A team at UCLA is introducing a biological yet non-photosynthetic CO2 reduction mechanism that has the potential to yield environmental and economic benefits via CO2-derived high-value products. Their paper is published in the journal Joule.
Most of us naturally associate biological CO2 conversion with photosynthesis in plants and algae. While engineering photosynthetic hosts to convert CO2into high-value products is sensible, dependence on sunlight limits its tractability and scalability. The productivity of photosynthesis is proportional to the surface area exposed to sunlight, a capricious source of energy in many regions. Furthermore, the maximum efficiency of solar energy conversion by photosynthesis is 5%, while typical solar panel efficiency reaches 20%. These shortcomings may be overcome if the Calvin cycle—the light-independent metabolic pathway in which CO2 is assimilated by the famous enzyme Rubisco—is introduced into non-photosynthetic organisms and driven by chemical energy instead of light.
Perhaps less familiar to most is a biological yet non-photosynthetic CO2 reduction mechanism: the reductive acetyl-CoA pathway, also known as the Wood-Ljungdahl pathway (WLP). Acetogenic microbes (e.g., A. woodii, C. ljungdahlii, and M. thermoacetica) can reduce two CO2 molecules, via the carbonyl and methyl branches of this pathway, to make one acetic acid. Unlike plants and algae, these acetogens do not depend on sunlight; they can instead derive energy from H2.
… An inevitable challenge associated with biological CO2 conversion is that typical bioprocesses are slow and must be sped up. This complication arises because cells require energy in two forms, reducing power and ATP, in balance. … Therefore, meta-bolic engineers must ensure that (1) minimum cellular ATP requirement is met and (2) cells have ATP and reducing power in the right stoichiometry for desired product synthesis. If these are achieved, carbon yield and productivity can be greatly accelerated (e.g., each gram of acetogenic M. thermoacetica cells can reduce 56 g of CO2 per day, ~50 times as fast as photosynthetic cells).—Erşan and Park
The Overall Schema for Non-photosynthetic CO2 Utilization toward a Sustainable Future. Using various sources of energy and integrated biological-inorganic catalysis may optimize non-photosynthetic CO2 conversion. Producing both high-margin products and high-volume products will ensure economic and global-scale CO2 utilization. Erşan and Park
Erşan and Park (2020) “Light-Independent Biological Conversion of CO2,” Joule doi: 10.1016/ j.joule.2020.08.007