Physiology and methane productivity of Methanobacterium thermaggregans

Accumulation of carbon dioxide (CO₂), 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 CO₂ recycling is to employ autotrophic and hydrogenotrophic methanogens for CO₂-based biological methane (CH₄) production (CO₂-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 CH₄ productivity. Moreover, inoculum size, sulfide feeding, pH, and temperature were optimized. Optimization of growth and CH₄ 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 CH₄ evolution rate (MER) of M. thermaggregans was compared to Methanothermobacter marburgensis, the CO₂-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 CH₄ cell factory highly suited to be applicable for CO₂-BMP.