Next, we set out to find the maximum phosphate storage capacity in our strains by supplying an excess of phosphate. We opted for a fed batch scheme, tuning phosphate delivery rate to roughly match phosphate uptake rate. To accomplish this, we utilized previously described custom photobioreactors with a working volume of 800mL, 750µE/m 2 /s illumination, and 5% CO2 sparged at 0.1L/min. Phosphate feeding was capped at ~1.4mL/hour from a peristaltic pump to minimize changes to reactor volume. Several phosphate concentrations were tested before settling on 200mM K2HPO4, which delivers ~350mmol/L/hr (Supp Fig ).