The primary role of the carboxysome is to enhance the carboxylation over oxygenation reactions by rubisco, and hence the local concentration of O2 inside the carboxysome is expected to be lower than the CO2 concentration under most physiological conditions. Residual O2 permeation across the carboxysome shell results in the limited photorespiration. The cytosolic O2 concentration will be relatively low, as O2 preferentially partitions to lipid membranes, and is generated during photosynthesis by photosystem II that is embedded within the thylakoid membrane . The carboxysome shell proteins are just as effective at reflecting O2 entering from the cytosol as they are in retaining CO2. The presence of the shell would act as a physical barrier to lower the O2 availability near the rubisco active site compared to CO2, thereby minimizing photorespiration. This is analogous to the reduction in O2 availability within protein nanocages, where the proteins inhibit O2 access to the nanocage interior . To be clear, as stated some O2 will permeate across the carboxysome shell, and if it does so, we would anticipate that rubisco will push RuBP towards the photorespiratory pathway and 2-phosphoglycolate production. The ability for some O2 to enter the carboxysome is consistent with previous experimental findings that intact photorespiration pathways are required for normal growth in cyanobacteria under standard atmospheric conditions .