Population dynamics of methanogens during acidification of biogas fermenters fed with maize silage

Abstract

AbstractLimitations in the supply of essential trace elements for methanogenic Archaea can arise in biogas production from renewable resources in flow‐through systems particularly when no manure is added. Without compensating supplementation, primarily Co and Na+ can become limiting in long‐term mono‐digestion of maize silage at threshold values of ca. 0.03 and 10 mg/L, respectively. These deficiencies apparently triggered process acidification. Using molecular biological methods (PCR‐SSCP and quantitative real‐time PCR), microbial population dynamics were monitored qualitatively and quantitatively at distinct stages during the experiments, investigating mcrA/mrtA, coding for a subunit of the key enzyme of methanogenesis. An exponential correlation between mcrA/mrtA copies and methane productivity was obtained. Members of obligatley acetoclastic Methanosaetaceae were found only at low acetate concentrations (below 1 g/L). At organic loading rates>1 g volatile solids/(L×d) and without acidification symptoms, obligately hydrogenotrophic (oh) Methanobacteriales and versatile Methanosarcinaceae were dominating, and an abundance of up to 1010 methanogens per mL fermenter content was determined. In the acidified process, however, ca. 4 orders of magnitude less methanogens were detected, and Methanomicrobiales (oh), more specifically Methanospirillum hungatei or Methanoculleus spp., were dominating. Species diversity at the DNA level was highest at efficient process performance without stress symptoms and at a relatively low organic loading rate (1–2 g volatile solids/(L×d)). According to the quantitative real‐time PCR data, the process was not sustained below an availability of 10−8 to 10−9 μg Co per methanogenic cell.