Nutrient acquisition and limitation for the photoautotrophic growth of Synechocystis sp. PCC6803 as a renewable biomass source.

Photoautotrophic microorganisms (cyanobacteria and algae) offer high promise as a source of biomass for renewable energy due to their rapid growth rates and high biomass yields. To provide a framework for evaluating the feasibility of growing phototrophic microorganisms with high biomass production rates, we operated a bench-scale photobioreactor using Synechocystis sp. PCC6803 and with light conditions imitating actual day-night light irradiance (LI). During the time of peak LI, PCC6803's specific growth rate (1.7 day⁻¹) and the nitrate uptake rate (0.46 g N/g DW day) were high compared to past reports. Analysis employing the stoichiometry of photosynthesis of PCC6803 and ionic speciation showed that bicarbonate and phosphate were driven to very low concentrations for the high-LI conditions. In particular, the systematic evaluation of rate-limiting factors identified when the CO₂-C(i) supply rate needed to be increased to mitigate HCO₃⁻ depletion and a large pH increase. It also showed that the traditional BG-11 medium needs to be augmented with phosphate to avoid severe P depletion. This work exploits quantitative understanding the stoichiometry and kinetics of cyanobacteria for the high-rate production of a renewable biomass.