Whole-cell biocatalysis for hydrogen storage and syngas conversion to formate using a thermophilic acetogen

Abstract

Background

In times of global climate change, the conversion and capturing of inorganic CO ₂ have gained increased attention because of its great potential as sustainable feedstock in the production of biofuels and biochemicals. CO ₂ is not only the substrate for the production of value-added chemicals in CO ₂-based bioprocesses, it can also be directly hydrated to formic acid, a so-called liquid organic hydrogen carrier (LOHC), by chemical and biological catalysts. Recently, a new group of enzymes were discovered in the two acetogenic bacteria Acetobacterium woodii and Thermoanaerobacter kivui which catalyze the direct hydrogenation of CO ₂ to formic acid with exceptional high rates, the hydrogen-dependent CO ₂ reductases (HDCRs). Since these enzymes are promising biocatalysts for the capturing of CO ₂ and the storage of molecular hydrogen in form of formic acid, we designed a whole-cell approach for T. kivui to take advantage of using whole cells from a thermophilic organism as H ₂/CO ₂ storage platform. Additionally, T. kivui cells were used as microbial cell factories for the production of formic acid from syngas.

Results

This study demonstrates the efficient whole-cell biocatalysis for the conversion of H ₂ + CO ₂ to formic acid in the presence of bicarbonate by T. kivui. Interestingly, the addition of KHCO ₃ not only stimulated formate formation dramatically but it also completely abolished unwanted side product formation (acetate) under these conditions and bicarbonate was shown to inhibit the membrane-bound ATP synthase. Cell suspensions reached specific formate production rates of 234 mmol g _protein ⁻¹ h ⁻¹ (152 mmol g _CDW ⁻¹ h ⁻¹), the highest rates ever reported in closed-batch conditions. The volumetric formate production rate was 270 mmol L ⁻¹ h ⁻¹ at 4 mg mL ⁻¹. Additionally, this study is the first demonstration that syngas can be converted exclusively to formate using an acetogenic bacterium and high titers up to 130 mM of formate were reached.

Conclusions

The thermophilic acetogenic bacterium T. kivui is an efficient biocatalyst which makes this organism a promising candidate for future biotechnological applications in hydrogen storage, CO ₂ capturing and syngas conversion to formate.