Bioprocess strategies for enhancing the outdoor production of Nannochloropsis gaditana: an evaluation of the effects of pH on culture performance in tubular photobioreactors

A priority of the industrial applications of microalgae is the reduction of production costs while maximizing algae biomass productivity. The purpose of this study was to carry out a comprehensive evaluation of the effects of pH control on the production of Nannochloropsis gaditana in tubular photobioreactors under external conditions while considering the environmental, biological, and operational parameters of the process. Experiments were carried out in 3.0 m tubular photobioreactors under outdoor conditions. The pH values evaluated were 6.0, 7.0, 8.0, 9.0, and 10.0, which were controlled by injecting pure CO on-demand. The results have shown that the ideal pH for microalgal growth was 8.0, with higher values of biomass productivity (P) (0.16 g L d), and CO use efficiency (ECO2) (74.6% w w); RCO2 value obtained at this pH (2.42 gCO2 g) was close to the theoretical value, indicating an adequate CO supply. At this pH, the system was more stable and required a lower number of CO injections than the other treatments. At pH 6.0, there was a decrease in the P and ECO2; cultures at pH 10.0 exhibited a lower P and photosynthetic efficiency as well. These results imply that controlling the pH at an optimum value allows higher CO conversions in biomass to be achieved and contributes to the reduction in costs of the microalgae production process.