# Effect of pH Change on the Microalgae-Based Biogas Upgrading Process
## 3 Results  
Figure 2 shows the photosynthetic activity and PSII quantum yield of the microalgae culture when the pH decreased from 7.0 to 5.0 after the addition of HCl, at two biomass concentrations of 0.5 and 1.3 g L\({}^{-1}\). The photosynthetic activity of the light-exposed culture decreased after 100 min for both biomass concentrations when the pH changed from 7.0 to 5.0 (Figure 2A,B). On the other hand, although the PSII quantum yield slightly decreased for both biomass concentrations, it remained around 0.6 and 0.7 (Figure 2C,D), such values correspond to healthy batch microalgae cultures [22]. When the pH was adjusted from pH 7.0 to pH 5.0 at 0.5 g L\({}^{-1}\) (Figure 2A), the photosynthetic activity of the culture recovered, demonstrating that the cells did not suffer permanent damage. On the other hand, when the pH was changed in the conditions of darkness, no significant changes in the photosynthetic activity were observed.  
Figure 3 shows the photosynthetic activity and PSII quantum yield of the microalgae culture when the pH was decreased from 7.0 to 5.8 by CO\({}_{2}\) injection, simulating the process occurring in the column (the DIC concentration in the microalgae culture was tripled because of CO\({}_{2}\) bubbling). In contrast to the situation of the pH change by only acid addition (Figure 2A,B), when the pH was reduced by CO\({}_{2}\) injection, the photosynthetic activity decreased immediately in the culture exposed to light and darkness (Figure 3A). This result could mean that CO\({}_{2}\) inhibits the microalgae activity by a different mechanism for pH decrease. The photosynthetic activity was recovered after CO\({}_{2}\) injection. However, once CO\({}_{2}\) injection was stopped, the pH increased and the DIC concentration decreased in the culture medium due to both carbon fixation and desorption processes. In this sense, similar results were obtained with _Chlorella sp._, whose growth was inhibited when it was exposed to high CO\({}_{2}\) concentrations, but the growth reappeared when the concentration was decreased [28].  
As shown in Figure 3B, the CO\({}_{2}\) injection caused an increase in the PSII quantum yield. An increase in PSII yield means that a higher percentage of the absorbed light energy was used in the photochemical process. This response could be attributed to an increase in the demand for reducing power (NADPH) to fix and reduce the higher carbon inorganic concentration in the culture medium [29].  
To study the preferred inorganic carbon source of _C. sonokiniana_, the apparent affinity constant for the dissolved inorganic carbon was determined. Figure 4 shows the photosynthetic activity as a function of the inorganic carbon concentration provided in the electrode cube at pH 7.0 and pH 5.0. The apparent affinity constant for dissolved inorganic carbon (KDIC) was 0.9 uM at pH 5 and 112.0 uM at pH 7. The NaHCO\({}_{3}\) added into the algal samples at pH 5 was mostly in the form of CO\({}_{2}\). Therefore, the lower apparent KDIC value at pH 5 than pH 7 suggests that this microalgae had a higher affinity for CO\({}_{2}\) than HCO\({}_{3}\)Figure 3: Effect of the pH and DIC concentration changes on microalgae activity. **(A)** Photosynthetic activity when CO\({}_{2}\) injection was applied. **(B)** PSII quantum yield when CO\({}_{2}\) injection was applied.  
Figure 2: Effect of the pH change on the microalgae activity by HCl addition. **(A)** Photosynthetic activity when a biomass concentration of 0.5 g L\({}^{-1}\) was used. **(B)** Photosynthetic activity when a biomass concentration of 1.3 g L\({}^{-1}\) was used. **(C)** PSII quantum yield when a biomass concentration of 0.5 g L\({}^{-1}\) was used. **(D)** PSII quantum yield when a biomass concentration of 1.3 g L\({}^{-1}\) was used.