The pH values of cultures affect the biochemical processes associated with microalgae, including the bioavailability of CO2 for photosynthesis and use of the medium nutrients.

The optimum pH is determined according to the type of microorganism. Some species have an optimum pH of around 7.0; however, some microalgae are tolerant to high pH (Spirulina, pH 11.0) or low pH (Chlorococcum, pH 4.0) (Kumar et al., 2010).

The optimum growth of the microorganism in an acidic or basic environment can be maintained if the intracellular pH is 7.5, regardless of the external pH. Living cells have the ability, within certain limits, to maintain internal pH by expelling hydrogen ions. The external pH generally has a drastic change before it affects the cell. The optimum pH of the cultures should be maintained, thereby preventing the collapse of cell cultures by the cellular process of rupture due to high pH. The control of pH must be integrated with the aeration system by the addition of alkaline solution to the culture (Wang et al., 2012).

Some microalgae have high productivity when maintained at an alkaline pH between 10 and 11. The high pH may be beneficial for outdoor cultivation because it inactivates patho­genic microorganisms and other microalgae (Kumar et al., 2010).

In the case of cultivation with addition of CO2, the concentration of this gas may be the dominant factor that will determine the pH of the culture. In this case, the CO2 demand results from the balance between the transfer of CO2 to the liquid and CO2 consumption by the cells (Wang et al., 2012). SOx and NOx, present in flue gas from burning coal, can also cause changes in pH, damaging microalgal cultivation. With high concentrations of CO2 the pH drops to 5.0, and when exposed to SOx and NOx this value is 2.6 (Westerhoff et al., 2010).

The pH also influences the removal of ammonia and phosphorus. The high pH may increase the removal of ammonia through its volatilization and phosphorus through its pre­cipitation (Craggs, 2005).