Photoautotrophic nutrient utilization and limitation during semi-continuous growth of Synechocystis sp. PCC6803

Microbial photosynthesis presents a valuable opportunity to capture abundant light energy to produce renewable bioenergy and biomaterials. To understand the factors that control the productivity of photosyntheticmicroorganisms, we conducted a series of semi-continuous experiments using bench-scale photobioreactor (PBR) systems, the cyanobacterium Synechocystis PCC6803 (PCC6803), and light conditions imitating actual day-night light irradiance (LI). Our results demonstrate that using normal BG-11 medium resulted in severe phosphate (P_i) limitation for continuous operation. Mitigation of Pi-limitation, by augmenting the Pi content of BG-11, allowed higher biomass productivity; however, once P_i-limitation was alleviated, limitation by inorganic carbon (C_i) or LI occurred. C_i-limitation was detected by a low total C_i concentration (<5mgC/L) and high and fluctuating pH. C_i-limitation was relieved by delivering more CO₂, which led to a stable pH in the range of 7-9 and at least 5mg/L of C_i in HCO₃^-. LI limitation, evidenced by an average LI <14W/m² for PCC6803, was induced by a high biomass concentration of 1,300mg/L. Thus, this work provides quantitative tools of stoichiometry and kinetics to evaluate limitation on PBRs.