Temperature increases from 55 to 75 degree C in a two-phase biogas reactor result in fundamental alterations within the bacterial and archaeal community structure

Agricultural biogas plants were operated in most cases below their optimal performance. An increase in the fermentation temperature and a spatial separation of hydrolysis/acetogenesis and methanogenesis are known strategies in improving and stabilizing biogas production. In this study, the dynamic variability of the bacterial and archaeal community was monitored within a two-phase leach bed biogas reactor supplied with rye silage and straw during a stepwise temperature increase from 55 to 75 degree C within the leach bed reactor (LBR), using TRFLP analyses. To identify the terminal restriction fragments that were obtained, bacterial and archaeal 16S rRNA gene libraries were constructed. Above 65 degree C, the bacterial community structure changed from being Clostridiales-dominated toward being dominated by members of the Bacteroidales, Clostridiales, and Thermotogales orders. Simultaneously, several changes occurred, including a decrease in the total cell count, degradation rate, and biogas yield along with alterations in the intermediate production. A bioaugmentation with compost at 70 degree C led to slight improvements in the reactor performance; these did not persist at 75 degree C. However, the archaeal community within the downstream anaerobic filter reactor (AF), operated constantly at 55 degree C, altered by the temperature increase in the LBR. At an LBR temperature of 55 degree C, members of the Methanobacteriales order were prevalent in the AF, whereas at higher LBR temperatures Methanosarcinales prevailed. Altogether, the best performance of this two-phase reactor was achieved at an LBR temperature of below 65 degree C, which indicates that this temperature range has a favorable effect on the microbial community responsible for the production of biogas.