Cellulolytic and Xylanolytic Microbial Communities Associated With Lignocellulose-Rich Wheat Straw Degradation in Anaerobic Digestion

Abstract

The enzymatic hydrolysis of lignocellulosic polymers is generally considered the rate-limiting step to methane production in anaerobic digestion of lignocellulosic biomass. The present study aimed to investigate how the hydrolytic microbial communities of three different types of anaerobic digesters adapted to lignocellulose-rich wheat straw in continuous stirred tank reactors operated for 134 days. Cellulase and xylanase activities were monitored weekly using fluorescently-labeled model substrates and the enzymatic profiles were correlated with changes in microbial community compositions based on 16S rRNA gene amplicon sequencing to identify key species involved in lignocellulose degradation. The enzymatic activity profiles and microbial community changes revealed reactor-specific adaption of phylogenetically different hydrolytic communities. The enzymatic activities correlated significantly with changes in specific taxonomic groups, including representatives of Ruminiclostridium, Caldicoprobacter, Ruminofilibacter, Ruminococcaceae, Treponema, and Clostridia order MBA03, all of which have been linked to cellulolytic and xylanolytic activity in the literature. By identifying microorganisms with similar development as the cellulase and xylanase activities, the proposed correlation method constitutes a promising approach for deciphering essential cellulolytic and xylanolytic microbial groups for anaerobic digestion of lignocellulosic biomass.

Keywords: anaerobic digestion; biogas; fluorometric enzyme assay; hydrolysis; lignocellulose; microbial adaptation; microbial community; wheat straw.

FIGURE 1

Methane production rates for the thermophilic reactor (R1), and mesophilic reactors (R2 and R3). MPRs are presented as weekly average. The reactors were fed with wheat straw from day 11–81 and organic loading rates (g VS ⋅ L^–1 ⋅ day^–1) are indicated on the graph. The MPR was not recorded during days 60–66 due to a technical error.

FIGURE 2

Cellulase (A) and xylanase (B) activities given by MUF release rate in the thermophilic R1, and the mesophilic reactors R2 and R3. The reactors were fed with wheat straw from day 11–81 and organic loading rates (g VS ⋅ L^–1 ⋅ day^–1) are indicated on the graphs.

FIGURE 3

Redundancy analysis. RDA analysis of the sampled duplicate reactors R1 (A,B), R2 (C,D), and R3 (E,F), constrained by either cellulase (A,C,E) or xylanase (B,D,F) activity. Samples are colored by enzymatic activity and a line is drawn between consecutive sampling points in the time series. The individual ASVs are shown on the models as gray dots. R1 is thermophilic, and R2 and R3 are mesophilic reactors.