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 (PM) systems, the cyanobacterium Syncchocystis PCC6803 ( PCC6803), and light conditions imitating actual day night light irradiance (1,1) Our results demonstrate that using normal BG-11 medium resulted in severe phosphate (P,) limitation for continuous operation Mitigation of P-1-limitation, by augmenting the P, content of BG-11, allowed higher biomass productivity; however, once P-1-limitation was alleviated, limitation by inorganic carbon (C-1) or LI occurred. C-1-limitation was detected by a low total C, concentration (<5 mg C/L) and high and fluctuating pH. C-1-limitation was relieved by delivering more CO2, which led to a stable pH in the range of 7-9 and at least 5 mg/L of C-1 in HCO3-. 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. Biotechnol, Bioeng. 2010,106 553-563. (C) 2010 Wiley Periodicals, Inc.