Enhancing Methanogenesis performance in the Power-to-Methane process: Addressing H₂ and CO₂ intermittency

Question

Power-to-Methane (PtM) technology stands at the forefront as a promising solution for the storage of surplus renewable energy aligned with CO₂ reduction. In the biological PtM process, methanogens utilize H₂ generated from water electrolysis powered by renewable energy and CO₂ from industrial waste gas to produce CH₄, namely methanogenesis. Our work aims to enhance the robustness of PtM processes by optimizing this methanogenesis step, addressing challenges related to intermittent gas feeding.

Methods

The model strain Methanococcus maripaludis MM901 was cultured in batch cultures to study its starvation-revival dynamics under H₂- vs. CO₂- starvation by monitoring OD_600nm and specific CH₄ production rate. To obtain mechanistic understanding of the observed difference, we also (1) measured NAD⁺ and NADH in the cells during starvation using bioluminescent assays, as well as (2) performed oxygen exposure experiments, where cultures were exposed to air for 1h during starvation followed by monitoring their revival dynamics after O₂ removal.

Results

M. maripaludis MM901 exhibited a shorter lag time and higher metabolic activity after H₂ starvation compared to CO₂ starvation, and an increase in starvation time amplified the difference. We hypothesize that cells experience a reductive stress under CO₂ starvation. We measured NAD⁺ and NADH in cells during starvation and observed significantly higher NADH/NAD⁺ ratio in CO₂ starved cells indicating accumulation of reducing equivalence. O₂ exposure also exerts greater inhibition on the revival of CO₂-starved cultures over H₂-starved cultures shown by a significantly longer lag phase.

Conclusions

In CO₂-starved cells where there is H₂ leftover, it is possible that O₂ reacts with accumulated reducing equivalence, resulting in the production of highly oxidative reactive oxygen species, and subsequently damaged enzymes and DNA in cells. To ensure the process stability and gas product quality, where H₂ leftover is preferred over CO₂ leftover, further efforts are needed to adapt methanogens to starvation under CO₂ limitation.