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

The value of rhodopsin proton pumps as light harvesters in electro-microbial CO2-to-biomass conversion depends on molecular photophysics, pathway energetics and reactor light delivery. We couple these scales for Cupriavidus necator, comparing Calvin-Benson-Bassham (CBB) and reductive glycine (rGlyP) assimilation. The model predicts a 17.9% dark rGlyP yield advantage, close to 16.5% measured in chemostats. At 568 nm, 100 PPFD and 45% membrane occupancy, light-derived ATP raises yield by 17% for rGlyP and 14% for CBB. Charging CO2 capture and formate electrosynthesis, a retinal-only reactor reaches 3.29% efficiency versus 2.56% in the dark. A carotenoid antenna raises this to 3.52% at the model-derived 70% transfer estimate and helps in 93.2% of uncertainty samples, but lowers efficiency at the thermodynamic limit by diverting photons from photovoltaics. Point-source CO2 cuts the relative light benefit from 38% to 8%. Rhodopsin is therefore most useful when upstream formate is energy-intensive, not as a general efficiency boost.