Effects of inorganic carbon and pH on growth kinetics of Synechocystis sp. PCC 6803

Growth pH and the concentration of bioavailable inorganic carbon (C_ib) are two essential factors controlling the rate of photoautotrophic growth of microalgae. We developed a novel experimental strategy to determine the effects of pH and C_ib independently by a pH-Stat and the use of NH₄NO₃ as the N source. We grew Synechocystis sp. PCC 6803 at pH values of 7.5, 8.5, and 9.5, with a stable inorganic carbon (C_i) concentration ranging from 0.08 to 3.3 mM, and a light intensity of ~ 202 μE m^− 2 s^− 1. Among the pH values tested, pH 8.5 supported the highest maximum specific growth rate (μ_max), 2.4/day that was statistically larger than μ_max values at pH 7.5 and 9.5, about 2.0/day. Using the sum of dissolved CO₂, carbonic acid, and bicarbonate (i.e., C_ib) to fit Monod kinetics, we found the half-maximum-rate concentrations (K_Cib) were in a narrow range: 86 μM for pH 8.5, 84 μM for pH 7.5, and 96 μM for pH 9.5, values that were statistically indistinguishable. All K_Cib values were small, indicating that Synechocystis has a high affinity for C_ib, attaining μ_max by ~ 1 mM. Analyzing the kinetics individually for each C_i species supports that bicarbonate best represented the rate of C_i uptake.