It's not. Photosynthesis is two steps: 1. Assimilating photons from sunlight to generate electrical potential by oxidizing water (ps that is kind of a crazy c

It's not. Photosynthesis is two steps: 1. Assimilating photons from sunlight to generate electrical potential by oxidizing water (ps that is kind of a crazy concept). 2. Fixing carbon dioxide by using the electron from step one to reduce CO2 to C-OH (ish) 1. Photosystem-II-I systems really suck, like, commercially available photocells are more efficient than fundamental quantum limits of the protein complex, beyond what it is likely for us to be able to do design or evolve for. Our photocells are getting better too. Also, photosynthetic organisms, especially algae, self-shadow. That's why you almost have to have those rediculous algae racetracks (look it up) and of course often the least valuable land + where there is sunlight so that it stands a chance to make economical sense is often in the desert, so for algae you're stuck shipping water to the desert :/ 2. Carbon fixation really really sucks. RuBisCo is the enzyme that is the bottleneck. It sucks so hard that RuBisCo is literally the most abundant enzyme on the planet to make up for how bad it is. Not only is it an annoyingly slow enzyme, it's oxygen sensitive, which is hilarious because step 1 of photosynthesis cracks water. Plants and algae do some crazy gymnastics to separate part 1 and part 2; look up C3 and C4 cycles, and algae had to invent colonial forms and terminal differentiation because their carbon fixers are kept segregated from their photosynthesizers; one algae -synechocystis 6803 has one of the biggest algae genomes, it probably mostly goes into a rigourous gene expression which is used to anticipate circadian cycles and prepare the cell to flip between day (photosystem) and night (carbon and nitrogen fixation) cycles.