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].