Responses of synechocystis sp. PCC 6803 to total dissolved solids in long-term continuous operation of a photobioreactor

This study evaluated how Synechocystis sp. PCC 6803 responds to high total dissolved solids (TDS) associated with eliminating nutrient limitation during long-term operation of a photobioreactor. The unique feature is that the TDS were not dominated by Na⁺ and Cl^-, as in seawater, but by HCO₃^- and NO₃^- from nutrient delivery. The TDS-stress threshold was about 10g/L. Whereas inorganic N and P limitations slowed the rate of inorganic C (C_i) uptake in the light, TDS stress was manifested most strongly as a substantial increase of endogenous respiration rate at night. Relief from TDS stress was incomplete when lowered pH led to a HCO₃^- increase (560mgC/L as a threshold). Impaired photosynthesis led to a cascade of reduced C_i-uptake, pH decrease, HCO₃^- accumulation, and HCO₃^--associated stress. Thus, long-term photobioreactor operation requires balancing the delivery rates of CO₂, N, P, and other TDS components to avoid general and C_i-associated TDS stresses.