초록
<P><B>Background</B></P><P>In times of global climate change, the conversion and capturing of inorganic CO<SUB>2</SUB> have gained increased attention because of its great potential as sustainable feedstock in the production of biofuels and biochemicals. CO<SUB>2</SUB> is not only the substrate for the production of value-added chemicals in CO<SUB>2</SUB>-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 <I>Acetobacterium woodii</I> and <I>Thermoanaerobacter kivui</I> which catalyze the direct hydrogenation of CO<SUB>2</SUB> to formic acid with exceptional high rates, the hydrogen-dependent CO<SUB>2</SUB> reductases (HDCRs). Since these enzymes are promising biocatalysts for the capturing of CO<SUB>2</SUB> and the storage of molecular hydrogen in form of formic acid, we designed a whole-cell approach for <I>T. kivui</I> to take advantage of using whole cells from a thermophilic organism as H<SUB>2</SUB>/CO<SUB>2</SUB> storage platform. Additionally, <I>T. kivui</I> cells were used as microbial cell factories for the production of formic acid from syngas.</P><P><B>Results</B></P><P>This study demonstrates the efficient whole-cell biocatalysis for the conversion of H<SUB>2</SUB> + CO<SUB>2</SUB> to formic acid in the presence of bicarbonate by <I>T. kivui</I>. Interestingly, the addition of KHCO<SUB>3</SUB> 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<SUB>protein</SUB><SUP>−1</SUP> h<SUP>−1</SUP> (152 mmol g<SUB>CDW</SUB><SUP>−1</SUP> h<SUP>−1</SUP>), the highest rates ever reported in closed-batch conditions. The volumetric formate production rate was 270 mmol L<SUP>−1</SUP> h<SUP>−1</SUP> at 4 mg mL<SUP>−1</SUP>. 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.</P><P><B>Conclusions</B></P><P>The thermophilic acetogenic bacterium <I>T. kivui</I> is an efficient biocatalyst which makes this organism a promising candidate for future biotechnological applications in hydrogen storage, CO<SUB>2</SUB> capturing and syngas conversion to formate.</P>