The mechanisms for converting water and atmospheric CO2 into oxygen evolved over a billion+ years before the Earth's atmosphere even supported aerobic organisms on land with every variable crucial to the survival of producer organisms painfully optimized. I don't think we even have a good quantum mechanical description of chlorophyll and its electron transport chain, which might have ridiculously high efficiencies compared to our solar panels (I think I ready this somewhere?). Once you add the chemical pathways for taking the energy and storing it with CO2/H2O, you increase the complexity many orders of magnitude. Even if we can replicate the pathway, scaling it up to actually impact the CO2/O2 ratios would be both economically and technologically difficult.
