Physiology and methane productivity of Methanobacterium thermaggregans.

Accumulation of carbon dioxide (CO), associated with global temperature rise, and drastically decreasing fossil fuels necessitate the development of improved renewable and sustainable energy production processes. A possible route for CO recycling is to employ autotrophic and hydrogenotrophic methanogens for CO-based biological methane (CH) production (CO-BMP). In this study, the physiology and productivity of Methanobacterium thermaggregans was investigated in fed-batch cultivation mode. It is shown that M. thermaggregans can be reproducibly adapted to high agitation speeds for an improved CH productivity. Moreover, inoculum size, sulfide feeding, pH, and temperature were optimized. Optimization of growth and CH productivity revealed that M. thermaggregans is a slightly alkaliphilic and thermophilic methanogen. Hitherto, it was only possible to grow seven autotrophic, hydrogenotrophic methanogenic strains in fed-batch cultivation mode. Here, we show that after a series of optimization and growth improvement attempts another methanogen, M. thermaggregas could be adapted to be grown in fed-batch cultivation mode to cell densities of up to 1.56 g L. Moreover, the CH evolution rate (MER) of M. thermaggregans was compared to Methanothermobacter marburgensis, the CO-BMP model organism. Under optimized cultivation conditions, a maximum MER of 96.1 ± 10.9 mmol L h was obtained with M. thermaggregans-97% of the maximum MER that was obtained utilizing M. marburgensis in a reference experiment. Therefore, M. thermaggregans can be regarded as a CH cell factory highly suited to be applicable for CO-BMP.