Perspectives for the biotechnological production of biofuels from CO2 and H2 using Ralstonia eutropha and other 'Knallgas' bacteria.

With global CO2 emissions at their highest in several years, mitigation and possibly reduction of greenhouse gas buildup and concomitant production of renewable fuel molecules for growing transportation fuel needs are urgent challenges for renewable energy scientists and engineers. Knallgas bacteria provide a biocatalyst platform for utilization of CO2 and production of diverse and some high-energy density biofuel molecules, requisite for drop-in transportation fuels. The most well-studied Knallgas bacterium, Ralstonia eutropha, has been engineered to produce n-butanol, isobutanol, and terpene molecules under chemolithoautotrophic conditions. There are other representatives of this group of bacteria that potentially have the capabilities for CO2-based fuel molecule synthesis. In principle, fermentative production of biofuel from CO2 could rival the "power-to-gas" (non-biological production of fuels using CO2 and H2) production methods. However, challenges remain for both methods in order to compete with currently priced petroleum-based fuels. With continued streamlining of processes and attention to Industrial Ecology principles, biofuel synthesis by Knallgas bacteria could represent a viable part of a nation's energy portfolio.