Stimulating effect of protein-rich matter on the biogenic conversion of CO2 to CH4

• Protein-rich matter such as protein powder from soybean can be as effective as yeast extract to stimulate the biogenic conversion of CO2 to CH4 via the reduction pathway with H2.
• The biogenic conversion of CO2 to CH4 with H2 supply from the metabolization of protein-rich matter can be reflected by a net reduction of CO2 levels.
• Moderate acidic pH levels (>5) enhances the biogenic conversion of CO2 to CH4 with H2 supply from the metabolization of protein-rich matter.
• H2 is mainly formed from the metabolization of carbohydrate compounds contained in protein-rich matter.

H2 supply constitutes the bottleneck for the biogenic recycling of CO2 to CH4 via the reduction pathway with H2. Past studies have reported that yeast extract has a significant effect on H2 formation from various types of organic matter. In order to elucidate whether or not less expensive protein-rich matter like protein powder from soybean can be used as substitute for yeast extract, a comparative assessment of biogenic conversion of CO2 to CH4 with H2 supply from the fermentation of yeast extract, protein powder from soybean, and glucose was done using a mixture culture of H2-forming fermentative and H2-using methanogenic microbes. Batch experiment results showed that under H2 limitation conditions, larger amounts of H2 is supplied at the expenses of less CO2 production with yeast extract and protein powder from soybean than with glucose. Protein powder from soybean has been found to have similar stimulating effect on H2 formation to yeast extract. The biogenic conversion of CO2 to CH4 with protein powder from soybean was reflected by a net reduction of CO2 levels. For the first time, it is shown that soybean protein is a potential candidate to stimulate the biogenic recycling of CO2 to CH4 in geological storage sites of CO2.

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