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Background  Replacing fossil fuels as energy carrier requires alternatives that combine sustainable production, high volumetric energy density, easy and fast refueling for mobile applications, and preferably low risk of hazard. Molecular hydrogen (H 2 ) has been considered as promising alternative; however, practical application is struggling because of the low volumetric energy density and the explosion hazard when stored in large amounts. One way to overcome these limitations is the transient conversion of H 2  into other chemicals with increased volumetric energy density and lower risk hazard, for example so-called liquid organic hydrogen carriers such as formic acid/formate that is obtained by hydrogenation of CO 2 . Many homogenous and heterogenous chemical catalysts have been described in the past years, however, often requiring high pressures and temperatures. Recently, the first biocatalyst for this reaction has been described opening the route to a biotechnological alternative for this conversion.    Results  The hydrogen-dependent CO 2  reductase (HDCR) is a highly active biocatalyst for storing H 2  in the form of formic acid/formate by reversibly catalyzing the hydrogenation of CO 2 . We report the identification, isolation, and characterization of the first thermostable HDCR operating at temperatures up to 70 °C. The enzyme was isolated from the thermophilic acetogenic bacterium  Thermoanaerobacter kivui  and displays exceptionally high activities in both reaction directions, substantially exceeding known chemical catalysts. CO 2  hydrogenation is catalyzed at mild conditions with a turnover frequency of 9,556,000 h −1  (specific activity of 900 µmol formate min −1  mg −1 ) and the reverse reaction, H 2  + CO 2  release from formate, is catalyzed with a turnover frequency of 9,892,000 h −1  (930 µmol H 2  min −1  mg −1 ). The HDCR of  T. kivui  consists of a [FeFe] hydrogenase subunit putatively coupled to a tungsten-dependent CO 2  reductase/formate dehydrogenase subunit by an array of iron–sulfur clusters.    Conclusions  The discovery of the first thermostable HDCR provides a promising biological alternative for a chemically challenging reaction and might serve as model for the better understanding of catalysts able to efficiently reduce CO 2 . The catalytic activity for reversible CO 2  hydrogenation of this enzyme is the highest activity known for bio- and chemical catalysts and requiring only ambient temperatures and pressures. The thermostability provides more flexibility regarding the process parameters for a biotechnological application.    Electronic supplementary material  The online version of this article (10.1186/s13068-018-1236-3) contains supplementary material, which is available to authorized users.

Hydrogenation of CO2 at ambient pressure catalyzed by a highly active thermostable biocatalyst