Uptake of phosphate by Synechocystis sp. PCC 6803 in dark conditions: Removal driving force and modeling

Rapid uptake of inorganic phosphate (Pi) by microalgae should occur through two processes operating in parallel: onto extracellular polymeric substances (EPS) and intracellular polymeric substances (IPS). Most previous studies focused only on overall Pi uptake and ignored the roles of EPS. We investigated the two-step removal of Pi by Synechocystis sp. PCC 6803 in dark conditions (i.e., without incorporation of Pi into newly synthesized biomass). We also developed a model to simulate both steps. Experimental results with Synechocystis confirmed that Pi in the bulk solution was removed by the two uptake mechanisms operating in parallel, but with different kinetics. All uptake rates decreased with time, and the Pi uptake rate by IPS was much higher than that by EPS at all times, but EPS had a larger maximum Pi-storage capacity -- 33-48 mgP/gCOD_EPS versus 15-17 mgP/gCOD_IPS. Synechocystis had a maximum Pi-storage capacity in the range of 22-28 mgP/g dry biomass. Protein in EPS and IPS played the key role for binding Pi, and biomass with higher protein content had greater Pi-storage capacity. Furthermore, biomass with low initial stored Pi had faster Pi-uptake kinetics, leading to more Pi removed from the bulk solution. This work lays the foundation for using microalgae as a means to remove Pi from polluted water and for understanding competition for Pi in microbial communities.