A conditional upper-bound design map for rhodopsin-augmented electro microbial CO2-to-biomass conversion

Researchers have developed a design map to optimize the process of converting CO2 to biomass using rhodopsin-augmented electro-microbial systems. The key factors influencing the value of rhodopsin proton pumps in this conversion are molecular photophysics, pathway energetics, and reactor light delivery. They investigated the Calvin-Benson-Bassham (CBB) and reductive glycine (rGlyP) assimilation pathways in Cupriavidus necator.

The model indicates a 17.9% advantage in dark rGlyP yield compared to CBB, which aligns closely with experimental measurements in chemostats. Under ideal conditions of 568 nm light intensity at 100 PPFD and 45% membrane occupancy, light-derived ATP increases yield by 17% for rGlyP and 14% for CBB. By simultaneously charging CO2 capture and formate electrosynthesis, a retinal-only reactor achieves a 3.29% efficiency rate, surpassing the 2.56% efficiency in a dark environment.

Incorporating a carotenoid antenna enhances this efficiency further, reaching 3.52% under the model's 70% transfer estimate. However, this enhancement comes at the cost of a slight decrease in efficiency by 0.04% when the thermodynamic limit is reached, as photons are diverted from photovoltaics. Notably, point-source CO2 delivery reduces the relative light benefit from 38% to 8%.

Thus, rhodopsin's utility is most pronounced when upstream formate is energy-intensive, rather than providing a universal efficiency enhancement.