Physiology and methane productivity of Methanobacterium thermaggregans.

Accumulation of carbon dioxide (CO ₂ ), 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 ₂ recycling is to employ autotrophic and hydrogenotrophic methanogens for CO ₂ -based biological methane (CH ₄ ) production (CO ₂ -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 ₄ productivity. Moreover, inoculum size, sulfide feeding, pH, and temperature were optimized. Optimization of growth and CH ₄ 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 ^-1 . Moreover, the CH ₄ evolution rate (MER) of M. thermaggregans was compared to Methanothermobacter marburgensis, the CO ₂ -BMP model organism. Under optimized cultivation conditions, a maximum MER of 96.1 ± 10.9 mmol L ^-1  h ^-1 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 ₄ cell factory highly suited to be applicable for CO ₂ -BMP.