Anaerobic digestion of wheat straw by alkaliphilic mixed cultures and their physiological and molecular characterization

Alkaline pre-treatment of lignocellulose is commonly used in bioethanol production and might also enhance the anaerobic digestion of lignocellulosic biomass to biogas. However, the elevated pH of the substrate may require an alkalitolerant microbial community for its effective digestion. Therefore, the aim of the present study was to enrich anaerobic, lignocellulolytic cultures batchwise from sediments of two soda lakes with wheat straw as substrate under alkaline (pH 9) and mesophilic (37°C) conditions. Two mixed cultures were characterized in detail. Their gas production ceased after four to five weeks and the gas was mainly composed of CO2 and CH4. Acetate and propionate were the main liquid intermediates, which were nearly completely consumed until the end of incubation. The physiological behaviour of the cultures was stable even after several transfers. The enrichment process was followed by molecular fingerprinting (TRFLP) of the bacterial 16S rRNA gene and of the mcrA functional gene for methanogens. Additionally, the enriched microorganisms were identified by cloning and sequencing of the same genes. The main shift in the community compositions occurred between the sediment samples and the first enrichment, whereas the structure was stable in the following transfers. The two communities mainly consisted of Sphingobacteriales and Spirochaeta, but differed at genus level. Methanosaeta and Methanosarcina were the predominant methanogenic genera in cultures from Lake Velencei and Szarvas, respectively. Single lignocellulolytic microorganisms were isolated and identified as members of the alkaliphilic or alkalitolerant genera Actinotalea, Alkaliflexus, Natronincola, Anaerovirgula, Alkaliphilus and Alkalitalea. Nine strains could not be affiliated to any described genera. However, the most predominant members of the enrichment cultures were not obtained in pure cultures. The results of the present study show that anaerobic, alkaline habitats harbour diverse microbial communities, which can degrade lignocellulose effectively and are therefore a potential resource for improving anaerobic digestion by bioaugmentation.