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 photosynthetic microorganisms, 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 P(i)-limitation, by augmenting the P(i) 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 (<5 mg C/L) and high and fluctuating pH. C(i)-limitation was relieved by delivering more CO(2), which led to a stable pH in the range of 7-9 and at least 5 mg/L of C(i) in HCO(3)(-). LI limitation, evidenced by an average LI <14 W/m(2) for PCC6803, was induced by a high biomass concentration of 1,300 mg/L. Thus, this work provides quantitative tools of stoichiometry and kinetics to evaluate limitation on PBRs.