Process parameters and changes in the microbial community patterns during the first 240 days of an agricultural energy crop digester

Nicolas Weithmann, Alfons Rupert Weig, Ruth Freitag

Commercial biogas production takes place by complex microbial communities enclosed in controlled "technical ecosystems". Once established, the communities tend to be resilient towards disturbances, although the relative abundance of their members may vary. The start-up phase, during which the community establishes itself, is therefore decisive for the later performance of the reactor. In this study, we followed the first 240 days of a standard agricultural energy crop digester consisting of a 400 m(3) plug flow fermenter and a 1000 m(3) agitated post digester, operated at 40-45 °C. The feed consisted of corn and later grass silage augmented by ground wheat. Changes in both the eubacterial and methanogenic archaeal communities were followed by automated ribosomal intergenic spacer analysis (ARISA). In addition the copy number of the methyl-coenzyme reductase A (mcrA)-genes found in all known methanogens were followed by quantitative PCR, while selected samples from two phases-one early, one late-of the community structure development were subjected to high throughput sequencing. Biogas volume and composition (CH4, CO2, H2, H2S, O2), pH, ammonia-N, and volatile fatty acids (VFA), were measured as part of the routine process control. VFA/TIC values were calculated on this basis. Whereas the total gas production of the plant established itself at about 2500 m(3) biogas per day within the first months, the composition of the microbial communities showed distinct spatial and temporal differences over the investigated time period. Absolute values for DNA isolation procedures are difficult to certify, hence comparative results on community structures obtained using standardized ARISA with identical primers are of value. Moreover, ARISA patterns can be statistically analyzed to identify distinct subgroups and transitions between them as well as serial correlations. Thereby the microbial community and its structural development can be correlated with statistical relevance to changes in operational (feed) and process parameters (pH-value, biogas composition). In particular when augmented by deep sequencing data of judiciously chosen samples, this allows a hitherto unknown level of insight into the performance of technical biogas plants.