play an important role in the transition to a climate neutral society. Despite the great abundance of lignocellulosic biomass, its use for biogas production is limited by its recalcitrant structure. The microbial degradability of lignocellulosic biomass in the biogas system can be enhanced by making use of the system’s native cellulose-degrading bacterial community. The development of such operational strategies for enhanced degradability of lignocellulosic biomass would, however, benefit from more knowledge on (i) the taxonomy of this community and (ii) its response to process parameters.

This study aimed to contribute to a better understanding of these two areas by (i) characterising a putative novel cellulose-degrading bacterial species (strain Dc1) isolated from an industrial biogas reactor and (ii) investigating how the abundance of cellulose-degrading bacteria correlates with process parameters. Physiological features of strain Dc1 were characterised according to current standards for classification of novel species. Furthermore, the abundance of a functional gene-marker for cellulose degradation, cel48, was quantified by quantitative PCR and correlated to a set of process parameters in Swedish farm-based biogas plants.

The results indicate that Dc1 is mainly cellulolytic and saccharolytic, with some ability to degrade amino acids and fumarate. Dc1 exhibited close similarity to a known cellulose-degrading bacterium, Acetivibrio cellulolyticus (98.5%), based on 16S rRNA gene sequence analysis, but more experiments are needed to validate whether it belongs to the same species or represents a novel one. The relative abundance of cel48 (copies/ng total DNA) correlated positively with hydraulic retention time (HRT) in the range of 16-49 days (p<0.001). There was also an indication of negative correlation of the relative amount of cel48 with the total ammonium-nitrogen (TAN) concentration in the digestate, in the ranges 1.7 – 3.9 g/L.

The results from both parts of the study must, however, be interpreted with care, as they were subject to shortcomings that reduced the reliability of the results. Nevertheless, the results still offer new insights into the substrate utilisation and taxonomy of Dc1, as well as an indication of a positive effect of increased HRT and a negative effect of TAN on the cellulose-degrading bacterial community in agricultural biogas processes. The absence of correlations with other process parameters suggests that the abundance of cellulose-degrading bacteria may depend on the overall environment created by several process parameters rather than on individual ones.