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 −1 protein h −1 (152 mmol g −1 CDW 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.

Overcoming Energetic Barriers in Acetogenic C1 Conversion

“…Therefore, acetogens are the most flexible organisms to be used for a biological approach to capture and store CO 2 in the dark and absence of O 2 (Daniell et al, 2012;Liew et al, 2016;Köpke and Simpson, 2020). Since CO 2 fixation can be driven by H 2 oxidation, these bacteria also capture and store H 2 , a key process in the biohydrogen economy (Bailera et al, 2017;Müller, 2019;Schwarz and Müller, 2020).…”

Carbon Dioxide Utilisation—The Formate Route

“…7). A different rearrangement of the same basic features (FDH plus Hase) is found in cytoplasmatic dihydrogen-dependent FDHs (better denominated as CO 2 reductases), that physiologically catalyse the reduction of CO 2 to formate with the simultaneous and direct oxidation of dihydrogen, that is, without the intervention of an external electron-transfer protein or molecule, as reviewed by Litty and Müller in this Book [152] and also [153][154][155][156][157][158][159]. The dihydrogen-dependent CO 2 reductase of the acetogen A. woodii is a tetramer (abcd), holding one FDH-like subunit comprising one molybdenum and one [4Fe-4S] centres, where CO 2 is reduced; the necessary electrons are transferred intramolecularly from an iron-iron hydrogenase-like (Fe/Fe-Hase) subunit (second active site), via two small electron-transfer subunits (each with four [4Fe-4S] centres) ( Fig.…”

“…4.2.2. ), as is reviewed by Litty and Müller in this Book [152] and also [153][154][155][156][157][158][159] and references herein (below).…”

“…[271] for details Adapted with permission from Ref. [271] Acetogens and methanogens are organisms that reduce (fix) CO 2 in vivo [274,275], and, as such, they have been the focus of intense research to develop new CO 2 converter devices, enzymatic and whole-cell systems, as is reviewed by Litty and Müller in this Book [152] and also [153][154][155][156][157][158][159]. Herein, we only highlight the dihydrogen-dependent CO 2 reductases (Sect.…”

“…1h −1 (% 150 mmol g cdw −1 h −1 ) and a volumetric formate production rate of 270mMh −1 ; high titres up to 130 mM of formate were reached, with the key advantage of having the unwanted acetate formation abolished [159].…”