Introduction
The globally increasing level of atmospheric greenhouse gases and its proven effect of global warming is an urgent incentive for the chemical industry to develop greenhouse gas neutral or even negative processes. Biotechnology offers a CO2 saving alternative to traditional chemical processes for the production of an ever-increasing range of carbon containing molecules, by consuming renewable rather than fossil carbon sources. Still, almost all biotechnological processes emit CO2 originating from the production of sugar as a so-called first generation carbon source (see e.g. Salim et al. , 2019), from the generation of utilities (power, heat, steam), as well as from the oxidation of part of the carbon source to generate metabolic energy. As a result, part of the CO2 that is fixed by the crops producing the carbon source returns to the atmosphere during the process, and these carbons are lost for the product.
A major step in further decreasing CO2 emissions by biotechnological processes would be to capture the emitted CO2, electrochemically reduce it to a suitable organic molecule using renewable electricity, and (co-)feed this carbon source back into the fermentation stage of the process (Noorman. Here we present formic acid as an example:
\begin{equation} \text{CO}_{2}\ +H_{2}\text{O\ }CH_{2}O_{2}+0.5\ O_{2}\text{\ \ \ \ \ }\left[eq.1\right]\nonumber \\ \end{equation}
Formic acid has been demonstrated as a suitable auxiliary energy source for several microbial species (Bruinenberg et al. , 1985; Overkampet al. , 2002; Geertman et al. , 2006; Harris et al. , 2007; Wang et al. , 2019), which can transfer the electrons from formic acid to NAD+, forming NADH and CO2 with a formate dehydrogenase enzyme (FDH):
\begin{equation} CH_{2}O_{2}+\text{NAD}^{+}\text{\ \ }\ \text{CO}_{2}+\ NADH\ +\ H^{+}\text{\ \ \ \ \ }\left[eq.2\right]\nonumber \\ \end{equation}
The cells can then use the NADH generated to either provide reducing power in biosynthetic pathways or generate metabolic energy (ATP) via aerobic respiration. This closed carbon cycle, where the emitted CO2 is continuously captured, reduced to formic acid and fed back into the fermentation, can theoretically provide all ATP via [eq.2] plus respiration. Such processes uniquely use the primary carbon source (e.g. glucose) for assimilation and therefore have significantly increased biomass and product yields on the primary carbon source. In essence, such a process is partially decarbonized by replacing a fraction of the glucose substrate by renewable electricity [Figure 1].